442 R. M. PACE. 



cells being differentiated in this manner (P). 22, fig. 5) and 

 developing into ova, while the remainder give rise to the 

 follicle cells. As maturation proceeds, the follicle cells in- 

 crease in number and appear to grow in among the primitive 

 ova, so that when these latter are ripe each ovum is surrounded 

 by a follicular membrane (PI. 22, figs. 2 and 3,Fo.). 



Generally speaking, all the ova contained in the ovai-y are 

 of about the same age (PI. 22, figs. 2-5). 



The Maturation of the Egg : the Yolk Nucleus. 



The chief point of interest in the process of the maturation 

 of the egg of Flustrella is the appearance of a "yolk 

 nucleus," apparently homologous with that type described 

 by van Bambeke (1) as occurring in the egg of Pholcus 

 phalangioides. The existence of a yolk nucleus does not 

 appear to have been hitherto recorded in any of the Ecto- 

 proctous Bryoza, although a similar structure has been figured 

 by Braem (4 and 6) and by Kraepelin (17) as being present 

 in the egg of Plumatella among the Entoprocta. 



The Yolk Nucleus. — The history of this body in the egg 

 of Flustrella hispida is briefly as follows : 



In very young ovaries in which the ovarian cells are only 

 just recognisable, there ai-e, in addition to the germinal 

 vesicle, certain darkly-staining granules surrounding the 

 nucleus and lying in close contact with the latter (PI. 22, figs. 

 4-5 ; PI. 23, fig. 29, Y.N.). These granules at a later period 

 coalesce to form the structure, which, following van Bambeke, 

 may best be termed the ''yolk nucleus.^' They originate, as 

 has been said, quite close to the germinal vesicle, and their 

 appearance is so very similar to that of certain intra-nuclear 

 elements as to suggest that they have originated from the 

 nucleus. In fact, in one case (PI. 22, fig. 5; PI. 23, fig. 29) 

 these granules seemed to be in process of actually passing out 

 from the germinal vesicle. The granules at this period are 

 homogeneous in appearance, and their behaviour with any of 



