WENRICH: spermatogenesis of PHRYNOTETTIX MAGNUS. 109 



somes which were distinctive of the species from which they were 

 derived. Tennent ('08) found in hybrids between Moira and Arbacia 

 a mixture of two kinds of chromosomes, each variety of which could 

 be distinguished. It is, indeed, difficult to understand how these 

 distinctive chromosomes could recur with such definite characteristics 

 in hybrid embryos, if there is no persistent identity for them. 



Variations from the general rule of chromosomal constancy have 

 been recorded from time to time, for example, in the shapes of tetrad 

 chromosomes. In many species there is a tendency for each of the 

 forms of the tetrads to be reproduced in the first spermatocyte meta- 

 phase. This is particularly true where the chromosomes are all of a 

 similar shape and size. But even in such cases, there is a variation 

 in the exact contour presented by different chromosomes. It has 

 been made apparent by many investigators, especially by McClung 

 and his students, that the shape of a metaphase tetrad is dependent 

 upon the extent and character of the movement on each other of the 

 constituent cliromatids. The work of these authors also shows that 

 homologous chromosomes tend to assume about the same shape in all 

 the cells at corresponding stages of mitosis, but that this condition of 

 similarity has its exceptions. Baumgartner ('04) called attention to 

 the constancy in the number of rings formed among the tetrads of 

 Gryllus, and others have noted similar conditions. However, such a 

 criterion for individuality is not always a safe guide, as was pointed 

 out by Foot and Strobell ('05). Commenting on Baumgartner's 

 paper, they say, in regard to chromosomes in Allolobophora foctida: — 

 " We find no constant form differences of the chromosomes, the simplest 

 form of the bivalent chromosomes is two rods«attached end to end, and 

 these present a variety of shapes, rings, figures 8, crosses, etc., without 

 any regularity or constancy. The free ends of the bivalent chromo- 

 somes show a tendency to unite into a ring and in some cases nearly 

 all the eleven chromosomes are rings, and sometimes not a single ring 

 is formed" (footnote, p. 222). A glance at figures 39 and 40 (Plate 4) 

 of this paper will also show a variability in shape of the eleven bivalent 

 chromosomes. In my account of tetrad A, I have shown that this 

 element may or may not form rings, so that this character could not 

 be used as a criterion for identification in the earlier postspireme 

 stages. But in spite of these exceptions, there does exist in many 

 cases a strong tendency for a chromosome to assume the same shape 

 at similar stages in all the cells of an animal, and the exceptions have 

 no significance in relation to the question of a variation in the funda- 

 mental organization of the tetrads. 



