20 (190) Society for Experimental Biology and Medicine. 



is much smaller than the corresponding one in the female (which 

 is in agreement with the observations of Stevens on the beetle 

 Tenebrio). These types may conveniently be designated as A and 

 B, respectively. The essential facts have been determined in three 

 genera of each type, namely (type A), Protenor belfragei, Anasa 

 tristzs, and Alydus pilosulus, and (type B), Lygceus turcicus, Eus- 

 chistus fissilis, and Ccenus delius. The chromosome groups have 

 been examined in the dividing oogonia and ovarian follicle cells of 

 the female and in the dividing spermatogonia and investing cells of 

 the testis in case of the male. 



Type A includes those forms in which (as has been known 

 since Henking's paper of 1890 on Pyrrochoris) the spermatozoa 

 are of two classes, one of which contains one more chromosome 

 (the so-called " accessory" or heterotropic chromosome) than the 

 other. In this type the somatic number of chromosomes in the 

 female is an even one, while the somatic number in the male is one 

 less (hence an odd number), the actual numbers being in Protenor 

 and Alydus Q 14, J 13, and in Anasa Q 22, cJ 1 21. A study of 

 the chromosome groups in the two sexes brings out the following 

 additional facts : In the cells of the female all the chromosomes 

 may be arranged two by two to form pairs, each consisting of two 

 chromosomes of equal size, as is most obvious in the beautiful 

 chromosome groups of Protenor, where the size differences of the 

 chromosomes are very marked. In the male all the chromosomes 

 may be thus symmetrically paired with the exception of one which 

 is without a mate. This chromosome is the 14 accessory" or het- 

 erotropic one ; and it is a consequence of its unpaired character 

 that it passes into only half the spermatozoa. 



In type B all the spermatozoa contain the same number of 

 chromosomes (half the somatic number in both sexes), but they 

 are, nevertheless, of two classes, one of which contains a large 

 and one a small " idiochromosome." Both sexes have the same 

 somatic number of chromosomes (14 in the three examples men- 

 tioned above), but differ as follows : In the cells of the female 

 (oogonia and follicle cells), all the chromosomes may, as in type 

 A, be arranged two by two in equal pairs, and a small idiochro- 

 mosome is not present. In the cells of the male, all but two may 

 be thus equally paired. These two are the unequal idiochromo- 



