484 EMBRYOLOGY OF THE TURTLE. Part III. 
At a much later period, (Pl. 8, fig. 5, 9,) these cells begin to press against each 
other, and to assume a more or less polygonal shape; yet this change does not at 
all correspond to the age or size of the egg, but appears to exhibit a considerable 
amount of variation as to shape and magnitude, since in a follicle inclosing an 
ovum invisible to the naked eye (Pl. 8, 12, 4; Pl. 9a, fig. 13, 6, 13a) the cells are 
closely set against each other, whilst in another and much larger egg, visible without 
a lens, (Pl. 9a, fig. 12, 12b, 17, ¢,) there is but very little mutual crowding. In 
addition to their change in shape, the cells last mentioned have also undergone an 
internal alteration: a darkenmg of their walls, and a slight increase in the conspic- 
uousness of the mesoblast (Pl. 8, fig. 5, 9; Pl. 9a, fig. 12b, 17, ¢). 
An ege about twice the diameter of that represented in Pl. 9a, fig. 12, when 
brought under the microscope, has the appearance of being covered by a network 
with polygonal meshes. These meshes, when more magnified, are found to be large, 
mutually compressed cells, belonging to the innermost layer, or tunica granulosa, of 
the Graffian follicle. Their size has considerably increased, and the contents have 
become very hyaline, especially the large mesoblast, which it is very difficult to 
detect (Pl. 9a, fig. 14, 14a). Each mesoblast occupies about one third the diameter 
of its ectoblast; as usually, before and afterwards, it has a central position, and is 
remarkable for its thickness (Pl. 9a, fig. 14) when compared with its breadth. The 
external surface of these cells is more or less flattened next to the inner stratum 
(PI. 9a, fig. 14, a) of the stroma, a feature more conspicuous still in older eges. 
When the egg has about one tenth of an inch in diameter, (Pl. 9a, fig. 18,) the 
exterior surface of these cells is closely pressed and flattened against the fibrous 
stroma (Pl. 9a, fig. 18a, 4); and the cells themselves have become internally so 
transparent that even the mesoblast is not visible, except when brought out by 
reagents. This hemispherical shape, with their rounded surface next to the zona 
pellucida, they retain for the rest of the imterovarian life of the egg (PI. 9d, dia- 
gram, fig. 2, 6). What further changes they undergo, after the egg has been 
expelled from their embrace, has not yet been investigated, and therefore the 
subject must be left for further research. 
The Zona pelluada. Although the zona pellucida is developed later than the 
vitelline sac, yet, on account of its connection with the Graafian follicle, we think 
it proper to consider it first. The earliest appearance of the zona pellucida which 
we have noticed is seen at a time when the ege has already become visible to 
the naked eye (Pl. 9a, fig. 12); it is then represented by a layer of excessively 
hyaline, large, flat cells, (Pl. 9, a, fig. 12a and 17, 4, 0',) resting on the outer sur- 
face of the yolk sac, and just within the tunica granulosa (c). For want of 
observations we can say nothing about the origin of these cells, excepting that, 
from their position, they must be developed from the Graafian follicle, and indi- 
