ANIMAL BIOLOGY. [Part I. 



(y^th in.) has but little; the larger ovum of the frog contains 

 a moderate amount ; in the much larger fowl's egg (for we will 

 here take the fowl as our typical bird) there is a relatively 

 enormous quantity. The distribution of the yolk-spheres also 

 differs in the two types in which they are freely developed. In 

 the frog they are larger and more closely aggregated at one pole, 

 than they are at the opposite pole, in which the yolk-spheres 

 are smaller. This protoplasmic pole, with the smaller yolk- 

 spheres, is termed the animal pole. In the fowl the yolk-spheres 

 are of two kinds, smaller and earlier developed white yolk- 

 spheres, larger and later developed yellow yolk-spheres. These 

 are arranged in somewhat concentric layers around a central 

 flask-shaped mass of the white yolk. 



The germinal vesicle, which is relatively large in the immature 

 ovum, occupies less relative space in the mature egg. In the 

 rabbit's ovum it is near, but not at the centre. With the 

 development of food-yolk it becomes thrust over towards the 

 animal pole, near which it lies in the frog's ovum; while in 

 the fowl's egg it travels to the periphery, where, together with 

 the protoplasm round it, which remains relatively free from 

 yolk-spheres, it constitutes the germinal disc. 



The phenomenon now to be described, termed the extrusion 

 of the polar cells, probably occurs in all three types. It consists 

 in the division of the germinal vesicle of the ovum (or the 

 nucleus of the cell) into two parts, of which one remains within 

 the ovum, while the other, carrying with it a portion of the 

 protoplasm, gives rise to a little protuberance, budded off from 

 the ovum, and known as the polar cell. The polar cell is then 

 constricted off, becomes separate from the ovum, and dies. A 

 second polar cell is then budded off in a similar way. Thus two 

 polar globules are produced by a process analogous to cell- 

 division. It is supposed that they remove from the ovum 

 matter detrimental to the further development of the egg. 

 Each contains a share of the nucleus of the ovum. The share 

 that remains to the ovum after the extrusion of the polar cells, 

 containing perhaps only one quarter of the original nuclear 

 matter, is called the female pronudeus. 



