112 C. M. CHILD. 



of the smaller to form larger masses also occurs frequently. Dur- 

 ing yolk-formation the nucleolus remains apparently unchanged, 

 but the spireme soon loses its deeply staining character and its 

 substance appears to spread throughout the nucleus in a more or 

 less recticular condition so that the nucleus resembles the nuclei 

 of most eggs before maturation. 



It is almost impossible to reproduce the nuclear structure of 

 this period with any degree of exactness : moi cover, it is not cer- 

 tain how far the visible structure is characteristic of the living cell 

 and how far it is the product of fixation. An attempt has been 

 made, however, in Figs. 2427 to indicate the changes in nuclear 

 structure. Although the spireme of earlier stages has disappeared 

 the nucleus differs in appearance from the ovarian nuclei before 

 the growth period in that a reticulum is now visible while in the 

 earlier stages the nuclei were almost entirely homogenous in ap- 

 pearance except for the nucleolus and other granules. An 

 account of maturation and fertilization will be given in another 

 paper. At this time it need only be said that typical maturation 

 spindles appear, but that it has been impossible to follow the 

 process of chromosome-reduction. Fig. 28 shows the first polar 

 spindle with its enormous centrosomes. 



VII. THE DEVELOPMENT OF THE VITELLARIUM. 



The " yolk-gland " differentiates from the posterior portion of 

 the ovarian mass of proliferating cells. During the earlier stages 

 its cells are indistinguishable from those of the ovary, but later 

 they can be distinguished by their slightly smaller size. The 

 nuclear division is almost wholly amitotic. Occasionally, how- 

 ever, though less frequently than in the ovary, a case of mitosis 

 is seen. Division seems to be somewhat more rapid than in the 

 ovary, amitoses being commonly more numerous in a given area. 

 As in the ovary the nuclei of earlier stages are surrounded by very 

 little cytoplasm and lie in the parenchymal tissue. Fig. 29 rep- 

 resents a group of nuclei at the stage when the yolk-gland is first 

 distinguishable from the ovary. Numerous amitoses are visible. 

 Fig. 30 shows several cases of division from a later stage. The 

 process of yolk formation in these cells differs from that in the egg 

 cells. It is not preceded by any marked increase in size of the 



