io6 MANUAL OF ELEMENTARY ZOOLOGY 



towards the poles of the spindle. There they arrange 

 themselves in a radiating manner. Finally, the chromo- 

 somes of each group thus formed constitute a daughter 

 nucleus by passing through a series of changes which 

 reverse those by which the mother nucleus broke up. 

 They unite end to end to form a skein, and this breaks 

 up into a nuclear meshwork, a nuclear membrane mean- 

 while forming around it. It will be seen that the result 

 of this process is an exact halving of the linin and 

 chromatin of the mother nucleus between the daughter 

 nuclei. It is often supposed that the object of this process 

 is the inheritance by each daughter nucleus of all the 

 powers of the parent nucleus, but the fact that most of the 

 cells exhibit a part only of the potentialities of the fertilised 

 ovum makes it difficult to accept this interpretation without 

 qualification. 1 



The nuclei of the ova and spermatozoa, destined to form 

 „ , by fusion a single nucleus from which the 



Cametogenesls. ' , .. P . , . , ,. 



body nuclei arise, contain each only half 

 the number of chromosomes found in the body nuclei. 

 This is the result of certain peculiarities in the cell 

 divisions by which the gametes arise. The formation of 

 ova and spermatozoa is known as gametogenesis. The 

 formation of spermatozoa is known as spermatogenesis, 

 that of the ovum as oogenesis. Gametogenesis comprises 

 two successive divisions of a cell, derived from the germinal 

 epithelium, known as the gametocyte. The gametocytes 

 which give rise to spermatozoa are known as spermatocytes ; 

 those which give rise to ova are oocytes. 



The first division of gametogenesis is known as the 

 meiotic division. It differs from ordinary mitosis in that 

 by it the number of chromosomes is halved. It is therefore 

 called the reducing division. The process is complicated by 

 the fact that the true chromosomes come together in pairs 

 in the gametocyte before the meiotic division, so that the 

 nucleus appears already to contain half the normal number 

 of chromosomes, but these are really double chromosomes. 

 Then the two chromosomes of each double chromosome 

 part in the meiotic division, so that the daughter nuclei 

 have each half the normal number of chromosomes, but 

 1 See footnote on p. 190. 



