TI1E DEVELOPMENT OF VERTEBRATA. 



217 



somes in pairs, so that the number is not actually reduced 

 until the reduction divisions have taken place. In other 

 cases the longitudinal halves are separate and are trans- 

 versely divided so that there is a tetrad of four rods. The 

 first reduction division has been called heterotype because 

 it differs from the type of somatic division, and the second 

 homotype because it is less different ; but there seems more 

 similarity between the two reduction divisions, and this 

 term itself best describes them. On the view that the 



Fig. 1106. Spermatogenesis in Axcaris megalocephala, variety with four chromo- 

 somes in somatic cells. A, a cell of the reproductive organ with four thread-like 

 chromosomes. B, a spermatocyte with two chromosomes each in the form of a 

 tetrad. C, the nucleus dividing. D, two spermatocytes formed, each with two 

 dyads. E, second reduction division. F, four spermatida formed, each containing 

 two simple chromosomes or monads. 



tetrad really consists of two somatic chromosomes united 

 but not mixed together, it is supposed that one of the 

 divisions consists in the separation of whole chromosomes, 

 but it is not certain whether this takes place in the first or 

 second division ; the fact that in parthenogenetic animals 

 eggs may develop without fertilisation when only one divi- 

 sion occurs, but not after the second, seems to show that 

 the reduction really takes place in the second division. 



