ORTHOPTERAN SPERMATOGENESIS 701 



definitely in many cases. The problem before us therefore, 

 according to the terms of the argument set down, is to analyse 

 the associated spermatogonia! derivatives and to determine their 

 relations as they prepare for, and undergo, division in the first 

 spermatocyte. 



In comparing the figures of the different investigators a strik- 

 ing uniformity in the outlines of the chromosomes is apparent. 

 Also in the written descriptions there is always mention of the 

 occurrence of chromosomes in the form of rods, crosses, rings, 

 V's, X's, U's, etc. The practical uniformity of chromosome 

 structure throughout the group seems to exist beyond question. 

 Variation inheres therefore in the interpretations and not in the 

 structures themselves. 



It will suffice for the present comparison to begin a considera- 

 tion of tetrad formation with the stage wherein the chromo- 

 somes have taken on their definitive condition and yet show their 

 constituent parts. For convenience of discussion also the differ- 

 ent chromosome forms will be taken up individually and the 

 opinions of various authors regarding them compared. 



1. Rod-shaped tetrads. Of all those appearing at this time the 

 rod-shaped chromosome is the most simple, and, according to 

 my view, is the fundamental type. It seems without doubt to 

 be constituted of two spermatogonial chromosomes united end 

 to end with their longitudinal clefts coincident and with their 

 points of contact indicated by more or less divergence of their 

 chromatids. This interpretation is given to the structure by 

 Sutton, Montgomery, Davis, Buchner, Robertson, Pinney, 

 Nowlin, Stevens and Wassilieff. 



So far as the actual composition of the rod is concerned it is 

 of no consequence whether there has been an earlier parasynapsis 

 or whether, later, there is pre- or post-reductuon. Should para- 

 synapsis occur, however, there is a possibility that the plane 

 of separation may be along the longitudinal cleft of the homol- 

 ogous elements instead of through the space between them. 

 Extended rods in the metaphase would then be almost completely 

 divided longitudinally at this time. Involved in this is also the 



