256 LILIAN SHELDON. 



wall on each side of both sacs there is a thickening of the cells, 

 which is the rudiment of the mesoderm. The peripheral yolk 

 is mostly absorbed, but the small round bodies are still present 

 in large quantities as before, outside the embryo, and among 

 its cells. 



10. The septum has become divided into two layers by a 

 process of ingrowth of the surrounding tissue, so that each 

 sac is completely enclosed by a protoplasmic layer, and the 

 embryo now consists of a sac doubled on itself in such a way 

 that the ventral face of the anterior part of the body is opposed 

 to that of the posterior part of the body. Indications of cavi- 

 ties have appeared in the mesoblastic bands, which are the 

 rudiments of the somites. 



11. The embryo begins to straighten itself out, the ventral 

 surface of the posterior end gradually removing itself farther 

 from that of the anterior. The mouth is present as an invagi- 

 nation of ectoderm on the ventral surface just behind the 

 praeoral lobes. The endoderm is present as a layer of nuclei 

 surrounded by a reticulum of protoplasm lying at the periphery 

 of the yolk. The somites are present in the anterior region 

 in the form of a series of definite cavities at the sides of the 

 body. The small round bodies outside the body of the embryo 

 are almost entirely absorbed. 



12. The embryo is still further straightened out, so that the 

 only indication of the doubling is in the fact that the posterior 

 end of the body is bent up at an angle to the anterior. The 

 anus is not yet formed. The somites are present throughout 

 the whole length of the embryo, and in the anterior ones the 

 somatic wall is thicker than the splanchnic. 



13. The peripheral food material is completely absorbed, so 

 that the embryo lies just within the vitelline membrane and 

 egg-shell. The appendages begin to appear as blunt rounded 

 protuberances on a lateral ridge which runs along each side of 

 the body. The antennae arise as buds on the prseoral lobes. 

 The anus is present, situated on a papilla at the posterior end 

 of the body, and consisting of a simple ectodermic invagination. 

 A primitive streak and groove are present, anterior to and on 



