no CYTOLOGY 



CHAP, 



division X divides equationally in those nuclei where it is present. The 

 four young spermatids formed from each primary spermatocyte remain 

 attached together, and it is easy to verify the fact that two have five 

 chromosomes and two have six. The chromosomes remain individually 

 distinguishable even in the ripe spermatozoon, so that fertilization of 

 the eggs by the two different kinds of spermatozoa can be traced. The 

 whole case can be followed as clearly as in a text-book diagram. 



Similar simple conditions (presence of a single X chromosome, no Y 

 chromosom.e) have been described by Gulick in five species of Heterakis and 

 Strongylus (191 1). 



Conditions are more complicated in the genus Ascaris. In A. lum- 



hricoides Edwards (1910) found that the X element consists of a group 



. of five chromosomes which pass undivided to one pole in anaphase I. 



There is no Y chromosome, and two classes of spermatozoa are thus 



formed, one with nineteen and the other with twenty-four chromosomes. 



In A. Cams (Walton, 1918) the X group consists of six chromosomes, 

 the two types of spermatozoa having respectively twelve and eighteen 

 chromosomes ; all the mature ova have eighteen. 



The problem of the sex chromosome in A. megalocephala has been 

 attacked by several workers. In this species the X chromosome appears 

 to be single, and there is no Y. As a rule the X element is attached to 

 one of the larger chromosomes, and hence difficult or impossible to 

 recognize. In rare individuals, however, it is a separate element. In 

 the $ the two X's can also be recognized, again generally, but not 

 always attached to the larger chromosomes. For further information 

 and literature in regard to this species the reader is referred to Frolowa 



(1913)- 



It is noteworthy that in Nematodes the division which is differential 



for the sex chromosomes varies as it does in insects. In most species 



so far described this is the first division, but in A . nigrovenosiis (described 



below) it is the second. 



Among Vertebrates, sex chromosomes have been studied principally 

 in Birds and Mammals. 



Sex chromosomes have been described in many species of the latter 

 group, but in most cases the evidence cannot be considered quite 

 conclusive. The clearest example is perhaps the Opossum (Jordan, 

 1912).^ A summary of the work done on mammalian sex chromosomes 

 will be found in Jordan (1914). 



1 The most circumstantial account of sex chromosomes in a mammal is probably that 

 of Wodsedalek (1913) for the pig. Until, however, some means is found of reconciling the 

 extraordinary discrepancy between this author's account of the chromosomes of the sperma- 

 togonia of this animal and that given by Hance (1918 a), it is difficult to appraise the value 

 of the evidence. 



