1886.] OVUM OF LEPIDOSIREN. 289 



the peripheral cells, from the germinal epithelium, but that they are 

 first invagiuated and subsequently surrounded by the peripheral 

 layer. 



In a later stage the formation of yolk has commenced, and the 

 cells of the central mass are in places separated from each other by 

 aggregations of yolk-particles, though for the most part the cells 

 remain closely adherent ; these latter, however, contain yolk-particles 

 in their interior, and the follicular cells, which still form two or three 

 layers, are also filled with yolk. The later stages have been already 

 referred to (p. 283). 



It is clear, therefore, that in Lepidosiren there are two kinds of 

 ova ; those which arise in the way just described may possibly be 

 confined to the postenibryonic period. 



The mass of central cells with the surrounding follicular layers is 

 clearly comparable to the " Ureiernester," described by Balfour and 

 others, in many Vertebrates, e.g. Elasmobranchs. But although 

 there is this general similarity between the ovary of Lepidosiren and 

 that of Scyllium, there is evidently a very great difference in detail. 



In the Elasmobrauch-ovary the nest of primitive germinal cells is 

 imbedded in a mass which consists of the general undifferentiated 

 cells of the germinal epithelium ; there is no definite follicular layer 

 at this period. The protoplasm of the primitive germinal cells 

 fuse together, and the nuclei multiply ; some of the nuclei dege- 

 nerate, while others undergo further development, and are eventually 

 separated off from the rest, together with a certain amount of proto- 

 plasm, to form ova ; the degenerating nuclei are absorbed and aid iu 

 the nutrition of the ova. When the ovum is formed, some of the 

 undifferentiated germinal cells range themselves round it and form 

 the follicular layer. A nest gives rise to a variable number of ova. 

 The yolk is formed much later. 



In Lepidosiren the nest is imbedded in a mass of cells which are 

 definitely marked off from the surrounding cells of the ovary, and 

 can be recognized as the future follicular epithelium ; the protoplasm 

 of at least some of the primitive germinal cells fuse together, and 

 the nuclei appear also to multiply, if not the cells themselves in 

 many cases ; some of the nuclei degenerate (show a paler colour 

 under the influence of borax carmine) and become irregular in shape ; 

 the formation of yolk commences extraordinarily early (as compared 

 with the Elasmobranch) in the mass resulting from the fusion of the 

 peripheral cells, in the remaining cells, and in the follicular layers ; 

 the nest gives rise to but a single ovum. 



It seems to me impossible to deny that the whole structure (Plate 

 XXVIII. fig. 7), which I have compared to the "nest" of the 

 Elasmobranch ovary, eventually becomes a single ovum ; the question 

 that must first be answered is, does the ovum in this case represent 

 a single cell or is it produced by the fusion of a number of cells? 

 The only answer to this question that the facts at my disposal enable 

 me to give is that these ova are formed by a coalescence of a number 

 of cells out of the tiest, the retnainder serving as pabulum. This 

 opinion is so far confirmatory of Gotte's observations on the deve- 



