Soon after the formation of the notochord, the protovertebræ begin to be formed along the sides of the medullary groove (Pl. 1, fig. 19, _pv_). Each new protovertebra (of those which are formed from before backwards) arises just in front of the anterior end of the primitive groove. As growth continues, the primitive groove becomes pushed further and further back, and becomes less and less conspicuous, till at about thirty-six hours only a very small and curved remnant is to be seen behind the sinus rhomboidalis; but even up to the forty-ninth Dursy has been able to distinguish it at the hinder end of the embryo.
The primitive groove in the chick is, then, a structure which appears very early, and soon disappears without entering directly into the formation of any part of the future animal, and without, so far as I can see, any function whatever. It is clear, therefore, that the primitive groove must be the rudiment of some ancestral feature; but whether it is a rudiment of some structure which is to be found in reptiles, or whether of some earlier form, I am unable to decide. 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. The fact that it is formed in the hinder part of the pellucid area perhaps tells slightly in favour of this hypothesis, since the point of involution of the epiblast not unfrequently corresponds with the position of the anus.
EXPLANATION OF PLATE 1, Figs. 6-8 and 13-19.
Figs. 6 and 7 are sections through an embryo rather earlier than the one drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove (_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque streak coming from the end of the primitive groove. The notochord is hardly differentiated, but the _complete_ separation of mesoblast and hypoblast under the primitive groove is clearly shewn. Fig. 7 passes through the anterior end of the primitive groove (_pr_), and shews the fusion between the mesoblast and epiblast, which is always to be found under the primitive groove.
Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a transparent object. Primitive groove (_pr_). Medullary groove (_md_), which passes off from the anterior end of the primitive groove, and is produced by the thickening of the mesoblast. Head fold (_pf_).