1886. ] OVUM OF LEPIDOSIREN. 289 
the peripheral cells, from the germinal epithelium, but that they are 
first invaginated and subsequently surrounded by the peripheral 
layer. 
"ia 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 
ia 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 postembryonic 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 Elasmobranch-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 in 
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 ¢hese ova are formed by a coalescence of a number 
of cells out of the nest, the remainder serving as pabulum. ‘This 
opinion is so far confirmatory of Gotte’s observations on the deve- 
