450 R. M. PACE. 



ripens, the germinal vesicle becomes more or less surrounded 

 by a differentiated mass of protoplasm, wliicli persists until 

 after fertilisation, but subsequently vanishes. 



Braem (4 and 6) mentions the presence of a light zone of 

 protoplasm surrounding the nucleus, which makes its appear- 

 ance in the ovum at an early stage. This zone is at first not 

 cleai'ly defined from the outer layer of the protoplasm, but 

 later on it becomes sharply demarcated. The outer zone takes 

 no part in segmentation. Small, darkly-staining bodies next 

 arise in the outer zone of the protoplasm. These bodies are 

 of varying sizes, and are each surrounded by a clear area. 

 Braem states that they are similar in appearance to the 

 nucleolus, and suggests that they resemble the latter in 

 chemical composition. He believes that they originate from 

 the outer zone of the cytoplasm, but similar bodies may also 

 occur in the inner zone. Braem suggests that these bodies 

 in the ovum of Plum at ell a may be homologous with the 

 yolk nucleus of certain other animal ova. 



In the case of the eggs of Flustrella hispida, the facts 

 so far obtained all point to the conclusion that the yolk 

 nucleus is a true cell organ originating from the nucleus at 

 an early stage in the history of the ovum ; that after under- 

 going a series of changes it finally disintegrates ; and that 

 the process of disintegration is in some manner intimately 

 connected with yolk formation. So far as can be seen, the 

 germinal vesicle plays no direct part in the formation of the 

 yolk. It has not yet been practicable sufficiently to investigate 

 the presence or otherwise of a yolk nucleus in the eggs of 

 other Bryozoa, and the results of this investigation must be 

 reserved for a future paper. 



The C en tro sphere. — No trace of a centrosphere has yet 

 been detected in the ovum of Flustrella hispida. 



The Polar Bodies. — The formation of the polar bodies 

 had only been observed in one case. The egg (PI. 24, fig. 51) 

 had already passed into the tentacle sheath, and the first polar 

 body {P.B.) was lying on the surface of the ovum, while the 

 second was in process of formation. 



