476 ' CHORDATA. 
surrounded by a mass of follicle-cells which are usually 
regarded as nutritive. These cells increase in number and 
the whole follicle grows rapidly. A split occurs between 
them, so that in a fully-formed ‘“ Graafian follicle” the 
ovum lies towards the centre surrounded by certain of the 
follicle-cells. A large cavity separates them from the outer 
layer of follicle-cells which form the outer tunic of the follicle 
and the two layers are connected by strands. 
When ripe, the follicle bursts and discharges the ovum at 
the surface of the ovary, whence it passes into the oviduct 
through its fimbriated opening. The ripe egg has a hyaline 
membrane around it, the zoza radiata; and inside this 
there is the delicate vitelline membrane. The mammalian 
Fig. 327.—THE MAMMALIAN GRAAFIAN FOLLICLE IN THE OVARY. 
Central Cavity. 
Outer Layer 
of Follicle 
Cells. 
Nucleus. 
A, Early stage. B, Later. 
egg so produced is always of minute size, often about ‘1 mm. 
in diameter (about the same as Amphioxus). Maturation is 
effected by the extrusion of two polar bodies and fertilisa- 
tion takes place high up in the Fallopian tube. 
SEGMENTATION.—The egg immediately commences to 
segment whilst it passes down the Fallopian tube. There is 
no yolk and the segmentation is total and nearly equal. 
The first division is into two blastomeres, of which one is 
very slightly the smaller. Each divides into two and then 
into four. The larger cells then become tucked inside 
the smaller, which on their part divide more rapidly and 
spread round them. Thus there is produced a stage 
in which the larger or hypoblast cells are enclosed on every 
