458 WESLEY R. COE, 



have moved nearer together but rather that the ends of the membrane 

 have been separated. Indeed, the asters have already begun to move 

 apart, but the nucleus elongates at a corresponding rate so that the 

 asters are always at its ends. 



Meanwhile the chromatic network of the cleavage-nucleus has 

 become rearranged and broken up into a definite number of slender, 

 rod-like chromosomes. Of these there are, so far as I can make out, 

 thirty-two both in C. marginatus and in Micrura caeca, which is 

 twice the number found in the polar spindles. As has been described 

 for other animals, a portion of the chromatic network remains after 

 the formation of the chromosomes in the shape of small, irregular 

 granules. These gradually lose their staining power, and are evidently 

 absorbed by the cytoplasm. 



As the asters at the ends of the nucleus increase in size, certain 

 of the fibres from opposite asters meet in the centre of the nucleus 

 to form the cleavage-spindle (Fig. 34). There is no doubt that the 

 spindle-fibres are derived from the linin-network of the nucleus, just 

 as the other fibres of the asters are formed from the cytoplasmic 

 reticulum. 



The nuclear membrane soon disappears, and the chromosomes are 

 arranged in the equator of the spindle. These chromosomes are very 

 much more slender than those seen in the polar spindles. The asters 

 move apart as they increase in size so that the spindle becomes much 

 more elongated, as is shown in Figs. 34 and 35. The rod-like chromo- 

 somes are split lengthwise, much as in Echinoderms and other 

 animals, and the resulting segments move from the equatorial plate 

 of the spindle towards the asters (Figs. 35, 36). 



During the growth of the cleavage asters, the centrosomes like- 

 wise increase slightly in size though they remain very minute in all 

 stages. Each centrosome soon becomes surrounded with a delicate, 

 homogeneous layer of protoplasm , or centrosphere (Figs. 31 — 33). 

 During the metaphase the centrosphere continues to increase in size 

 until it becomes more than half the diameter of the spindle ''■). In 

 its growth the centrosphere encroaches to some extent upon the region 

 previously occupied by the inner ends of the aster-fibres. This leads 



1) I have no doubt that the centrospheres appear somewhat larger 

 in the sections and, consequently, in the drawings than they actually 

 are in the living egg, but I do not believe that they are swollen very 

 considerably by the reagents. 



