148 



HEREDITY AND ENVIRONMENT 



In other cases Miss Stevens as well as Wil- 

 son discovered that two accessory chromo- 

 somes, differing in size, might be present in 

 the male whereas in the female they are of 

 equal size (Fig. 35) . In such cases two types 

 of spermatozoa are produced in equal num- 

 bers, one containing a large and the other a 



Oocyte 



Protenor Type 

 Ova 



Fertilized Egg 



Spermatocyte Seduction Spermatids 



Division 



Fig. 34. Diagrams of Sex Differentiation in the Bug, 

 Protenor. The oocyte contains 6 chromosomes and the sper- 

 matocyte 5 chromosomes which are not yet united into synap- 

 tic pairs; the "sex" chromosomes are shown in black, two are 

 present in the oocyte, but only one in the spermatocyte. In 

 the reduction division the synaptic pairs separate, giving rise 

 to two types of spermatids, one of which has the sex chrom- 

 osome and the other lacks it; all ova are alike in this regard. 

 If an egg is fertilized by a sperm without the sex chrom- 

 osome a male results; if fertilized by a sperm containing the 

 sex chromosome a female results. (After Wilson with 

 modifications.) 



