68 THE STRUCTURE OF THE FOWL 



periods of division or multiplication, growth, 

 and maturation. 



During the period of multiphcation the 

 spermatogonia divide repeatedly and rapidly 

 by that process of mitosis which is undergone 

 by animal cells in general. Innumerable small 

 cells are produced, each containing in its 

 nucleus the number of chromosomes, or 

 stainable elements, typical of the ordinary 

 body or somatic cells. In the second period 

 growth in size takes the place of multiplication, 

 and spermatocytes of the first order result. 

 Then comes the period of maturation during 

 which two rapidly succeeding divisions occur. 

 The first division results in the formation of 

 two cells exactly alike, spermatocytes of the 

 second order, but differing from the somatic 

 cells in containing only half the typical number 

 of chromosomes. The second division pro- 

 duces two similar spermatids from one sperma- 

 tocyte of the second order. That is to say, 

 four spermatids precisely alike can claim descent 

 from one spermatocyte of the first order.-^ 



^ In the maturation period of oogenesis an important difference 

 will be noted (see p. 110). From one oocyte of the first order four 

 cells are produced, but three of these, the polar bodies, are small 

 and disappear ; wliile one cell, the mature ovum, is very large and 

 possesses the power to produce and embryo after fertilisation has 

 taken place. 



