244 LILIAN SHELDON. 



elements, which are also very plentiful lying among the cells 

 of the embryo, especially in the prseoral lobes, and in the 

 somites. These bodies, which are represented drawn under a 

 high power in fig. 16, have a somewhat different form in this 

 egg to that which they have in the previously described ones ; 

 they are still round in shape, but they contain in their interior 

 a larger or smaller amount of vacuoles. The region in which 

 the embryo is doubled on itself is shorter in this egg than in 

 the preceding stage. 



As was mentioned before the main difference between this and 

 the previously described embryo is in the complete separation of 

 the two sacs in that region where they are superposed upon 

 one another. This change seems to have been effected by the 

 ingrowth of the surrounding tissue, which by pushing in the 

 septum causes it to become double. This process had already 

 begun in the anterior region of the ovum of the previous stage, 

 where in fig. 17 a the ventral sac is seen to be surrounded by 

 a complete layer of ectoderm, while more posteriorly in the 

 egg it is only separated from the dorsal sac by a single septum, 

 as is shown in fig. 17 b. 



In the next stage, sections of which are figured in figs. 19 

 a — d, the separation between the cavities has progressed still 

 farther, the anterior tip of the ventral cavity, i. e. the posterior 

 tip of the embryo, lying at some distance from the ventral wall 

 of the dorsal one, as is shown in fig. 19 h. The region in 

 which the embryo is doubled on itself is also shorter than 

 before, so that it seems to be gradually straightening itself out. 

 The embryo has also advanced considerably in other respects 

 — the mouth is present as an ectodermic invagination (fig. 19 

 b, M.), the inner end of which forms the pharynx ; the prseoral 

 lobes are united in front of the mouth by the cerebral commis- 

 sure (fig. 19 a, Cer. Com.) ; the somites are present as a series 

 of paired, hollow, thin-walled vesicles lying on the lateral faces 

 of the body below the ectoderm, which in this region is slightly 

 thickened In the posterior portion of the body the somites are 

 not yet present (fig. 19 c, Mes.), the mesoblast being still in 

 the form of a pair of proliferating ridges of cells. The endoderm 



