DEVELOPMEINT OF FLUSTRBLLA HISPIDA. 449 



the conclusion that the yolk nucleus, although of nuclear 

 origin, does not consist of chromatin, as has been maintained 

 by several writers, owing to its apparent origin from and simi- 

 larity of appearance to the chromatin granules of the germinal 

 vesicle; but that it is either purely albuminous or consists of 

 nucleo-albumin containing a large percentage of nucleic acid 

 constituents. According to this author, accounts of the 

 origin of the yolk from the cytoplasm at a point distant from 

 the nucleus, or from several centres, or of its formation all 

 over the egg, refer only to its later history, and do not take 

 into account an earlier stage, which is marked by the appear- 

 ance of the yolk nucleus. If Crampton's view be the correct 

 one, a study of younger eggs should in such cases lead to the 

 discovery of this supposed early stage. 



Wallace (26) describes in the egg of Zoarces a yolk 

 nucleus of the type occurring in Flustrella; and by Mr. 

 Wallace's courtesy, I have been enabled to examine many of 

 his preparations and drawings, which show that the yolk 

 nucleus agrees in almost all respects with that of Flustrella 

 hispida. Wallace, however, found that fixing reagents 

 containing nitric acid dissolved out the yolk nucleus, while, 

 as has already been stated, this is not the case with the egg 

 of Flustrella. Further, Wallace agrees with Bambeke 

 that the formation of oil-drops precedes true yolk formation. 



The Yolk Nucleus and Yolk Formation in Bryozoan 

 Eggs. — As has been stated earlier in this paper, no yolk 

 nucleus has hitherto been noted in the eggs of any of the 

 Ectoprocta.^ Among the Entoprocta, however, a structure, 

 which appears to be similar to the yolk nucleus occurring in the 

 egg of Flustrella hispida, has been figured by Kraepelin 

 and by Braem as present in the ovum of Plumatella. 



Ki'aepelin (17) points out that shortly before the egg 



^ It has lately been possible bj the courtesy of Miss A. Heath to examine 

 some preparations of a species of Alcyonidium which contained young ova. 

 The material, which was collected at Millport in September, 1905, contained 

 abundance of young ova, and these were found to contain "yolk nuclei" 

 apparently similar to that which has been described in Flustrella. 



