I go 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



meiotic division, so that of the four resulting microgametes two are 

 provided with sex chromosomes while two are without. 



The same end-result is arrived at in Fig. 86, B, although here it is the 

 second meiotic division in which the sex chromosome passes over bodily. 



A sex chromosome occurs also in the egg before meiosis but in this 

 case it is distributed in the ordinary manner at each division and one is 

 therefore present in every ripe macrogamete. 



Syngamy taking place at random between large numbers of micro- 

 gametes and macrogametes will clearly result in the formation of two 

 types of zygote in approximately equal numbers — the one type differing 





Fig. 86. 



Diagram showing the behaviour of the sex chromosome (x) during the meiotic divisions in the 

 male Ascaris. A, Sex chromosome remains undivided in the first meiotic division ; B, sex chromo- 

 some remains undivided in the second meiotic division. 



from the other in having an extra sex chromosome brought in by the 

 microgamete. It follows that the adult individuals into which the 

 zygotes develop will similarly be divided into two types the one differing 

 from the other in the fact that each of its cells contains two sex chromo- 

 somes in place of one. The former are the female individuals, the latter 

 the male : and the sex of the individual would appear to be the result of 

 pure chance — according as the macrogamete is fertilized by a microgamete 

 containing the extra sex chromosome or by one which is devoid of it. 



It will be seen that the recognition of these additional facts con- 

 cerning the sex chromosome involves an emendation of the statement 

 on p. 182 that in A. megalocephala the diploid number of chromosomes 



