Part 5 (1/2)
[Ill.u.s.tration: _a_ Transparent area. _b_ Primitive trace.]
As development advances, the cicatricula expands. ”We are indebted to Pander,”[26] says Dr. Allen Thomson in his admirable essay above quoted, ”for the important discovery, that towards the twelfth or fourteenth hour, in the hen's egg the germinal membrane becomes divided into two layers of granules, the serous and mucous layers of the cicatricula; and that the rudimentary trace of the embryo, which has at this time become evident, is placed in the substance of the upper-most or serous layer.” ”According to this observer, and according to Baer, the part of this layer which surrounds the primitive trace soon becomes thicker; and on examining this part with care, towards the eighteenth hour, we observe that a long furrow has been formed in it, in the bottom of which the primitive trace is situated; about the twentieth hour this furrow is converted into a ca.n.a.l open at both ends, by the junction of its margins (the _plicae primitivae_ of Pander, the _laminae dorsales_ of Baer:) the ca.n.a.l soon becomes closed at the cephalic or swollen extremity of the primitive trace, at which part it is of a pyriform shape, being wider here than at any other part.
According to Baer and Serres, some time after the ca.n.a.l begins to close, a semi-fluid matter is deposited in it, which on its acquiring greater consistence, becomes the rudiment of the spinal cord; the pyriform extremity or head is soon after this seen to be partially subdivided into three vesicles, which being also filled with a semi-fluid matter, gives rise to the rudimentary state of the encephalon.” ”As the formation of the spinal ca.n.a.l proceeds, the parts of the serous layer which surrounds it, especially towards the head, become thicker and more solid, and before the twenty-fourth hour we observe on each side of this ca.n.a.l four or five small round opaque bodies, these bodies indicate the first formation of the dorsal vertebrae.
[Ill.u.s.tration: _a_ Transparent area. _b_ Laminae dorsales. _c_ Cephalic end. _d_ Rudiments of dorsal vertebrae. _e_ Serous layer. _f_ Lateral portion of the primitive trace. _g_ Mucous layer. _h_ Vascular layer. _k_ Laminae dorsales united to form the spinal ca.n.a.l.]
”About the same time, or from the twentieth to the twenty-fourth hour, the inner layer of the germinal membrane undergoes a farther division, and by a peculiar change is converted into the vascular mucous layers.” (A.
Thomson, _op. cit._) It will thus be seen, that the germinal membrane is that part of the ovum in which the first changes produced by impregnation are observed. The rudiments of the osseous and nervous systems are formed by the outer or serous layers; the outer covering of the foetus or integuments, including the amnois, are also furnished by it. ”The layer next in order has been called _vascular_, because in it the development of the princ.i.p.al parts of the vascular system appears to take place. The third, called the _mucous_ layer, situated next the substance of the yelk, is generally in intimate connexion with the vascular layer, and it is to the changes which these combined layers undergo, that the intestinal, the respiratory, and probably also the glandular systems owe their origin.”
(A. Thomson, _op. cit._ p. 298.)
[Ill.u.s.tration: _a_ Serous layer. _b c_ Vascular layer. _d_ Mucous layer.
_e_ Heart.]
The embryo is therefore formed in the layers of the germinal membrane, and becomes, as it were, spread out upon the surface of the ovum: the changes which the ovum of mammalia undergoes appear from actual observation, to be precisely a.n.a.logous to those in the inferior animals. (_Baer_, _Prevost_ and _Dumas_.) From the primitive trace, which was at first merely a line crossing the cicatricula, and which now begins rapidly to exhibit the characters of the spinal column, the parietes of the head and trunk gradually approach farther and farther towards the anterior surface of the abdomen and head until they unite; in this way the sides of the jaws close in the median line of the face, occasionally leaving the union incomplete, and thus appearing to produce in some cases the congenital defects of hare-lip and cleft palate. In some way the ribs meet at the sternum; and it may be supposed that sometimes this bone is left deficient, and thus may become one of the causes of those rare cases of malformation, where the child has been born with the heart external to the parietes of the thorax. In like manner the parietes of the abdomen and pelvis close in the linea alba and symphysis pubis, occasionally leaving the integuments of the navel deficient, or, in other words, producing congenital umbilical hernia, or at the p.u.b.es a non-union of its symphysis with a species of inversion of the bladder, the anterior wall of that viscus being nearly or entirely wanting.
The cavity of the abdomen is therefore at first open to the vesicula umbilicalis or yelk, but this changes as the abdominal parietes begin to close in; in man and the mammalia merely a part of it, as above mentioned, forms the intestinal ca.n.a.l, whereas, in oviparous animals the whole of the yelk-bag enters the abdominal cavity, and serves for an early nutriment to the young animal. Another change connected with the serous or outer layer of the germinal membrane is the formation of the _amnion_. The foetal rudiment which from its shape has been called _carina_, now begins to be enveloped by a membrane of exceeding tenuity, forming a double covering upon it; the one which immediately invests the foetus is considered to form the future epidermis; the other, or outer fold, forms a loose sac around it, containing the liquor amnii. Whilst these changes are taking place in the serous layer of the germinal membrane, and whilst the intestinal ca.n.a.l, &c. are forming on the anterior surface of the embryo, which is turned towards the ovum, by means of the inner or mucous layer, equally important changes are now observed in the middle or vascular layer. ”In forming this fold,” says Dr. A. Thomson, ”the mucous layer is reflected farthest inwards; the serous layer advances least, and the s.p.a.ce between them, occupied by the vascular layer, is filled up by a dilated part of this layer, the rudiment of the heart.” (_Op. cit._ p. 301.)
Whilst this rudimentary trace of the vascular system is making its appearance, minute vessels are seen ramifying over the vesicula umbilicalis, forming, according to Baer's observations, a reticular anastomosis, which unites into two vessels the vasa omphalo-meseraica.
(_British and Foreign Med. Rev._ No. 1.) These may be demonstrated with great ease in the chick: the cicatricula increases in extent; it becomes vascular, and at length forms a heart-shaped net-work of delicate vessels, which unite into two trunks, terminating one on each side of the abdomen.
[Ill.u.s.tration: _b_ Is a portion of the convexity of the amnion, upon which, at _a_ is the fundus of the diminutive human allantois.
_c_ The duct of the vesicula umbilicalis, dividing into two intestinal portions; and besides this duct are two vessels which are distributed upon the vesicula umbilicalis, and form a reticular anastomosis with each other. _From Baer._]
The umbilical vesicle now begins to separate itself more and more from the abdomen of the foetus, merely a duct of communication pa.s.sing to that portion of it which forms the intestinal ca.n.a.l. The first rudiment of the cord will be found at this separation; its foetal extremity remains for a long time funnel-shaped, containing, besides a portion of intestine, the duct of the vesicula umbilicalis, the vasa omphalo-meseraica (the future vena portae,) the umbilical vein from the collected venous radicles of the chorion, and the early trace of the umbilical arteries. These last-named vessels ramify on a delicate membranous sac of an elongated form which rises from the inferior or caudal extremity of the embryo, viz. the _allantois_; whether this is formed by a portion of the mucous layer of the germinal vesicle, in common with the other abdominal viscera, appears to be still uncertain: in birds this may be very easily demonstrated as a vascular vesicle, arising from the extremity of the intestinal ca.n.a.l; and in mammalia, connected with the bladder by means of a ca.n.a.l called _urachus_: from its sausage-like shape, it has received the name of _allantois_.
The existence of an allantois in the human embryo has been long inferred from the presence of a ligamentous cord extending from the fundus of the bladder to the umbilicus, like the urachus in animals. But from the extreme delicacy of the allantois, and from its function ceasing at a very early period, it had defied all research, until lately when it has been satisfactorily demonstrated in the human embryo by Baer and Rathke. It occupies the s.p.a.ce between the chorion and amnion, and gives rise occasionally to a collection of fluid between these membranes, familiarly known by the name of the liquor amnii spurius, which, strictly speaking is the liquor allantoidis.
The function of the allantois is still in a great measure unknown. In animals it evidently acts as a species of receptaculum urinae during the latter periods of gestation; but it is very doubtful if this be its use during the earlier periods. It does not seem directly connected with the process of nutrition, which at this time is proceeding so rapidly, first by means of the alb.u.minous contents of the vitellus, or vesicula umbilicalis, and afterwards by the absorbing radicles of the chorion; but, from a.n.a.logy with the structure of the lower cla.s.ses of animals, it would appear that it is intended to produce certain changes in the rudimentary circulation of the embryo, similar to those which, at a later period of pregnancy, are effected by means of the placenta, and after birth by the lungs, const.i.tuting the great functions of respiration.
In many of the lower cla.s.ses of animals, respiration (or at least the functions a.n.a.logous to it) is performed by organs situated at the inferior or caudal extremity of the animal: thus for instance, certain insect tribes, as in hymenoptera, or insects with a sting, as wasps, bees, &c.; in diptera, or insects with two wings, as the common fly; and also the spider tribe, have their respiratory organs situated in the lower part of the abdomen. In some of the crustacea, as, for instance, the shrimp, the organs of respiration lie under the tail between the fins, and floating loosely in the water. Again, some of the molusca, viz. the cuttle-fish, have the respiratory organs in the abdomen. We also know that many animals, during the first periods of their lives, respire by a different set of organs to what they do in the adult state: the most familiar ill.u.s.tration of this is the frog, which, during its tadpole state, lives entirely in the water.
[Ill.u.s.tration: _a_ Bronchial processes. _b_ Vesicula umbilicalis. _c_ Vitellus. _d_ Allantois. _e_ Amnion. _From Baer._]
As the growth of the embryo advances, other organs whose function is as temporary as that of the allantois, make their appearance: these also correspond to the respiratory organs of a lower cla.s.s of animals, although higher than those to which we have just alluded,--we mean bronchial processes or gills. It is to Professor Rathke (_Acta Naturae Curios._ vol.
xiv,) that we are indebted for pointing out the interesting fact, that several transverse slit-like apertures may be detected on each side the neck of the embryo, at a very early stage of development. In the chick, in which he first observed it, it takes place about the fourth day of incubation: at this period the neck is remarkably thick, and contains a cavity which communicates inferiorly with the oesophagus and stomach, and opens externally on each side by means of the above-mentioned apertures, precisely as is observed in fishes, more especially the shark tribe; these apertures are separated from each other by lobular septa, of exceedingly soft and delicate structure. Rathke observed the same structure in the embryo of the pig and other mammalia; and Baer has since shown it distinctly in the human embryo. It is curious to see how the vascular system corresponds to the grade of development then present: the heart is single, consisting of one auricle and one ventricle; the aorta gives off four delicate, but perfectly simple branches, two of which go to the right, and two to the left side; each of these little arteries pa.s.ses to one of the lobules or septa at the side of the neck, which correspond to gills, and having again united with the three others, close to what is the first rudiment of the vertebral column, they form a single trunk which afterwards becomes the abdominal aorta. In a short time these slit-like openings begin to close; the bronchial processes or septa become obliterated, and indistinguishable from the adjacent parts; the heart loses the form of a single heart; a crescentic fold begins to mark the future division into two ventricles, and gradually extends until the septum between them is completed. It is also continued along the bulb of the aorta, dividing it into two trunks, the aorta proper and pulmonary artery; at the upper part the division is left incomplete, so that there is an opening from one vessel to the other, which forms the ductus arteriosus.[27] A similar process takes place in the auricles, the foramen ovale being apparently formed in the same manner as the ductus arteriosus; these changes commence in the human embryo about the fourth week, and are completed about the seventh.
At first the body of the embryo has a more elongated form than afterwards, and the part which is first developed is the trunk, at the upper extremity of which a small prominence less thick than the middle part, and separated from the rest of the body by an indentation, distinguishes the head. There are as yet no traces whatever of extremities, or of any other prominent parts; it is straight, or nearly so, the posterior surface slightly convex, the anterior slightly concave, and rests with its inferior extremity directly upon the membranes, or by means of an extremely short umbilical cord.
The head now increases considerably in proportion to the rest of the body, so much so, that at the beginning of the second month, it equals nearly half the size of the whole body: previous to, and after this period, it is usually smaller. The body of the embryo becomes considerably curved, both at its upper as well as its lower extremity, although the trunk itself still continues straight. The head joins the body at a right angle, so that the part of it which corresponds to the chin is fixed directly upon the upper part of the breast; nor can any traces of neck be discerned, until nearly the end of the second month.
The inferior extremity of the vertical column, which at first resembles the rudiment of a tail becomes shorter towards the middle of the third month, and takes a curviture forwards under the r.e.c.t.u.m, in the fifth week the extremities become visible, the upper usually somewhat sooner than the lower, in the form of small blunt prominences. The upper close under the head, the lower near the caudal extremity of the vertebral column. Both are turned somewhat outwards, on account of the size of the abdomen; the upper are usually directed somewhat downwards, the lower ones somewhat upwards.
[Ill.u.s.tration: _Diagram of the foetus and membranes about the fourth week._
_a_ Vesicula umbilicalis already pa.s.sing into the ventricular and r.e.c.t.u.m intestine at _g_. _b_ Vena and arteria omphalo-meseraica. _c_ Allantois springing from the pelvis with the umbilical arteries. _d_ Embryo. _e_ Amnion. _f_ Chorion. _From Carus._]
The vesicula umbilicalis may still be distinguished in the second month as a small vesicle, not larger than a pea, near the insertion of the cord, at the navel, and external to the amnion. From the trunk, which is almost entirely occupied by the abdominal cavity, arises a short thick umbilical cord, in which some of the convolutions of the intestines may still be traced. Besides these it usually contains, as already observed, the two umbilical arteries and the umbilical vein, the urachus, the vasa omphalo-meseraica, or vein and artery of the vesicula umbilicalis, and perhaps, even at this period, the duct of communication between the intestinal ca.n.a.l and vesicula umbilicalis, the foetal extremity of which, according to Professor Oken's views, forms the processus vermiformis.
[Ill.u.s.tration: _Diagram of the foetus and membranes about the sixth week._