The Cellular Basis 



161 



a small accessory (the Y chromosome) it gives rise to a male with 

 the formula XY (Fig. 54). 



In other animals one may not be able to distinguish separate 

 X or Y chromosomes and yet such structures may be joined to 

 one or two ordinary chromosomes. This is the case in the thread 

 worm, Ascaris (Fig. 55), 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 pres- 

 ent. 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 type of spermatozoon which enters the egg (Fig. 55). 



Recurring to the comparison of digits and chromosomes, the 

 chromosomal theory of sex determination may be illustrated 

 by assuming that all males have lost a particular digit, say the 



Atcaris Type 



Jterf action 

 Division 



Mature Egg and 

 Polar U 



W r -^5 



FIG. 55. DIAGRAMS OF SEX DETERMINATION IN THE THREAD WORM, 

 Ascaris. The X chromosomes (black) are here joined to ordinary chro- 

 mosomes, 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.) 



