Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated with silver nitrate, shewing the small rounded cells frequently found at the meeting-points of several larger cells which are characteristic of the upper layer. EXPLANATION OF PLATE 1. Figs. 6-8 and 13-19. (III. p. 41.) 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. Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is twenty-three hours old; fig. 19 twenty-five hours. They both shew how the medullary canal arises entirely independently of the primitive groove and in front of it, and also how the primitive groove gets pushed backwards by the growth of the medullary groove. _pv_, Protovertebræ; other references as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were cut. EXPLANATION OF PLATE 2. (IV. p. 47.) Fig. 1 is taken from the anterior part of the pellucid area of a thirty hours' chick, with four protovertebræ. At _n_ is a nucleus with two nucleoli.