DEVELOPMENT OF THE CHICK 111 



centric arrangement of the fibres of the ad- 

 jacent stroma. Examination of the ovary of a 

 laying hen shows that there is a regulation of 

 the degree of growth of the ova, since all sizes 

 are present from the smallest to one or two of 

 such dimensions as to lead to the conclusion 

 that they are almost or quite ready to escape 

 into the oviduct. 



The third period of oogenesis, or stage of 

 maturation, takes place partly while the ovum 

 is still within the ovarian follicle and partly 

 after it has escaped into the oviduct and is 

 undergoing fertilisation. Maturation consists 

 of two unequal divisions. By each division 

 the ovum is split into a small cell — known as 

 the polar body — and a large cell which will be 

 regarded as the ovum proper. The two polar 

 bodies do not contribute to the formation 

 of the embryo, but degenerate and disappear. 

 Their production has for its object a reduction 

 in the number of chromosomes, or stainable 

 elements, of the nucleus of the ovum. The 

 mature ovum, therefore, is deficient in chromo- 

 somes, the normal number being restored 

 only when the nucleus of the spermatozoon — 

 which has also lost chromosomes during a 

 comparable process of maturation — combines 



