636 



1. Lepidosiren. ì\\ my paper already referred to Î have doscribcd 

 the forination of yolk in the cells of the follicular epithelium, and 

 the proliferation of these cells into the interior of the ovum. These 

 facts have been confirmed by further researches, but the stage in the 

 development of the ovum which is characterised by the enormous 

 increase of the vascular sheath and the concomitant formation of yolk 

 in the follicular epithelium, proves not to be a further development of 

 the stage which I have described as the first. The ovary of Lepi- 

 dosiren contains two kinds of ova which folloAv a different 

 course of development. In the one kind the ovum is the equiva- 

 lent of a single cell and its structure and development are so similar to 

 what is found in the Amphibia that no further description is requisite 

 here; Ay er s has figured such ova in his account of the visceral ana- 

 tomy of Lepidosireîi and Ceratodus. The other kind of ova are 

 present in the ovary in equal abundance, and each is the resul- 

 tant of a number of distinct cells which combine to form 

 the mature ovum. Theearlieststage which I have been able to disco- 

 ver has the following structure ; the germinal epithelium appears in pat- 

 ches, Avhich are several cells thick, and in a condition of active multiplica- 

 tion ; in connection with these aggregations of cells on the outside of the 

 ovary (corresponding to the »Epithel-inseln« of Waldeyer /) is a hollow 

 sphere of cells evidently derived from them ; this forms the future fol- 

 licular layer of the ovum as well as a layer lying outside the vascular 

 sheath — the secondary follicle layer of Balfour — which is thus seen to 

 be derived from the germinal epithelium. The interior of the sphere 

 is partly occupied by a mass of cells rather different in appearance 

 from those which constitute the follicle. The whole structure appears 

 to be the equivalent of the «Ureiernest« of Semper and Balfour. It 

 differs however in the details of its structure and more particularly in 

 the course of its development. Instead of one or more of the central 

 cells developing at the expense of the rest and separating off from the 

 nest with some of the follicular cells to form as many distinct ova, 

 the whole of the cells in the interior of the sphere com- 

 bine to form one ovum. There is however not a direct fusion of 

 all the central cells ; round the periphery of the central mass there 

 appears a layer of a more or less homogeneous substance which is 

 deeply stained by Borax Carmine, and resembles closely the liquor 

 folliculi of the Graafian follicle; this substance also appears in the 

 interior of the mass of cells and divides it up into isolated groups of one 

 or more cells. But this protoplasmic mass appears to be the result 

 of a fusion of some of the central cells, and in any ca^e yolk particles 

 are deposited in it as well as in the remaining cells and in the cells of 



