462 WESLEY E. COE, 



Wilson finds in Toxopneustes that the cleavage-plane almost 

 always passes very nearly through the point at which the spermato- 

 zoon entered the egg. This is often approximately true of Cerehra- 

 tulus for the very reason that the spermatozoon usually enters that 

 part of the egg lying farthest from the region of the polar-bodies. 



Let us now turn back and follow the course of the cleavage- 

 spindle itself. In the anaphase, the fibres of the spindle which pre- 

 viously passed directly from one centrosphere to the other become 

 thickened at the equator, acquire in that region a distinct affinity for 

 chromatin-stains, and eventually break up into small granules. The 

 mantle fibres from opposite poles cross extensively at the equator 

 during the early anaphase, but later they do not cross at all. The 

 appearance is the same as if their extremities were mutually repellant, 

 for, on approaching the equatorial region, they bend outwards in all 

 directions towards the periphery of the cell, as is shown in Figs. 3b, 

 39. In a section of an egg at this stage these fibres are seen in 

 cross-section in the equatorial region of the spindle, and appear as 

 an equal number of darkly-stained granules. As the cell elongates 

 in anticipation of cleavage, the spindle becomes more elongated and 

 at the same much less definitely marked off from the surrounding 

 cytoplasm. Its fibres become more irregular and spread out loosely 

 over a broad area (Fig. 39). They gradually break up into rows of 

 small granules and then disappear. In the final stages of the cleavage, 

 the few fibres which have not disappeared are brought together into 

 a small bundle. The thickening of these fibres in their middle portions 

 gives rise to a small "Zwischenkörper", from which the ends of the 

 fibres may be seen passing into each of the two cells (Fig. 40). The 

 "Zwischenkörper" is much smaller than in many other animals, and 

 soon disappears completely. 



After the completion of the first cleavage, the primary aster- 

 fibres still extend, as irregular row^s of granules, nearly to the peri- 

 phery of each of the two cells. These degenerating fibres become 

 fainter and fainter, until they can no longer be distinguished from the 

 general reticulum. The secondary asters increase rapidly in size, and 

 separate to form the second cleavage-spindle. 



Centrosoine and Centrosphere. 



A rapidly growing aster in its early stages consists merely of 

 a few delicate fibres radiating about a minute central granule, or 



