66 bulletin: museum of comparative zoology. 



Efforts were first directed to a study of the postspireme stages in 

 the hope of discovering how the segments of the pachytene spireme 

 became the tetrads exhibiting the shapes of V's, X's, 8's, crosses and 

 rings. Such a variety of shapes and forms presented themselves at 

 any one of the tetrad stages, however, that it was impossible to decide 

 which were the more primiti^'e and which the derived forms. Figure 

 38, a-g (Plate 3); for example, shows some of the different shapes of 

 tetrads seen in a single stage and, indeed, in the same cyst. The only 

 method that seemed to offer a means of securing decisive evidence on 

 the problem was that of following the history of individual chromo- 

 some-pairs through a large number of stages. For this purpose it was 

 necessary to find pairs which possessed individual characteristics by 

 which they could be recognized in all the stages concerned. Fortu- 

 nately, at least three pairs were found which fulfilled these require- 

 ments. For convenience in description they have been designated 

 'M," "5," and "C." 



1. Chromosome-pair A. — This element was first distinguished in 

 the pachytene stage, where it is a very deeply stained spireme segment. 

 Examples of it are shown in figures 56 and 57, (Plate 5). Its differen- 

 tial staining property is so marked and constant that it can be recog- 

 nized by this character alone up to the later postspireme stages. But 

 there is an additional means of identification. Like most of the pachy- 

 tene threads, this one normally makes a loop the two ends of which 

 approach to, or attach at, the proximal pole of the nucleus (Plate 5, 

 fig. 56). One or both ends may become free from entanglements, but 

 more frequently only one. In the latter case the free end, or if both 

 ends are free, one of them, is nearly always terminated by two knobs, 

 of which one is usually larger and less deeply stained than the other 

 (Plate 5, fig. 57.(7; P'ate 10, fig. 113). These knobs, I believe, may be 

 identified as the polar granules described by Miss Pinney ('08). But 

 in this instance, as shown by numerous observations, the more promi- 

 nent granules occur at the distal end of the chromosome instead of 

 the proximal end, where they are found on the majority of the other 

 chromosomes. That the expanded condition of one of the granules 

 furnishes a means of identification, will be apparent from an examina- 

 tion of figure 62 (Plate 6). 



As an exceptional occurrence these two terminal granules may be 

 equal in size, neither one being expanded. In order to test the relative 

 frequency of these two conditions, some counts were made and 

 tabulated for both the spireme and postspireme stages, as follows: — 



