DIFFERENTIATION, HEREDITY, SEX 311 



" accessory chromosome" and the determination of sex. The 

 work of Wilson, Stevens, Montgomery, Payne, Guyer, Morgan, 

 and many others, has made known the presence of these elements 

 in a whole host of Insects, including most of the orders, in 

 Myriopods, Arachnids, and Copepods. Among the lower 

 forms, Nematodes (Boveri, Edwards, Boring, Gulick), Sagitta 

 (Stevens), and Echinoderms (Baltzer) are now known to pos- 

 sess idiochromosomes. And more recently some of the Chor- 

 dates have been added to the ever increasing list, for idiochro- 

 mosomes have been described in the common fowl, guinea fowl, 

 and rat (Guyer), the guinea pig (Stevens) and even in man, 

 where Guyer has reported two idiochromosomes, half the sperm 

 containing twelve, and half ten, chromosomes; the number of 

 chromosomes in the human somatic cells is, therefore, twenty- 

 two in the male, and twenty-four in the female. 



Not all of the forms included in the above list exhibit this 

 phenomenon as simply as it occurs in the case of Anasa; we may 

 mention two general modifications of this typical condition. 

 In many Coleoptera, Diptera, and Hemiptera, the idiochrbmo- 

 some is not strictly an unpaired element for during the sperma- 

 tocyte divisions, and in the spermatids, it is paired with a very 

 small chromosome called the Y-element; together with X this 

 makes up an XY bivalent chromosome which behaves like any 

 bivalent chromosome in the preliminaries to the first spermato- 

 cyte division (Fig. 145). In the spermatozoa, therefore, half 

 the nuclei contain the large idiochromosome (X), and half the 

 small one (Y). The relation to sex is what might be expected, 

 namely, the females contain the large X, the males the small Y. 

 In Metapodius, one of the Orthoptera, this small Y-element may 

 be either present or absent apparently, although it is possible 

 that it may be present and fused with another chromosome 

 when it is said to be absent. 



In Ascaris megalocephala it seems clear that a small X- 

 element, no Y-element being present, may thus appear either as 

 a separate body, or fused with one of the other chromosomes 

 (Boring, Boveri, Edwards). Such an attachment of the idio- 

 chromosome to a certain one of the ordinary chromosomes is 



