OF MONOTREMATA AND MARSUPIALIA. 
471 
From the nucleus of each follicular cell, and from the germinal vesicle, streams of 
these yolk granules travel into the body of the ovum. The result of this double mode 
of origin of the yolk is that the yolk is interrupted at one place, viz., where the 
germinal vesicle lies. The yolk in Monotremata has the same arrangement as that of 
Birds, where a central bottle-shaped mass of smaller spheres is continued from the 
germinal disc to the centre of the ovum. The streams of yolk granules passing into 
the ovum, immediately on entering, are more or less parallel to each other, and give a 
radially striated appearance to the peripheral layer of the ovum (Plate 29, fig. 1). 
A similar appearance in other Vertebrate eggs has been described as a definite 
membrane, and named the zona radiata. The time during which the greater number 
of yolk granules are formed in the cells of the follicular epithelium corresponds with a 
marked change in the appearance of the layer itself. When the ovum lias reached a 
diameter of '2 mm. the cells of the follicular epithelium, already much more columnar 
than in the younger stages, divide rapidly and form a layer three to four cells deep 
(Plate 29, fig. 1 ,fe.). When the ovum has increased to ‘5 mm. diameter it has received 
from the follicular cells such a number of yolk granules as would be nearly sufficient, if 
the granules were swollen to yellow yolk spheres, to completely fill a ripe ovum 
measuring 3 mm. in Echidna and 2 - 5 mm. in Ornithoi'hynchus. Between the stages of 
'32 mm. diameter and '5 mm. diameter the follicular epithelium again becomes one 
cell deep. This change corresponds with the change in the manner of growth of the 
yolk. Up to stage '32 mm. the formation of new granules was the chief function of 
the epithelium. After the granules are formed the absorption of fluid by osmosis 
becomes relatively more important; consequently, the yolk granule forming period 
may be conveniently spoken of as the first period of the activity of the follicular 
epithelium, while the period of absorption of fluid may be described as the second period. 
Both processes, of course, go on during the whole of both periods. Each period 
includes the maximum activity of one of the two processes, granule formation or 
absorption of fluid .* 
The active change of granules into yellow yolk spheres by the absorption of fluid is 
marked by the return of follicular epithelium to the condition of a single row of cells. 
Plate 29, fig. 2, shows the condition of the follicular epithelium ( fe .) and the vitelline 
membrane (vm .) in an ovum measuring 1 mm. diameter. 
The follicular epithelium becomes so flat that Beddard (2), who has recently 
described the ovary of Echidna, failed to find it ; while Guldberg, as has been 
already mentioned, stated that the follicular cells fused together to form a clear 
“ chorion.” 
When the ovum has reached its maximum diameter the follicular epithelium again 
wakes up ; and the ceils increase enormously in size, and each nucleus becomes larger 
than an entire cell in the previous stage, Plate 29, fig. 3 (fe.). This renewed activity 
* It is to be understood that tbis account of the origin of yolk is only a description of the structural 
appearances presented by preserved material. The physiological changes maybe much more complicated. 
