146 GERMINAL DISC. 



investigations has been to shew that the variations in the early 

 development of different Birds are comparatively unimportant. 

 In the sequel the common Fowl will be employed as type, atten- 

 tion 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 

 expanded extremity of the oviduct, is a large yellow body en- 

 closed 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 some- 

 what 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 

 protoplasmic region, containing the remains of the germinal 

 vesicle, which forms the germinal disc (fig. 86). It overlies the 



or 



/ 



^v *- _ -.-a. rf , r , r-^j- 



" 3 * 



FIG. 86. SECTION THROUGH THE GERMINAL DISC OF THE RIPE OVARIAN OVUM 



OF A FOWL WHILE YET ENCLOSED IN ITS CAPSULE. 



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

 surface of which nearest the ovum lies the vitelline membrane ; c. granular material 

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

 very well 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 ; y. space originally completely filled up by the germinal 

 vesicle, before the latter was shrivelled up. 



funnel-shaped disc of white yolk, into which it is continued with- 

 out any marked line of demarcation. It contains numerous 



