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 chromatids. 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 foetida: — 
“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. 
