152 EZRA ALLEN 



later from the nucleolus by its oval rather than spherical form, 

 and its somewhat lighter staining reaction. In cells of the 

 pachytene stage, it appears as an unevenly bipartite body, the 

 larger part staining more heavily than the smaller usually, what- 

 ever nuclear stain may be employed (figs. 1 and 53). Its posi- 

 tion close to the nuclear wall, isolated from the other chromosomes 

 by a well-niarked space, is another distinguishing characteristic. 



During the early tetrad stages of the euchromosomes, the 

 accessory maintains much the character just described. It 

 gradually assumes the form shown in figure 27, a woolly,, elon- 

 gated, curved, rod-like chromosome showing a constriction near 

 the middle. It does not stain quite as densely as the other 

 chromosomes. During this whole period I have not been able 

 to see a longitudinal split. When it appears on the spindle, how- 

 ever, the split is manifest when viewed en face or on end (figs. 

 18, 19, and 54). At this time it is readily distinguishable from 

 the other chromosomes by its rod form, its lighter staining reac- 

 tion, and its position on the spindle, even when not favorably 

 located or stained for observing its diad natiire (figs. 18, 19, 20, 

 21, 54, 55, 56). 



In the first maturation division this ehromosome does not divide 

 (figs. 21 and 22). Figure 21 is from a poorly fixed specimen. 

 The view shown in figure 22 was seen many times. Under favor- 

 able circumstances this chromosome can be seen in very late 

 anaphase or early telophase maintaining its characteristic posi- 

 tion, i.e., extending to one side like a curved finger. Its appar- 

 ently partial division transversely is a distinguishing mark. 



Its failure to divide in the first maturation division produces 

 daughter cells of 18 and 19 chromosomes, respectively, and thus 

 estabUshes a dimorphism of the spermatozoa. 



Cells in interkinesis show frequently a narrow U-shaped 

 chromosome as illustrated in figure 26. This may be the acces- 

 sory. It should occur in one-half the cells of this stage, but its 

 identification is questionable unless viewed en face, as all the 

 chromosomes have assumed the rod form in this stage. It fre- 

 quently seems to possess the partial transverse division in each 



