blackman: the spermatogenesis of scolopexdra. 



43 



already noted as characteristic of the ordinary chromosomes during the 

 first mitosis. The behavior of the other elements is quite different. 

 These are arranged with their long axes parallel to that of the spindle, 

 the separation of the chromatids occurring along the equatorial plane 

 at the place of constriction. This very evidently accomplishes a cros8 

 division of the chromosome. 



At the beginning of the anaphase the cell becomes lengthened along the 

 axis of the spindle. This elongation is useful in two ways. It aids in 



l 



Fig. F. 



Fig. D. 



Fig. E. 



Fig. D. Metaphase of second spermatocyte. The difference in shape and orien- 

 tation existing between the accessory and the other chromosomes is evident. 

 X960. 



Fig. E. High magnification of same stage, showing the differences between the 

 accessory chromosome and the other chromosomes, in the relations of chromatids 

 and in the attachment of mantle fibres. X 1.920. 



Fig. F. Slightly later stage ; showing the effect of the contraction of the mantle 

 fibres on the orientation of the accessory chromosome. X 1,920. 



the division of the chromosomes, and is also of great advantage in the 

 later division of the cytosome. During the early stages of chromatin 

 division the mantle fibres apparently do not shorten at all. The pulling 

 apart of the chromosomes is accomplished by the separation of the cen- 

 trosomes and the elongation of the cell, while the relative position of cen- 

 trosome and chromatid remains about the same (Figs. 43, 44). Later, 



