DEVELOPMENT OF FLUSTRELLA HISPIDA. 445 



plialangioides, occurs also in the egg of Flustrella, a 

 numbei" of eggs were preserved Avitli Dr. Allen's cliromo- 

 nitro-osmic mixture; these were sectioned without previous 

 bleaching, so that any fatty matter might remain intact, but 

 the results obtained have so far proved somewhat difficult of 

 interpretation. Even in very young eggs in Avhich the yolk 

 nucleus is still in quite the initial stages of development, 

 large drops of a fatty substance are found to be present in 

 the region of the developing yolk nucleus ; and as the egg 

 enlarges, the number of the fatty drops also increases, but 

 the latter always remain in the immediate neighbourhood of 

 the yolk nucleus, while similar globules are also visible within 

 that body itself (PI. 23, figs. 26, 27). The amount of fatty 

 material increases as degeneration proceeds. It collects 

 especially towards the periphery of the egg, and at the same 

 time the development of the yolk commences. The two sub- 

 stances increase in quantity side by side, so that the mature 

 egg has the appearance of a mass of yolk spherules inter- 

 spersed Avith fat globules (PI. 23, fig. 28). This condition, 

 the presence of fat and yolk side by side, continues so long 

 as any yolk is discernible in the larva — that is until after the 

 degeneration which precedes metamorphosis has commenced. 

 At no period are the fat globules arranged in any definite 

 relation to the nucleus. It is hoped that it may be possible 

 to elucidate this question of the relation of the fat globules 

 and the food yolk to one another and to the yolk nucleus, by 

 investigating in greater detail the history of the yolk nucleus 

 in the eggs of such other Bryozoa in which it may prove to 

 be present. 



Prolonged treatment with xylol will cause the fat globules 

 to disappear from the yolk. 



The Grerminal Vesicle. — To determine whether there is 

 any connection between the yolk nucleus and the germinal 

 vesicle, the latter body has also been carefully studied. PI. 23, 

 fig. 29, shows a young germinal vesicle to which particles of 

 the yolk nucleus are in close approximation, and it will be 

 seen that one of these latter appears to be actually in process 



