The life history of Diplodiscus temporatus Stafford. 631 



mitotic figure shows several points of difference from a common 

 segmentation mitosis. In some instances (Fig-. 37) the spindle extends 

 through nearly the entire length of the nucleus. In most instances, 

 however, the mitotic figure occupies only a comparatively small 

 part of the nucleus (Fig. 36). 



The chromosomes in the maturation division are verj- different 

 in appearence from those seen in the segmentation stages. On 

 comparing Fig. 35, where the chromosomes are arranged in an 

 equatorial plate for the maturation division, with Fig. 45, a section 

 through the plate of the first segmentation division, the difference 

 in the size and shape of the two sorts of chromosomes is strikinglj'- 

 shown. In any of the maturation divisions the size of the chromosomes 

 is scarcely more than one half that of those present in a segmen- 

 tation division. It should, however, be pointed out. when such a 

 comparison is made, that in segmentation mitoses there are great 

 differences in the size, and to a lesser extent in the shape, in the 

 chromosomes found in different nuclei, even those within the same 

 germ ball. 



In the shape of the chromosomes during maturation there is as 

 striking a difference as was noted in their size. In a polar view 

 of the equatorial plate of a maturation mitosis (Fig. 35). the 

 chromosomes are nearly circular in outline. In a side view of the 

 spindle (Fig. 34), they are somewhat elongated in the plane of the 

 plate; but this elongation is very slight when compared with the 

 chromosomes in a segmentation mitosis as is shown in Figs. .2(5 

 and 45. 



The achromatic part of the mitotic figure is made up of finer 

 fibres in the maturation than in the segmentation division. In some 

 eggs it can be only imperfectly made out. This may in part be 

 due to the fact that the contents of both the egg cell and its 

 nucleus are denser than are those of the segmentation cells of the 

 embryo. 



The centrosomes alone, of all the structures within the nucleus, 

 appear during mitosis to be of the same size and character in both 

 the maturing eggs and in the segmenting cells of the embryo. The 

 location of one centrosome within the nucleus at the time of the 

 maturation division is often some distance from the corresponding 

 end of the latter; but otherwise there is apparently, so far as 

 concerns the centrosomes, no difference between the two types of 

 division. 



