144 



HEREDITY AND ENVIRONMENT 



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, 

 whereas all ova have this element, and in this 

 case also sex is probably determined by the 



Ascaris Type 



Mature Egg and 

 Polar Body 



FIG. 36. DIAGRAMS OF SEX DIFFERENTIATION IN THE THREAD 

 WORM, Ascaris. The X chromosomes 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.) 



