OVARIES IN TELEOSTEAN AND ELASMOBRANCH FISHES. 179 



usually characterised as the '^yolk nucleus^' iu fishes, but 

 which is probably the centrosphere — has any connection 

 with the formation of yolk seems improbable, if we consider 

 that it may degenerate at very different times in different 

 species. 



3. Egg Membranes and Follicular Epithelium. ^' 



In regard to egg membranes it is of most interest and 

 importance to know — (1) whether they are formed from the 

 follicular epithelium or at the expense of the egg itself; (2) 

 if they are of the nature of secretions, or are produced by a 

 metamorphosis of living protoplasmic substance; (3) what 

 are the homologies subsisting between the various membranes 

 found in different groups of vertebrates ? (4) is there a 

 living protoplasmic connection between the cytoplasm of the 

 egg and that of the cells of the follicular epithelium by 

 means of which nutrient substances secreted by these cells 

 might pass into the egg ? The question whether the deuto- 

 plasm is manufactured by the egg itself or in the cells of 

 the follicular epithelium naturally suggests itself in this 

 connection. 



It will be well, before referring to some observations of 

 my own on this head, to make a brief statement of the chief 

 opinions of the more recent workers. Perhaps the two most 

 important papers of comparatively recent date on the structure 

 and development of egg membranes in fishes are those of 

 Mark (1890) and Eigenmann (1890). Since the first-men- 

 tioned observer gives a critical review of all the previous 

 literature on the subject of egg membranes, it is unnecessary 

 to attempt anything of the kind here. 



Classification of Egg Membranes. — Mark recognises 

 four kinds of egg membranes in fishes : 



(1) The innermost : a true vitelline membrane, representing 

 the cell membrane of the egg. Its existence has been 

 demonstrated by Scharlf (1887), Fulton (1898), and others 

 iu the ripe ovarian ova of certain teleosteau fishes. On 



