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MANUAL OF ELEMENTARY ZOOLOGY 



has therefore the normal number, which it imparts to the 

 body cells during the mitoses by which it joins them. 



Spermatogenesis takes place in the testis. Oogenesis 

 begins in the ovary and finishes after fertilisation. 



The actual course of gametogenesis differs greatly in the 

 two cases. In spermatogenesis the cytoplasm is equally 

 divided and each of the products of division becomes a 

 spermatozoon, so that from each spermatocyte four 

 spermatozoa arise. In oogenesis each division is unequal. 

 At the first division there are formed a large cell and 

 a small one, the latter containing very little cytoplasm 



and being known as the 

 first polar body. At the 

 second division, while 

 the first polar body forms 

 two very small cells, 1 the 

 large cell forms again a 

 large and a small pro- 

 duct. The large product 

 is an ovum ; the small 

 is called the second polar 

 body. Thus, instead of 

 four ova the oocyte 

 forms one ovum and 

 three vestigial cells which 

 come to nothing. The 

 formation of the polar 

 bodies is known as the 

 maturation of the ovum. 

 The meaning of these 

 processes is very ob- 

 scure. It is clear, however, (i) that, whatever be the 

 reason for the occurrence of two divisions in gametogenesis, 

 the fact of these being the same for both kinds of gametes 

 makes the ovum and spermatozoon equivalent bodies, 

 which contribute equally to the zygote nucleus ; (2) that the 

 reduction division prevents the number of chromosomes 

 from being increased by doubling at each conjugation ; (3) 

 that in this division the nuclear substance is not evenly 

 divided by the splitting of each chromosome as in ordinary 

 1 In many animals the first polar body does not divide. 



Fig. 67. — Sections of the egg of a frog 

 during maturation and fertilisation. — 

 Semi-diagrammatic. 



l, x, 3, Successive stages. 



««, Nucleus of ovum; p.i.i, p.b.z, first and 

 second polar bodies ; pig. , pigmented 

 protoplasm ; $ , female pronucleus ; 6 , 

 male pronucleus. 



