150 



HEREDITY AND ENVIRONMENT 



yet such structures may be joined to one or 

 two ordinary chromosomes. This is the case 

 in the thread worm, Ascaris (Fig. 36), where 

 two such accessory elements are present in the 

 female, each being joined to the end of an 

 ordinary chromosome, whereas in the male 

 only one such element is present. Here also 

 two classes of spermatozoa are found one with 

 and the other without the accessory element, 



Ascaris Type 



Mature Egg and FcTtilized 

 Polar Body 

 Division /^^Jn 



Reduction 



J 



Sperms 



Fig. 36. Diagrams of Sex Differentiation in the Thread 

 Worm, Ascaris. The X chromosomes (black) are here joined to 

 ordinary chromosomes, there being two in the egg mother 

 cell and one in the sperm mother cell. All eggs contain one 

 of these X chromosomes, while half of the spermatozoa have 

 it and half do not. Eggs fertilized by one type of sperm 

 produce females, those fertilized by the other type produce 

 males. (From Wilson.) 



