338 PROCEEDINGS OF THE AMERICAN ACADEMY. 



distinct chromosomes, and some of these already show plainly their tetrad 

 nature. In Figures 15 and 16 not all of the chromatin has as yet left 

 the nucleolus, several diffuse flaky masses being still spread out over its 

 surface. 



After leaving the karyosphere, the chromatin soon breaks up into a 

 number of diffusely granular segments, each representing a chromosome. 

 These segments are not at first very similar to those found in S. heros, as 

 they are not long slender chromatic filaments, but much shorter and thicker. 

 They, however, undergo the same changes (Figure 19) ; i. e., they are 

 divided first longitudinally and later crosswise. The latter division takes 

 place at the point where in the preceding telophase the spermatogonial 

 elements united. These soon come to resemble exactly the tetrads found 

 in the sister species. Many variations and distortions of the tetrad are 

 to be found, just as in S. heros, but, as in that species, they all may be 

 referred to the typical cruciform or double-V figures described by 

 Paulinier ('99) and McClung (:00). 



These quadripartite bodies arise in the following manner. In the 

 early prophase the chromatin of the cell is in the form of a number of 

 granular segments, each one of which represents one of the bivalent 

 chromosomes obtained by the end to end union in pairs of the univalent 

 chromosomes of the spermatogonium. Each of these segments splits 

 longitudinally throughout its entire length, thus forming a double thread 

 of chromatin. Soon after this cleavage is effected, each half of the 

 chromosome divides near the middle of its length, — i. e., at the point at 

 which the component elements united to form the bivalent chromosome, — 

 and the adjacent ends produced by this cross division become drawn out 

 in the same direction and perpendicular to the axis of the original thread, 

 but the adjacent ends resulting from the cross division of one half of the 

 thread move in a direction opposite to that of the other half, thus form- 

 ing the cross-shaped figures so common in the spermatocyte prophase 

 (Figure 19). 



In later stages the chromosomes come together more closely, thus 

 causing the chromosome to become more compact and to lose its granular 

 appearance. By this condensation the planes of cleavage separating the 

 four component chromatids, which are quite pronounced in earlier stages 

 (Figure 19), become so masked that they can no longer be distinguished. 

 The chromosomes of this stage — the late prophase — have typically the 

 four-lobed appearance represented in Figure 20 ; but some of them may 

 be so distorted as to show little evidence of relationship with the tetrads 

 of an earlier stage. 



