I.] THE OVARIAN OVUM. 15 



ovum this is in its turn embraced by a highly vascular 

 connective-tissue capsule. 



The youngest ova ai-e, as has already been stated, 

 quite naked. In ova of about 1"5 mm. the superficial 

 layer of the ovum becomes converted into a radiately 

 striated membrane called the zona radiata. At a later 

 period a second membrane, j^laced between the zona 

 radiata and the cells of the follicle, makes its appearance, 

 but its mode of origin is still unknown. As the ovum 

 approaches maturity the zona radiata disappears, and in 

 the ripe ovum the second membrane, which has already 

 been spoken of as the vitelline membrane, alone 

 remains. 



From what has just been stated it follows that in 



an egg which has been laid the yolk alone constitutes 



the true ovum. The white and the shell are in fact 



" accessory structures formed during the passage of the 



ovnm down the oviduct. 



When the ovarian ovum is ripe and about to be 

 discharged from the ovary, its capsule is clasped by 

 the open infundibulum of the oviduct. The capsule 

 then bursts, and the ovum escapes into the oviduct, its 

 longer axis corresponding with the long axis of the 

 oviduct, the germinal disc therefore being to one 

 side. 



In describing the changes which take place in the 

 oviduct, it will be convenient, following the order pre- 

 viously adopted, to treat first of all of the formation 

 of the accessory parts of the egg. These are secreted 

 by the glandular walls of the oviduct. This organ 

 therefore requires some description. It may be said to 

 consist of four parts : 1st. The dilated infundibulum 



