From what we have now stated, a close analogy will appear between the ova of the mammalia and those of the lower classes, more especially birds, which from their size afford us the best opportunities of investigating this difficult subject.
In birds, the covering of the vitellus is called yelk-bag; whereas, in mammalia and man it receives the name of vesicula umbilicalis. Its albuminous covering, which corresponds to the white and membrane of the shell in birds, is called chorion: by the time that the ovum has reached the uterus, this outer membrane has undergone a considerable change; it becomes covered with a complete down of little absorbing fibrillæ, which rapidly increase in size as development advances, until it presents that tufted vascular appearance, which we have already mentioned when describing this membrane.
The first or primitive trace of the embryo is in the cicatricula or germinal membrane, which contained the germinal vesicle before its disappearance. In the centre of this, upon its upper surface, may be discovered a small dark line;[25] “this line or primitive trace is swollen at one extremity, and is placed in the direction of the transverse axis of the egg.”
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 canal open at both ends, by the junction of its margins (the plicæ primitivæ of Pander, the laminæ dorsales of Baer:) the canal 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 canal 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 canal 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 canal four or five small round opaque bodies, these bodies indicate the first formation of the dorsal vertebræ.
a Transparent area. b Laminæ dorsales. c Cephalic end. d Rudiments of dorsal vertebræ. e Serous layer. f Lateral portion of the primitive trace. g Mucous layer. h Vascular layer. k Laminæ dorsales united to form the spinal canal.
“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 fœtus 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 principal 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.)
