EXPLANATION OF i'LATE I. 



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 i. Figs. 6—8 and 13—19. (III. p. 4'-) 



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 (/«</), 

 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 (/r), 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' blastodenn, seen as a transparent 

 ol)ject. Primitive groove (/r). Medullary groove (w(/), which passes off from the 

 anterior end of the primitive groove, and is produced by the thickening of the meso- 

 blast. Head-fold (//). 



Figs. 13 — 17 are sections through the blastoderm, drawn in fig. 18 through the 

 lines I, 2, 3, 4, 5 respectively. 



The first section (fig. 13) passes through the true medullary groove («/<"); 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 {in) under the medullary groove. 

 There is no adherence between the mesoblast cells and the epiblast under the me- 

 dullary groove. 



The second (fig. 14) section passes through the medullary groove where it has 

 become wider. Medullary folds. A, A ; notochord, f/i. 



In the third section (fig. 15) the notochord (i/i) 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 {/ir). 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, /iv, Proto-vertebrje ; other references as above. Fig. 18 is the blastoderm from 

 which sections figs. 13 — 17 were cut. 



EXPLANATION OF PL.\TE 2. (IV. p. 47.) 



Fig. 1 is t.aken frnm ihe anterior part of the pelUicid area of a thirly hours' chick, 

 ilh four ))rolo-verlebi.e. .\t u i> a iiucleu> with two nucleoli. 



