WENRICH: spermatogenesis of PHRYNOTETTIX MAGNUS. 105 



As to which of the two maturation divisions is equational and which 

 is reductional, no absohite rule can be laid down. The evidence, 

 however, points to the probability that generally chromosomes with 

 terminal spindle-fiber attachment are not separated from each other 

 until the second division, while those that have a non-terminal at- 

 tachment are separated in the first, and that consequently in the 

 former the reduction occurs at the second division, in the latter at the 

 first division. 



B. Individuality. 



The theory of individuality was early championed by Van Beneden 

 ('83), Rabl ('85), and Boveri ('88). In more recent years the theory 

 has been supported by many wi'iters, who have accepted as substan- 

 tial evidence in its favor the constancy in the number, size, and 

 shape of the chromosomes reappearing in the mitotic spindle of any 

 one species of animal or plant. On the other hand, some eminent 

 zoologists have attacked the theory on the ground that the individual 

 chromosomes cannot be traced through the so-called "rest" period 

 between mitoses. It will, therefore, be convenient to discuss the two 

 topics: — (a) constancy in metaphase chromosomes, and (b) persistent 

 organization of chromosomes. 



a. Constajici/ of Metaphase Chromosomes. 



1. Constancy in number. — The constancy in number of chromo- 

 somes for any species is among the most commonplace of cytological 

 observations. It will therefore be unnecessary to make any exten- 

 sive references to the literature. Some exceptions to the general 

 rule occur, however, and should receive attention. Supernumerary 

 chromosomes have been reported from time to time, and have been 

 studied especially by Stevens and Wilson. Wilson ('09) found in 

 Metapodius variations in chromosome-number from 21 to 26, though 

 the number for each individual animal was constant. The number of 

 chromosomes was dependent neither on sex, nor locality of habitat, 

 nor w^as it correlated with constant differences of size or of visible 

 structures in the adults. But the variation affects only particular 

 classes of chromosomes (the small idiochromosomes) and all exhibit 

 the same behavior. Furthermore, Wilson found a few cases of mitoses 

 in which both members of a pair of small chromosome were going to 



