538 



GENERAL CONSIDERATIONS 



In man, and perhaps in all mammals, there are an a;-chromosome and 

 a ^-chromosome in the male and two x-chromosomes in the female. Man 

 also has 46 autosomes. The formulae for the two sexes may be written: 

 cT = 46 + .T + 2/; 9 = 46 + 2x. The sex cells are 23 + x or 23 + y. 



Formula 9=8+2x 



Formula 6 = 8+x 



Primary 

 Spermatocyte 



S\ Primary 

 Zy Oocyte 



Two kinds of sperm^ ^^{J^ , 

 Produces 



Formula S = 8+2x 



One /<ino/ ofeqq cell 

 I 



Formula 9 = d-f-x 



Primary /^ 

 Spermatocy te ^'* 



B 



Produces 



Formula cf - S + x-t-y Formula 9= 8+2x 



Primary 

 Spermatocyti^ 



Primary 

 Oocytes 



Two /<inds _<? 

 Produces 



One kind ofeag cell 



6 



This may be reversed as in the case of A and B 



Fig. 327. — Diagrams illustrating sex determination in three different cases. 



609. Twins. — The production of twins may be mentioned in connec- 

 tion with sex. There are two types, known respectively as identical and 

 fraternal twins. Identical twins agree precisely in their characteristics 

 and are always of the same sex. For this reason it is assumed that they 

 are due to the separation of blastomeres developed from a single zygote, 

 each group of blastomeres developing into a complete individual. In 

 the case of the nine-banded armadillo it has been found that the four 



