1 6 STUDIES IN SPERM ATOGENESIS. 



ance in the evolution of a higher to a lower chromosome number." 

 (3) That of Miss Wallace, who suggests that in the spider only the 

 one out of each four spermatids which contains the accessory chro- 

 mosome is capable of developing into a functional spermatozoon, while 

 the other three degenerate, as do the polar bodies given off by the egg. 

 McClung is inclined to believe that the accessory chromosome is an 

 element common to all of the male reproductive cells of Arthropods, 

 and probably to vertebrate spermatocytes as well ('02). 



Of the insects considered in this paper Aphis and Termopsis have 

 no "accessory chromosome " or " heterochromosome " of any kind. 

 The fact that no males develop from the fertilized eggs of Aphis might 

 be offered as a reason for its absence, but such an argument would 

 not apply to Termopsis. The sex-character may indeed be repre- 

 sented in the chromatin of some one of the pairs of paternal and 

 maternal chromosomes of the spermatocytes, but there is no evident 

 peculiarity by which one-half of the spermatozoa can be said to be 

 different from the other half. As to McClung's statement ('02) ' ' that 

 if there is a cross-division of the chromosomes in the maturation 

 mitosis, there must be two kinds of spermatozoa, regardless of the 

 presence of the accessory chromosome," it appears to me that in a 

 case like the aphid, where the paired elements of the five bivalent 

 chromosomes are separated in the first maturation mitosis, there may 

 be as many as seventeen kinds of spermatozoa instead of two. If, 

 however, we suppose that the sex characters are segregated in the 

 first maturation mitosis, there would, of course, be two equal classes 

 of spermatozoa with reference to that character. 



In Stenopelmatus the element x in certain stages closely resembles 

 the " accessory chromosome " of McClung, and especially that 

 described by Baumgartner for Gryllus domesticus, but its origin and 

 fate are different. It first appears attached to an end of the spireme 

 in the growth stage of the young spermatocytes, where it is much 

 smaller than in later growth stages. It gradually increases in size, is 

 a conspicuous element in the first maturation spindle, goes into one 

 of each pair of spermatocytes of the second order, and there degenerates 

 during the rest stage between the two maturation mitoses. The whole 

 history of this element suggests that it may be rejected chromatin 

 analogous to that observed in the ovogenesis of many forms. In 

 Sagitta, for example, a considerable quantity of chromatin granules is 

 given off by the chromosomes and cast out into the cytoplasm near 

 the close of ovogenesis (Stevens, '03). Riickert ('92) has described a 

 similar casting out of chromatin material by the chromosomes of the 

 o ocy tes of Pristiurus. 



