190 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



. so that of the four resulting microgametes two are 

 led with sex chromosomes while two are without. 



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



,tic division in which the sex chromosome passes over bodily. 



A sex clin.inosi.me occurs also in the egg before meiosis but in this 



^rikiU'd in the ordinary manner at each division and one is 



ore present in every ripe macrogamete. 



ainy taking place at random between large numbers of micro- 



uametes and marrogametes will clearly result in the formation of two 



..f zygote in approximately equal numbers the one type differing 



FIG. 86. 



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

 < hromosome remains undivided in the first meiotic division ; B, sex chromo- 

 some rciii.iin> undivided in the second meiotic division. 



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

 iinete. It follows that the adult individuals into which the 

 Inp 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 

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

 pure chance according as the macrogamete is fertilized by a microgamete 



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

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

 sex chromosome involves an emendation of the statement 

 on j). is- that in A. megalocephala the diploid number of chromosomes 



