ROBERTSON: CHROMOSOME STUDIES IV. 277 



cases (figs. 18, 19) they are traveling to opposite poles, 

 but separating from each other much like the members of auto- 

 some pairs that have been in synapsis. 



(5) When the conjugated pairs of chromosomes (tetrads) 

 arrange themselves in the metaphase of the first spermatoc\i:e 

 division, the supernumerary body passes like an accessory, un- 

 di\ided to one pole, going either to the same pole as the access- 

 ory^ chromosome (figs. 17, 21), or to the opposite pole (figs. 18, 

 19, 20). 



(6) The supernumerary' appears in one-half of the second 

 spermatocAi:es either with the accessory or without it (figs. 

 22 to 27). 



(7) It di\ides in this division as does the accessory chromo- 

 some (figs. 26, 27). One-half the spermatids get it and one- 

 half do not. Four sorts of spermatozoa accordingly are 

 formed. 



From the characteristics here given, it is e\ddent that 

 this body is probably an accessory chromosome. If it oc- 

 curred in these cells in the absence of the normal accessory 

 (No. o.v) it certainly would be taken for that body. It fills 

 all the specifications for the accesson* of Tettigidea except 

 size. An idea of the difference in size may be gotten by com- 

 paring the lengths of the two chromosomes in figures 8, and 15 

 to 21. The proximal end, from which the spindle fiber springs, 

 seems to be normal, but the distal end appears somewhat ab- 

 normal (figs. 14a. 15, 17, 18). It seems likely that this chro- 

 mosome is a descendant of an X-chromosome which at some 

 previous polar body formation failed to separate properly from 

 its mate, losing thereby a portion from its distal end. An 

 incomplete daughter chromosome such as this might result. 

 In this connection it may be said that it behaves verj^ much 

 like the deficient No. 4 chromosome described in Studies III 

 (Robertson. 1915), whose peculiar structure and behavior was 

 accounted for in this way. 



In the literature on supernumeraries, the nearest approach 

 to this chromosome are the cases described by Stevens in 

 Diabrotica soror and D. 12-punctata (1908 b, 1912 a) and in 

 Ceuthophilus (1912 b) . Normally in these species there is a 

 large unpaired heterochromosome (accessory) in the male 

 and two such in the female similar to what occurs in Orthop- 

 tera. Stevens found about 50 per cent of the males of the 



