OVARIES IN TELEOSTEAN AND ELASMOBRANCH FISHES. 193 



but later the zonoid layer is marked off as a distinct mem- 

 brane. The zonoid layer is identical with Balfour's zona 

 radiata. 



(2) The origin of the definitive egg membrane (zona 

 radiata) in Zoarces, and other teleostean species, and of 

 the outermost membrane in Zeus was not ascertained. The 

 egg membranes of Zeus, with the possible exception of this 

 outer layer, are derived from the protoplasm of the ovum 

 apparently by a matting together and subsequent chemical 

 metamorphosis of cytoplasmic fibrils somewhat as Gathy 

 has shown for Clepsine. 



(3) The egg membrane of C hi mas r a appears to be inti- 

 mately connected with the follicular epithelium, and is pro- 

 bably formed — in part at least — at the expense of this 

 layer. 



(4) In Spiuax and Chimasra the cells of the follicular 

 epithelium are provided with delicate processes, which 

 penetrate the egg membrane and convey nutriment to the 



4. Histology op Egg Absorption. 



The details of the histological changes which accompany 

 the degeneration and absorption of ova have been studied 

 chiefly by Brunn (1882) in birds, and Ruge (1887) in 

 amphibia. Of those who have worked at the structure 

 of the teleostean ovary, few have failed to notice the obvious 

 fact that a certain proportion of the ova come to nothing, but 

 undergo a fatty degeneration. The degeneration of ovarian 

 ova in fishes is mentioned by His (1873), Brock (1878), 

 Emery (1880), Owsiannikov (1885), Barfurth (188G), Cun- 

 ningham (1897), and other writers. 



An immigration into the substance of the degenerating egg 

 of foreign elements, usually described as cells or nuclei, has 

 also been noted by various observers, and has been variously 

 interpreted. The earlier attempts to explain this occurrence 

 of intra-vitelline nuclei or cells were very wide of the mark. 



Barfurth gives an account of the degeneration of ova in 



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