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_). Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through the lines 1, 2, 3, 4, 5 respectively. The first section (fig. 13) passes through the true medullary groove (_mc_); the two medullary folds (_A_, _A_) are seen on each side with the thickened mesoblast, and the mesoblast cells are beginning to form the notochord (_nc_) under the medullary groove. There is no adherence between the mesoblast cells and the epiblast under the medullary groove. The second (fig. 14) section passes through the medullary groove where it has become wider. Medullary folds, _A_, _A_; notochord, _ch_. In the third section (fig. 15) the notochord (_ch_) is broader, and the epiblast is raised in the centre, while the medullary folds are seen far apart at _A_. In section fig. 16 the medullary folds (_A_) are still to be seen enclosing the anterior end of the primitive groove (_pr_). Where the primitive groove appears there is a fusion of the epiblast and mesoblast, and no appearance of the notochord. In the last section, fig. 17, no trace is to be seen of the medullary folds.