374 



throughout the yolk were numerous rounded cells with large distinct 

 nuclei containing more deeply stained nucleoli ; these cells were in 

 most cases limited by a distinct membrane and contained refracting 

 particles exactly similar to the yolk spherules. These cells could be 

 observed in process of division ; some were evidently degenerating hav- 

 ing lost the refracting particles in their interior ; the nucleus of such 

 cells was usually pale in colour (less deeply stained) and of a homo- 

 geneous appearance Avithout any deeply stained granules in its in- 

 terior ; this seems to me also to be a sign of degeneration of the cell. 

 These cellular structures in the interior of the ovum cannot be con- 

 founded with the white yolk spherules of birds ~ in as much as they are 

 furnished with a clearly defined nucleus and a distinct limiting mem- 

 brane. In every respect they ressemble the follicular cells and the only 

 conclusion I can come to is that they are derived from the 

 follicular epithelium. The contents of these cells like the con- 

 tents of the follicular cells consist of bodies which are indistinguish- 

 able from yolk spherules ; hence their function is probably to assist in 

 the formation of the yolk in the interior of the ovum Avhile the folli- 

 cular cells themselves elaborate the yolk from the blood and pass it on 

 to these cells and directly to the more peripheral regions of the ovum ; 

 indeed these cells were so numerous that it appears doubtful whether 

 they and the follicular cells do not form the entire mass of the yolk. 



The fact that the cells of the follicle share in the formation of the 

 yolk is known from the investigations of many naturalists ; it has been 

 shown to be the case in Elasmobranchs, Mammals and many inverte- 

 brate groups . In the Cephalopoda Lankester has described^ a method 

 of yolk formation which is analogous to that which has first been de- 

 scribed for Lepidosireti] the follicular epithelium becomes folded and 

 the folds project into the interior of the ovum; its cells proliferate 

 and pass into the substance of the ovum discharging their contents. 



I am not aware however that any method of yolk formation has 

 been described which is precisely like that of Lepidosiren and in any 

 case I believe I am right in saying that it is new to Lepidosiren. 



In older ova the follicular epithelium has shrunk to its former 

 limits and is now separated from the ovum by a distinct membrane 

 which although extremely thin has an obvious double contour. The 

 yolk is difierentiated into a narrow peripheral layer of different appea- 

 rance from a central mass. The germinal vesicle in transverse section 

 shows an outer thick homogeneous layer, within this a row of deeply 

 stained nuclear spots, within this again a central core of finely granu- 



2 Foster and Balfour, Elements of Embryology, p. 15. 



3 Philosoph. Trans. Roy. Sec. Lond. 1875. p. 43. PI. 12 fig. 23, 25. 



