476 CHORD AT A. 



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 zona radiata ; and inside this 

 there is the delicate vitelline membrane. The mammalian 



Fig. 327. — The Mammalian Graafian Folijcle in the Ovary. 



B 



/Central Cavity. 



A, Early stage. B, Later. 



egg so produced is always of minute size, often about 'i 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 I 

 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 



