304 THEOPHILUS S. PAINTER 



Many embryos, besides those figured, were studied, the results 

 being consistent throughout. Sceloporus spinosis embryos are 

 of two types, one carrying ten large V-shaped macro-chromosomes 

 (and the smaller V-shaped 'b' chromosomes) and the other 

 carrying twelve large V-shaped elements and the 'b' chromo- 

 somes. The former are destined to be males, the latter are 

 females. 



DISCUSSION 



1. Sex-chromosomes 



At the present time the point of most general interest in works 

 dealing with vertebrate spermatogenesis is the question: How 

 is sex determined, what is the condition of the sex-chromosomes? 

 Ever since the sex-chromosomes were discovered in the insects, 

 a number of investigators^ have worked upon nearly all of the 

 common vertebrates with a view of determining this question. 

 In some cases a sex-chromosome has been reported, in other 

 cases the results were negative, but it may be said with fairness 

 that in only a few instances has the evidence for either the pres- 

 ence or the absence of the X-chromosome been complete enough 

 to be convincing to one critically disposed toward the sex-chro- 

 mosome theory. In perhaps the majority of cases the evidence 

 for the existence of an X-chromosome has been based on obser- 

 vations of usually first spermatocyte cells, in which typically 

 a bipartite chromosome was seen lying well towards one pole of 

 the spindle and going presumably undivided to it. Only rarely 

 have such observations been extended to the second maturation 

 and proper check counts made here. . And yet in view of all we 

 know about the irregular way (in point of time) in which verte- 

 brate chromosomes divide, it would seem that we must accept 

 with some reserve those works in which the presence or absence 

 of the sex-chromosome was concluded from a study of the first 

 maturation division alone. It has been a matter of common 



^ No better review of this subject could be given than is contained in a recent 

 paper by Ethel Brown Harvey ('20). To the papers listed by Mrs. Harvey one 

 should add the recent work of Wodsedalek ('20), dealing with the spermato- 

 genesis of cattle. 



