42 



PHYSICAL BASIS OF HEREDITY 



between them so that two cells are formed, each cell con- 

 taining twelve chromosomes, six paternal and six mater- 

 nal. Thus, through fertilization, the whole number of 

 chromosomes is restored to the egg. This number remains 

 through all subsequent divisions of the cells of the embryo. 

 The male oi Ancyracanthus has only eleven (Fig. 17, a) 

 chromosomes ; because the male has only one sex-chromo- 



(Z 



Fig. 17. — Spermatogenesis of Ancyracanthua. Spermatogonia! cell, a; cell after growth 

 period with tetrads, b; first spermatocyte division, c; two cells resulting from first division 

 with six and with five chromoaomes, respectively, d; four cells resulting from the next 

 division, e; ditto,/; mature spermatozoa, one with six, the other with five, chromosomes,g; 

 ditto, living spermatozoa, h. (After MulsOw.) 



some, while the female has two sex-chromosomes. Both 

 sexes have ten other chromosomes, sometimes called auto- 

 somes. Just before the maturation divisions take place, 

 there are six rods in each sperm-cell, five of which (the 

 autosomes) condense into tetrads, the sixth (the sex- 

 chromosome) into only a double body (Fig. 17, b). A 

 spindle develops about these and each of the five auto- 

 somes divides. The sex-chromosome does not divide, but 

 passes to one pole of the spindle (Fig. 17, c). The result 



