232 N. M. Stevens 



maturation mitosis is always paler than the polar mass of fused 

 chromosomes, and is apparently about equal in bulk to one of the 

 two lagging elements seen in Fig. 37. I should therefore think that 

 the most probable interpretation of the irregular number of chro- 

 mosomes in the second spermatocytes (13) and of the two lagging 

 chromosomes is that the first (Figs. 37 and 38) is a precocious divis- 

 ion of the smaller heterochromosome (x^), and that the second 

 (Figs. 44-46) is one of the products of such a division, already uni- 

 valent and therefore not to be expected to divide again. In my 

 material the irregularities in division and in numbers are compara- 

 tively infrequent. As the number 24 has always been found in the 

 spermatogonia and 12 in the first spermatocytes, it would seem 

 that only the spermatids which are the result of two regular divi- 

 sions and therefore contain 12 chromosomes can become func- 

 tional. The occasional cases of a lagging chromosome in the first 

 spermatocyte mitosis suggest that the smaller heterochromosome 

 in Forficula is in somewhat the same uncertain condition as to its 

 behavior in maturation as is the case with the supernumerary 

 heterochromosomes in the Diabroticas, which divide sometimes 

 late in the first spermatocyte division and sometimes in the second 

 division, thus giving rise to irregular numbers. 



The heterochromosomes of two sizes can be distmguished in the 

 spermatids as shown in Figs. 47, o and b and Figs. 48, a and b, each 

 pair from one section of the same cyst. 



DISCUSSION 



In respect to the unequal heterochromosome pair, which judg- 

 ing from analogy with other insects, is probably to be associated 

 with the determination of sex, my results differ from those of all 

 others who have worked on the spermatogenesis of Forficula auric- 

 ularia. Either the inequality in this pair of chromosomes has 

 been overlooked, or the species is variable as to the character of 

 its heterochromosomes in different localities. I thought at first 

 that the difference in size of the. heterochromosomes (x^, and .V2) 

 in the Helgoland material was somewhat more conspicuous than 

 in that collected in Eisenach, but on further examination the dif- 



