I conclude this paper with a concise statement of what appears to me the probable nature of the much-disputed organ, the primitive streak, and of the arguments in support of my view.
In a paper on the primitive streak in the Quart. Journ. of Mic. Sci., in 1873 (p. 280) [This edition, p. [45]], I made the following statement with reference to this subject:—“It is clear, therefore, that the primitive groove must be the rudiment of some ancestral feature.... It is just possible that it is the last trace of that involution of the epiblast by which the hypoblast is formed in most of the lower animals.”
At a later period, in July, 1876, after studying the development of Elasmobranch fishes, I enlarged the hypothesis in a review of the first part of Prof. Kölliker's Entwicklungsgeschichte. The following is the passage in which I speak of it[451]:
"In treating of the exact relation of the primitive groove to the formation of the embryo, Professor Kölliker gives it as his view that though the head of the embryo is formed independently of the primitive groove, and only secondarily unites with this, yet that the remainder of the body is without doubt derived from the primitive groove. With this conclusion we cannot agree, and the very descriptions of Professor Kölliker appear to us to demonstrate the untenable nature of his results. We believe that the front end of the primitive groove at first occupies the position eventually filled by about the third pair of protovertebræ, but that as the protovertebræ are successively formed, and the body of the embryo grows in length, the primitive groove is carried further and further back, so as always to be situated immediately behind the embryo. As Professor Kölliker himself has shewn it may still be seen in this position even later than the fortieth hour of incubation.
"Throughout the whole period of its existence it retains a character which at once distinguishes it in sections from the medullary groove.
"Beneath it the epiblast and mesoblast are always fused, though they are always separate elsewhere; this fact, which was originally shewn by ourselves, has been very clearly brought out by Professor Kölliker's observations.
"The features of the primitive groove which throw special light on its meaning are the following:—
"(1) It does not enter directly into the formation of the embryo.
"(2) The epiblast and mesoblast always become fused beneath it.
"(3) It is situated immediately behind the embryo.