I.] THE OVARIAN OVUM. 19 



of which forms the bed on which the blastoderm rests. The 

 whole yolk is invested with the vitelline membrane, this 

 again with the white ; and the whole is covered with two 

 shell-membranes and a shell. 



11. 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 

 ripe ovarian ovum in the ovary of a hen up to the time when 

 it is laid. 



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. Examined 

 with care the ovum, which is tolerably uniform in appearance, 

 will be found to be marked at one spot (generally lacing the 

 stalk of the capsule and forming the pole of the shorter axis 

 of the ovum) by a small disc differing in appearance from 

 the rest of the ovum. This disc is known as the germinal 

 disc or discus proligerus. It consists of a lenticular mass of 



w.y. 



SECTION THROUGH THE GERMINAL Disc OF THE RIPE OVARIAN OVUM OP 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 by the action of the chromic acid. The material enclosed in the 

 membrane of the vesicle is in the hardened specimens finely granular, 

 y, space originally completely filled up by the germinal vesicle, before the 

 latter was shrivelled up by the action of the chromic acid. 



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