AVES. 121 



Fowl will be employed as type, attention being called when necessary 

 to the development of the other forms. 



The ovum of the Fowl, at the time when it is clasped by the ex- 

 panded extremity of the oviduct, is a large yellow body enclosed in a 

 vitelline membrane. It is mainly formed of spherules of food-yolk. 

 Of these there are two varieties ; one known as yellow yolk, and the 

 other as white. The white yolk spherules form a small mass at the 

 centre of the ovum, which is continued to the surface by a narrow 

 stalk, and there expands into a somewhat funnel-shaped disc, the edges 

 of which are continued over the surface of the ovum as a delicate 

 layer. The major part of the ovum is formed of yellow yolk. The 

 yellow yolk consists of large delicate spheres, filled with small 

 granules (fig. 85 A); while the white yolk is formed of vesicles of a 

 smaller size than the yellow yolk spheres, in which are a variable 

 number of highly refractive bodies (fig. 85 B). 



In addition to the yolk there is present in the ovum a small proto- 

 plasmic region, containing the remains of the germinal vesicle, which 

 forms the germinal disc (fig. 86). It overlies the funnel-shaped disc 



it-.y. 



1'ici. 80. SECTION THROUGH THE GERMINAL DISC OF THE EIPE OVARIAN OVUM OF A 



FOWL WHILE YET ENCLOSED IN ITS CAPSULE. 



.. Connective-tissue capsule of the ovum ; b. epithelium of the capsule, at the 

 surface of which nearest the ovum lies the vitelliue membrane; <. granular material 

 of the germinal disc, which becomes converted into the blastoderm. (This is not 

 very w T ell represented in the woodcut. In sections which have been hardened in 

 chromic acid it consists of fine granules.) w.y. white yolk, which passes insensibly 

 into the fine granular material of the disc; x. germinal vesicle enclosed in a distinct 

 membrane, but shrivelled up; ?/. space originally completely filled up by the germinal 

 vesicle, before the latter was shrivelled up. 



of white yolk, into which it is continued without any marked line of 

 demarcation. It contains numerous minute spherules of the same 

 nature as the smallest white yolk spherules. 



Impregnation takes place at the upper extremity of the oviduct. 



In its passage outwards the ovum gradually receives its acces- 

 sory coverings in the form of albumen, shell-membrane, and shell 

 (fig. 87). 



The segmentation commences in the lower part of the oviduct, 

 shortly before the shell has begun to be formed. It is mero- 

 blastic, being confined to the germinal disc, through the full depth 

 of which however the earlier furrows do not extend. It is mainly 

 remarkable for being constantly somewhat uiisymmetrical (Kolliker) 

 -a feature which is not represented in fig. 88, copied from Coste. 



