240 LILIAN SHELDON. 



the original yolk segmentation, and contains a good many 

 nuclei (fig. 15 a). 



Cell outlines are not distinct at this stage, but indications 

 of them are present between the nuclei of the protoplasm 

 bounding the sac. There are no traces of any cell boundaries in 

 the proliferating mass of nuclei. 



The structure of the egg at this stage may be briefly described 

 as follows : — The egg is surrounded by a vitelline membrane ; 

 beneath this is a peripheral layer of yolk containing small 

 round, highly refractive bodies ; within this is a sac ending 

 blindly at both ends, bounded by a layer of protoplasm, which 

 is one cell thick, except along one line, where, throughout most 

 of its length, there is a longitudinal ridge composed of a mass 

 of proliferating nuclei, among which many of the small round, 

 highly refracting bodies are scattered. At one end this ridge 

 thins out and gradually disappears; towards the other it in- 

 creases in thickness, and by the parting of its lateral walls 

 comes to enclose a secondary cavity, which lies below the 

 primary sac, and ends blindly shortly before the latter. The 

 sac is filled with food yolk, in which a few scattered nuclei are 

 present. 



By a comparison of this embryo with those of later stages 

 it is found that the internal sac, together with the proliferating 

 ridge, represents the embryonic part of the ovum, the single 

 layer of protoplasm being the ectoderm of the embryo. The 

 mesoderm and ectoderm are not yet definitely differentiated. 

 The peripheral yolk does not form any part of the future 

 embryo, but seems to be absorbed as food material. 



The most tenable hypothesis, whereby this stage can be con- 

 nected with the previous one, is that the cells at the edges of 

 the protoplasmic plate of the latter grow round the ovum in a 

 normal epibolic manner, except that, instead of spreading over 

 its surface, they grow round slightly internal to it, so as to leave 

 a peripheral layer of yolk outside them. A small quantity of 

 this peripheral yolk inserts itself between the protoplasmic 

 plate and the vitelline membrane, so that the whole embryo is 

 surrounded by yolk. The protoplasmic plate itself probably 



