1886. ] OVUM OF LEPIDOSIREN. 291 
large share in the building up of the ovum; in later stages, but 
before the follicular cells have dwindled down to a single layer, there 
is a considerable proliferation and migration inwards of these cells 
(Plate XXVIII. figs. 5, 6); finally, when the ovum is surrounded by 
only a single layer of follicular cells, there is still here and there a 
proliferation of these cells. 
VI. Résumé. 
The observations recorded in the present paper may be briefly 
summed up as follows. 
The ovum is in the earliest stages observed composed of a mass of 
deeply staining granular protoplasm ; it contains a germinal vesicle 
limited by a distinct membrane, inside of which is a peripheral layer 
of germinal spots; the ovum is separated from the surrounding 
follicular epithelium by a thin structureless membrane. The follicular 
epithelium is composed of a single layer of cells, each with a large 
deeply staining nucleus, which retains the same character through- 
out the whole development of the ovum. In the second stage the 
protoplasm of the ovum is arranged in a reticulate fashion; the 
germinal vesicle is not defined by a limiting membrane ; within the 
vitelline membrane, which is still delicate and structureless, is a well- 
defined zona radiata with closely arranged vertical pores. 
In the third stage the external vitelline membrane is much thicker 
and shows a radial striation, the pores being further apart than 
those of the external zona radiata; the latter is commencing to 
disappear. The formation of yolk is in active progress and takes 
place within the ovum, though not necessarily in its peripheral 
layers. 
In the more mature ova the yolk occupies the whole extent of the 
ovum ; occasionally a peripheral layer is to be distinguished from a 
central mass; the vitelline membrane alone is present, and is 
excessively thin and apparently homogeneous, with no trace of radial 
striation. The germinal vesicle has undergone certain changes, 
chiefly in the centripetal movement of the germinal spots. 
A single section frequently contained ova of all the above mentioned 
stages ; besides these there were present in the ovary a vast number 
of ova which undergo a different development; in size these ova 
were generally larger than those of Stages 2 and 3. The follicular 
epithelium of the ova is composed of a single layer of large, more or 
less columnar cells filled with granules exactly like the yolk-particles ; 
these cells proliferate and migrate into the interior of the ovum ; 
eventually they disappear: the yolk of these ova appears to be, at 
least in a great measure, formed by these cells, whose contents are 
round granules quite indistinguishable from the yolk. The follicular 
epithelium rests directly upon the ovum and is separated from it by 
no membrane. The earlier stages of these ova seem to indicate that 
they are formed by the fusion of a number of germinal cells. 
At a later stage these ova were indistinguishable from other 
mature ova; the follicular epithelium decreases in importance, and 
a delicate membrane is formed between it and the ovum. 
