88 KANSAS UNIVERSITY QUARTERLY. 



resting condition between two consecutive mitoses. These are the 

 points which I wish to consider here. 



Already the general methods of tetrad formation have been out- 

 lined. Now I wish to take up in a more detailed way the work done 

 upon insect material.* 



Wilcox (19), (20), (21), from a study of the " Spermatogenesis of 

 Caloptenus femur -rub nun, and Cicada tlhicen," concludes that the 

 tetrads are formed by segmentation of the spireme thread into parts 

 equal in number to the chromosomes of the spermatogonia. These 

 subsequently unite by twos, and, by a concentration of the chromatic 

 substance at the ends of the two rods, produce the tetrads. As each 

 element of the tetrad is considered a chromosome, no form of division 

 is possible except a cross-division separating whole chromosomes. 

 This fact is recogniz("d by Wilcox, when he states that, "According 

 to my interpretation of the Vierergruppen in Caloptemis, the formula 



alb 



would be c I d • Both the divisions following the formation of a 

 Vierergruppe would therefore be reduction divisions. ... If my 

 description of the ring formation be accurate, there may be two re- 

 ductions." 



I have examined numerous species of Melanoplus, including /e?/iw;'- 

 ruhrum, and I have never found anything to support the statements 

 of Wilcox. I am convinced that he is mistaken in his view of tetrad 

 formation. 



His error arises principally in interpreting the segments formed 

 from the spireme thread. In all the material that I have studied 

 there exists an indisputable longitudinal splitting of the chromatic 

 thread. This may be traced down through the rings and similar 

 structures into the chromosomes themselves. This fact has escaped 

 Wilcox's observation, for in his later paper (20) he reiterates his be- 

 lief in the absence of any longitudinal division. 



Having convinced himself of this fact, he proceeds to explain the 

 origin of the tetrads by means of repeated cross-divisions of the 



* It has not been my purpose, in this paper, to carry a comparison of iny results mucli be- 

 yond the work done upon insect material. I cannot, however, avoid a brief reference to the 

 articles of some investigators where the conclusions agree so closely with my own. I would 

 mention especially tuose of Moore (19) upon the Elasmobranch spermatogenesis. Griffin (25) 

 upon Thahtxxenia and Zirphd'ii, and Atkinson (24) upon Ariswrnn and Trillium. The close 

 concordance of these results upon materials derived from so widely different forms speaks inos-t 

 strongly for the existence of a type form of division which, I believe, will be exemplified more 

 and more widely as different species of animals and plants are studied. 



Th(^ clear and accurate conclusions of Griffin receive so strong a confirmation from the ap- 

 pearances noted in Hippisciii as to make a reasonable doubt of their correctness almost impos- 

 sible. Owing to the favorable character of the early prophase stages and the progressive 

 ciianges of the different metaphase chromosomes in Hippi.ycuK, any uncertainty that might 

 have attached to Griffin's conclusions regarding the sequence of the longitudinal and cross- 

 division is disiiellcd. 



Besides these instances where the observer has clearly understood and definitely remarked 

 the character of the maturation divisions, there are other cases where the figures presented by 

 different authors would indicate the existence of a modified form of this type division, although 

 their interpretations of them lead to different conclusions. 



