216 LILIAN SHELDON. 



forms the proliferating ridge. The small round, deeply staining 

 bodies found in the peripheral yolk have no obvious rudiment 

 in the previous stage; they present no definite structure, and, 

 except for their property of staining deep red with picrocarmine, 

 they resemble the yolk-spheres. It is possible that they may 

 be derived by the breaking down and alteration of the nuclei 

 which were present in the yolk in the previous stage. This 

 view as to their origin is supported by the fact that they are 

 very much more numerous in the peripheral than in the central 

 yolk, which was also the case with the nuclei. Whatever their 

 origin may be, they probably function as food material, as to 

 a certain extent at this stage, and very largely in later ones, 

 they are found lying among the cells of the embryo. 



By the next stage agaiu the ovum has undergone considerable 

 changes ; the peripheral yolk is mostly absorbed, but the small 

 round bodies are still very numerous, lying both outside the 

 embryo, i. e. between it and the vitelline membrane, and also 

 among the cells of the embryo, thereby rendering the exact 

 boundaries of the latter difficult to distinguish. In a transverse 

 section through the egg, near its anterior end, the embryo is 

 seen as a sac surrounded by a layer of ectoderm, which is ren» 

 dered somewhat indefinite by the intrusion of the small round 

 bodies. At the ventro-lateral corners there is a pair of pro- 

 liferating masses of nuclei, which are the rudiments of the prse- 

 oral lobes. As in the last stage, the whole embryo is filled 

 with yolk. A section slightly posterior to this is shown in 

 fig. 17 a; in it the rudimentary praeoral lobes {p.o.l.) are 

 present, though they are rather smaller than they were in the 

 section last described ; lying between them, on the ventral face 

 of the embryo, is the transverse section of the tip of a small 

 second sac (post. Em.), which is bounded by a fairly definite 

 layer of nuclei, and contains in its interior yolk- spheres, small 

 round bodies, and a few large nuclei. In a section through 

 the middle region of the ovum, such as is figured in fig. 17 b, 

 the second sac is found to have increased in diameter, and to 

 lie on the ventral face of the primary sac, from which it is 

 separated by a protoplasmic septum. Proliferating masses of 



