SPERMATOGENESIS IN THE ALBINO RAT 145 



members of the complex show their composition also in the 

 metaphase, after the chromatin has become homogeneous or 

 nearly so. The crosses differentiate their four chromatids in 

 prophase (fig. 29) and in metaphase (fig. 57). The small rings 

 of the complex are not large enough to decipher minutely, but 

 judging from their appearance as outlined in figure 27, we may 

 conclude that they are not exceptional in organization. 



Further proof of the tetrad composition is found in the ana- 

 phase forms. These are simple Vs. They are shown in figures 

 22, 41, and 42. The large ones are readily seen. The smallest 

 are obscure, but their V outline is often discernible, as shown in 

 figure 22. 



The history of chromosome B is illustrated in figures 34 to 42. 

 The account must begin with diakinesis, as so far I have beeii 

 unable to trace any pair of chromosomes through the synaptic 

 period. The critical point in diakinesis is of course the observa- 

 tion of the longitudinal split in the original pair of chromosomes. 

 That- this split has occurred is shown in figure 34. The same 

 figure shows also the arrangement of the chromatids in the tetrad^ 

 or the architecture of the tetrad. It is evident that there is no 

 crossing of the chromatids. While this tetrad does not show 

 throughout its extent this longitudinal split, the arrangement of 

 the chromatids is sufficiently seen to demonstrate, from what we 

 know of this same form of chromosome in forms more favor- 

 able for study (the Orthoptera, for example), that the tetrad 

 formation has been accomplished. The two portions of the 

 double ring lie at right angles to each other. The side of the 

 larger is divided and each member of this division passes to 

 opposite sides of the smaller ring. This constitution is main- 

 tained throughout the first spermatocycte phase, as seen in 

 figures 35, 36, 37, 18(A), 40, and 51. The architecture is shown 

 diagrammatically in figures 43 to 45, taken from McClung ('14). 

 The knobs in figure 35 represent the fused ends of the chromatids. 

 The elliptical ring appearing in this figure is formed of two chro- 

 matids with their ends thus fused. The mirror image of this ring; 

 lies behind, one end seen at the right. A side view is given in 

 figure 36, which shows that the fused ends have diverged in pairs,. 



JOURNAL OF MORPHOLOGY, VOL. 31, NO. 1 



