INTERNAL CAUSES OF VARIATION 160 



^ 



the ovary. More commonly, as in annelids, gasteropods, and 

 nematodes, they are not formed until after the spermatozoon 

 has made its entrance ; while in a few cases one polar body 

 may be formed before fertilization and one afterward, as in the 

 lamprey eel, the frog, and in Amphioxiis. In all these cases 

 the essential phenomena are the same. Two minute cells are 

 formed, one after the other, in rapid succession and near the 

 upper or animal pole of the ovum ; and in many cases the first 

 of these divides into two as the second is formed." 



To what extent this division is qualitative is unknown. Of 

 one thing we are certain : somewhere in the process the number 

 of chromosomes has been reduced to exactly one half the number 

 characteristic of the species. 



It was formerly supposed by Van Beneden, Weismann, and 

 Boveri that reduction consists in the casting out and degenera- 

 tion of half of the chromosomes. " Later researches conclusively 

 showed, however, that this view cannot be sustained, and that 

 reduction is effected by a rearrangement and redistribution of the 

 nuclear substance, without loss of any of its essential constitu- 

 ents." 1 This is said because the groups tetrads, rods, rings, 

 etc. arise spontaneously in the nucleus in the reduced number. 

 The loss occurs later in the extrusion of the polar bodies, but 

 no corresponding loss takes place on the male side because all 

 four cells are functional, though not all alike. 



Reduction in the male. 2 The maturation processes in the male 

 and female are practically identical in their results, with two 

 exceptions ; namely, first, in the male the four cells resulting 

 from the maturation divisions are all alike functional, and second, 

 they are exceedingly small in size as compared with the ovum, 

 being almost destitute of cytoplasm. 



The spermatogonia, corresponding to the oogonia of the 

 female, arise in the testes by mitotic division, with the full 

 somatic number of chromosomes. As in the female, they reach 

 a stage where division ceases for a time and enlargement ensues, 

 in which condition the cells are known as spermatocytes (corre- 

 sponding to oocytes in the female). 



1 Wilson, The Cell, p. 233. 



2 Ibid. pp. 241-242. 



