282 W. G. VanName — Embryology of Eustylochus. 



only the portion left colored after a partial extraction of the stain, 

 the iron-haematoxylin method may evidently mislead us as to the 

 actual size of this organ. In the present case the deeply stained 

 center of the aster may be reduced by continued extraction with 

 the alum solution from a body apparently occupying the whole 

 centrosphere, down to one so minute as to be scarcely visible with 

 the highest powers of the microscope, or it may be entirely dissolved 

 away. 



The enormous centrosomes, often unsuri'ounded by any indication 

 of a centrosphere, which appear in some of the illustrations of 

 Klinckowstrom and Francotte, are probably due to the whole centro- 

 sphere being stained. 



Figs. 12 and 13 are later stages of the spindle, showing the 

 separation of the derived chromosomes and the formation of the 

 polar body. The divergence of the centrosomes of the inner pole 

 in preparation for the second polar spindle may already be noticed. 

 The rapidity with which they move apart, or the time when they 

 start to do so, is not dependent upon the rate of separation of the 

 chromosomes. This will readily be seen by comparing the progress 

 made in these processes in the eggs shown in Figs. 12 and 13. Often 

 as the centrosomes separate a constriction appears in the outline of 

 the centrosphere between the centrosomes (Fig. 12). This appears 

 to happen only when the latter move apart at an early stage. If 

 their separation does not proceed far before the first polar body is 

 separated the constriction does not appear. The centrosphere simply 

 elongates and the new spindle forms as described below. 



The segments resulting from the division of each chromosome 

 remain connected by distinct and rather thick fibers even after they 

 have become widely separated (Figs. 12 and 13). In the late phases 

 they are generally closely grouped together. In the final stage of 

 the spindle no true cell-plate is found, although such a structure is 

 often quite noticeable in the second polar spindle. This was also 

 noted by Wheeler (30) in Mysostotna and by Coe (1) in Cerehratulus. 



The Number- of Chromosomes. 



In Eustylochxis ellipticus the number of chromosomes in the polar 

 spindles is ten, and therefore twenty in the cleavage spindles, though 

 they are then much more difticult to count. I have determined the 

 number to be the same in Planocera nebxdosa with almost as great 

 a degree of certainty. 



