BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 99 



At the time when the first yolk is being formed, the germinative 

 vesicle has a full rounded outline and its maximum diameter of about 

 45 fx (Plate 6, Fig 48). As the formation of the yolk continues, and 

 the ovum grows, the germinative vesicle shrinks, its membrane becomes 

 wavy, and before all of the cytoplasmic reticulum has become trans- 

 formed into yolkj the vesicle has shrunk to a diameter of about 30 or 

 35 fi. In the next stage (Fig. 49), the yolk extends up to the membrane 

 of the germinative vesicle, but the central yolk clump is not distinctly 

 separated into the yolk elements. In ova of this stage the germinative 

 vesicle has an average diameter of about 25 ^. Finally (Plate 6, 

 Fig. 50), when the ovum is larger still and the central clump of un- 

 divided yolk has broken up, the germinative vesicle has shrivelled to a 

 diameter of about 20 /x (see Table, p. 100). 



During the whole of this process, the methyl green and acid fuchsin 

 stain differentiates the nucleolus within the shrivelling germinative 

 vesicle. With some other stains however it cannot be seen, for the ger- 

 minative vesicle is so deeply stained that no structure at all can be dis- 

 tinguished within it. Iron hsematoxylin usually has this effect, and the 

 stain is so intense that excessive decoloration does not differentiate a 

 vesicle, but works from the periphery inward, removing all the stain from 

 around the edges, while in the centre it persists with undiminished in- 

 tensity. Occasionally, however, colonies are encountered in which the 

 stain does not work this way, but differentiates a vesicle, and in these 

 cases (Plate 6, Figs. 49, 50) the nucleolus and the remains of the retic- 

 ulum are plainly seen. 



In order to make perfectly sure that the sequence noted above is cor- 

 rect, I established four classes, based upon the sequence mentioned, and 

 then measured all the eggs in these stages contained in several series, 

 placing each egg in the class to which it belonged according to the 

 structure of the yolk, and germinative vesicle. If my sequence is cor- 

 rect, by this means I should get a progressive increase in the size of the 

 ova in the successive classes, and when passing from one class to the 

 next, and corresponding to this a diminution in the size of the germina- 

 tive vesicle. This condition is actually what I did obtain. 



The classes were : — 



1. Yolk bodies formed at the periphery only. Germinative vesicle 

 with a full rounded outline. 



2. Central part of the reticular cytoplasm not yet differentiated into 

 yolk. Germinative vesicle with a wavy outline. 



3. Yolk extends from the periphery to the germinative vesicle, but 



