EMBRYOLOGY OF THE CHICK 



That a new individual may be brought into existence, there 

 must be accomplished the union of the male element, or spermatozoon, 

 with the female element, the ovum. This union is called fertilization. 

 In the fowl this fertilization is accomplished at the anterior portion 

 of the oviduct, after the calyx has ruptured and discharged its yolk, 

 and before the albumen has been formed around it. The blasto- 

 derm is found on the surface of the yolk. One spermatozoon is all 

 that is required; in fact, only one can be used in this union. 



Spermatogenesis. The spermatozoa are formed by the seminifer- 

 ous tubules of the testis. From these cells, called the spermatogonia, 

 are formed other cells called spermatocytes, which in turn form the 

 spermatids, the immediate forerunners of the spermatozoa. During 

 the period of multiplication the spermatogonia divide repeatedly by 

 mitosis. Numbers of small cells are thus produced, each containing 

 in its nucleus the number of chromosomes typical of the somatic cell 

 of that fowl. In the second period the cells become larger and 

 spermatocytes of the first order are formed. Then comes the 

 period of maturation, during which two succeeding divisions rapidly 

 occur. The first division results in the formation of two cells exactly 

 alike. These are spermatocytes of the second order. They differ 

 from the somatic cells in that they contain only one-half the typical 

 number of chromosomes. The second division produces two similar 

 spermatids from one spermatocyte of the second order. Therefore 

 four spermatids exactly alike may be formed from one spermatocyte 

 of the first order. From these spermatids the spermatozoa are 

 formed (Fig. 55). The heads of the spermatozoa contain the nuclei 

 derived from the spermatids; the necks contain the centrosomes; and 

 the tail, consisting of three parts is probably formed from the proto- 

 plasm. Three parts of the tail are as follows: first, the pars conjunc- 

 tionis, which unites the tail to the neck; second, the pars principalis, 

 which constitutes the main length of the tail; and, third, the pars 

 terminalis, which consists of an axial filament which transverses the 

 entire tail and is surrounded by a protoplasmic sheath. 



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