24 SEGMENTATION AND FOEMATION OF THE LAYERS. 



ectoderm cells is shown in PI, IV^ fig. 12, made from an 

 ovum of the same age and prepared in the same way as the 

 last. This drawing represents one corner of the ectoderm 

 patch ; three whole cells and parts of three others are repre- 

 sented, and they are all seen to be connected by a loose reti- 

 culum. The protoplasm immediately round the nucleus has a 

 granular appearance owing to the closeness of the reticulum. 

 The connection between the ectoderm patch with the larger 

 endoderm masses, as seen with a lower power, is shown in 

 fig. 9, while fig. 6 represents two small endoderm masses con- 

 nected together by, and giving off in all directions, fine strands 

 as seen under a higher power. 



The endoderm masses now begin to draw together {vide figs. 

 10 — 13, PI. I), and form a ring-like mass applied all round 

 the edge of the ectoderm patch. This ring-like mass is thicker 

 at each end of the ectoderm disc than in the centre (PL I, 

 fig. 12), where, indeed, it is sometimes interrupted (PI. I, 

 fig. 13). PI. IV, fig. 16, represents a transverse section 

 through the edge of an ovum at this stage. 



The process of drawing together of the endoderm masses is 

 still further continued and the ectoderm cap becomes bent 

 round the concentrated solid mass so formed (PL I, fig. 15). 

 PL IV, fig. 20, represents a transverse section through an 

 ovum at a slightly later stage, in which a cavity, the future 

 mesenteron, has begun to appear. 



The ectoderm cap now gradually (PL I, fig. 18) grows round 

 the endoderm mass, and almost completely encloses it. The one 

 unenclosed point persists as the blastopore (PL I, fig. 20). While 

 this process has been taking place the cavity in the endoderm 

 mass has become larger, and on the completion of the process 

 of epibole opens to the exterior through the blastopore. The 

 ovum has now reached the gastrula stage {vide PL I, figs. 

 19 and 21). 



Before passing on to consider the structure of the gastrula 

 and the formation of the mesoderm, I desire to call attention 

 to certain remarkable features in the preceding development. 



1. The embryo at the gastrula stage, and in all the 



