174 WILLIAM WALLACE. 



point to the following as the most probable mode of formation 

 of the ova and follicular epithelium in teleostean fishes. Ova 

 and follicular epithelium have a commou origin in the ovarian 

 epithelium. The whole nest appears to arise by division of a 

 single mother cell, some of the elements giving rise to oogonia, 

 others to follicular ceils. The successful oogonium ingests 

 its immediate neighbours, and thereby increases to a certain 

 size. Very early, however, the oocyte is surrounded by a 

 complete follicular epithelium, the cells of which secrete fluid 

 nutriment derived from the lymph. After formation of a 

 permanent ovum and its follicular epithelium the surplus 

 oogonia and follicular cells go to the ground. 



2. Yolk Nuclei. 



A "yolk nucleus" of the type often noticed in invertebrate 

 eggs,^ and described particularly by Bambeke (1898) for 

 Pholcus phalangoides, and by Calkins (1895) and Foot 

 (1896) for earthworms, was met with in Zoarces. The 

 complete history of this body in fishes does not appear 

 to have been elucidated, although one finds allusions to 

 an inner perinuclear zone of dark protoplasm by Scharlf 

 (1887), to numerous "Dotterkerncn " in the cytoplasm of the 

 egg of Zoarces by Stuhlmann (1887), to deeply staining 

 granules "like nucleoli" outside the nucleus by Fulton 

 (1898), etc. The body has been confounded and connected 

 Avith another and quite distinct structure, the " centro- 

 sphere." It is to the latter, in fishes' eggs, that the name 

 '^vitelline nucleus" or "yolk nucleus " is usually but erro- 

 neously applied. 



Figs. 6 — lo and 37 illustrate the history of the true yolk 



^ Miss R. M. Clark, workiiij^ at I'lymoiitli, has kindly drawn my attention 

 to the occurrence of a yolk nucleus of this type in the oocytes of F lust roll a 

 hispida. Her preparations and figures of developing ova show very clearly 

 the same succession of appearances as 1 have figured and described for 

 Zoarces. 



