442 



HISTOLOGY 



a demonstration of the more ordinary features of this process in the ani- 

 mals. The more superficial features are discussed; but as in the majority 

 of cases among animals the chromatin does not show all the changes it 

 probably goes through, or allow us to explain the probable meaning of 

 such changes as we can see. Dr. H. E. Jordan has very kindly per- 

 mitted us to use the following account he has written, and the figures he 

 has drawn of his studies on the spermatogenesis of Aplopus Mayeri, a 

 giant "walking stick" of the Florida Keys, to demonstrate these features. 



FIG. 404. Half developed spermatozoa of Raja ocellata. Middle pieces are visible. The 

 members of each sperm column have been drawn in or have moved in to bury their tips 

 in the nurse cells. X 1000. 



The accessory chromosome and its relation to the phenomenon of 

 sex. 1 The accessory chromosome (so named by McClung, '02) was 

 first reported by Henking in a paper on the spermatogenesis of Pyr- 

 rhocoris apterus (Hemipter) published in 1890. Henking here noticed 

 that in one of the spermatocyte divisions one chromosome did not divide, 

 thus giving rise to two kinds of spermatozoa, one group with and the 

 'other without the odd element. McClung ('oo-'o2) studied a number 

 of forms among the Locustidae and Acrididae, and reported uniform 

 results in regard to the presence and behavior of the accessory chromo- 



1 Written by Dr. H. E. Jordan of the University of Virginia. 



