I.J THE OVARIAN OVUM. 11 



form of a flask-shaped mass in the interior of the yolk, 

 the upper somewhat expanded top of the neck of 

 which forms the bed on which the blastoderm rests. 

 The whole yolk is invested with the vitelline mem- 

 brane, this again with the white ; and the whole is 

 covered with two shell-membranes and a shell. 



Such an egg has however undergone most important 

 changes while still within the body of the hen; and 

 in order to understand the nature of the structures 

 which have just been described, it will be necessary to 

 trace briefly the history of the egg from the stage when 

 it exists as a so-called ovarian ovum in the ovary of a 

 hen up to the time when it is laid. 



In birds the left ovary alone is found in the adult ; 

 and is attached by the mesovarium to the dorsal wall 

 of the abdominal cavity, on the left side of the vertebral 

 column. It consists of a mass of vascular stroma in 

 which the ova are imbedded, is covered superficially 

 by a layer of epithelium, continuous with the epithelial 

 lining of the peritoneal cavity. The appearance of the 

 ovary varies greatly according to the age of the indi- 

 vidual. In the mature and sexually active females 

 it is almost wholly formed of pedunculated and highly 

 vascular capsules of various sizes, each containing a more 

 or less developed ovum ; in the young animal however 

 it is much more compact, owing to the absence of 

 advanced ova. 



If one of the largest capsules of the ovary of a hen 

 which is laying regularly be opened, it will be found to 

 contain a nearly spherical (or more correctly, ellipsoidal 

 with but slightly unequal axes) yellow body enclosed in 

 a delicate membrane. This is the ovarian ovum or egg. 



