306 ALLEN— SEX INHERITANCE IN SPH.EROCARPOS. 



animals — especially of such insects as Lygcciis and Euschistus (Wil- 

 son, 1906, 1912; Montgomery, 191 1) — that in the remainder of the 

 present discussion they will be designated, as commonly are the 

 apparently corresponding bodies in animals, the " X- " (large) and 

 "Y-" (small) chromosomes. This terminology is borrowed for 

 the sake of simplicity, in spite of the fact that it may require revision 

 in the light of future investigations. 



Whatever may later appear in this respect, at present the prob- 

 lem of the relation of the X- and Y-chromosomes to the complexes 

 of characters that distinguish male from female seems to be similar 

 in its broadest terms in Sphcerocarpos and in those animals whose 

 possession of sex chromosomes has been demonstrated. The sug- 

 gestion of an underlying similarity, however, does not involve main- 

 taining that the precise relation of these particular chromosomes to 

 certain hypothetical sex-determining factors is identical in all organ- 

 isms in which X- and Y-chromosomes are found. Indeed, there 

 appear at present to be, as regards the relation between special 

 chromosomes and characters distinctive of sex, three quite different 

 types of cases : 



(a) Those in which, in terms of the chromosomes, the female 

 is homozygous because it possesses what Wilson (1909) has called 

 two " X-elements " — the X-element in different instances being a 

 chromosome or a group of chromosomes ; and the male heterozygous 

 because it possesses either one X-element only, or one X- and one 

 Y-element ; the Y-element, if present, sometimes consisting of one 

 body, sometimes of a group. To this class are now referable a 

 large number of animals representing several phyla. 



{h) Those in which the male is homozygous, possessing two 

 X-elements, and the female heterozygous, possessing one X-element 

 only, or one X- and one Y-element. This class, apparently smaller 

 than the first, is established upon the basis of suggestive cytological 

 observations which in no case as yet cover the whole life history, 

 and of a larger amount of evidence derived from experimental 

 breeding. 



(c) Those in which the female possesses one X-element (or 

 chromosome), the male one Y-element, and in which the diploid 

 generation (the sporophyte) is heterozygous in terms of the chro- 



