208 WM. REES B. ROBERTSON 



ing has disappeared, probably due to an unwinding process. 

 Each chromosome consists of four parts and may be considered 

 as being split in two longitudinal planes, one at right angles to 

 the other (figs. 151, 152 a-c, 154.) This is more evident in some 

 chromosomes (fig. 149, nos. 1, 4) than in others (fig. 149, nos. 

 5, 10, 11), and usually more pronounced in later prophases (fig. 

 150) than in earlier ones (fig. 149). Figure 154 represents 

 chromosome no. 4 in five stages of the process of splitting lead- 

 ing up to the metaphase condition. 



In figure 155 the twelve chromosomes are shown in a stage 

 just preceding the breaking up of the nuclear wall. The four 

 parts and the more or less characteristic shape for each chromo- 

 some (tetrad) are now evident. No. 11, however, is not in its 

 typical form. This may be seen in figures 156 d and 156 f. Fig- 

 ures 156 a-f show a number of forms assumed by prophase chromo- 

 somes. Their form depends much upon the length of the sper- 

 matogonial rods which conjugated to produce them. Figures 

 156 a and 156 b are views of two conditions which are assumed 

 by a no. 1, 2, or 3 pair. The attraction fibers probably arise 

 at X, X, for in the metaphase of this division such chromosomes 

 are rods, frequently constricted in the middle, to whose ends the 

 attraction fibers are attached. During the preceding prophase 

 stages the chromosome has become split in one plane (fig. 156, 

 /, I) and the halves thus produced have separated at the end 

 which bears the knobs and have rotated around the opposite 

 end as a fixed center, each through an arc of 90° until the two 

 halves, still attached to each other at the end opposite the knobs, 

 form a nearly straight rod. While this separation and rotation 

 of the halves resulting from the first split is going on, each half 

 begins to show its secondary split (//, //, fig. 156b), or else has 

 already acquired it before the rotation begins. Without giving 

 my reasons here, I will simply say that I believe the first or pri- 

 mary split (7, /) produces the reductional division; i.e., separates 

 homologous spermatogonial chromosomes, and that the second 

 split {II, II) is an equational division of the now end-to-end 

 conjugated spermatogonial pair. This four-part chromosome 



