1 88 FERTILIZATION OF THE OVUM 



nomena in the sea-urchin Toxopneiistes (Fig. 94). As described at 

 page 197, the tail is in this case left outside, and only the head and 

 middle-piece enter the egg. Within a few minutes after its entrance, 

 and while still very near the periphery, the lance-shaped sperm-head, 

 carrying the middle-piece at its base, rotates through nearly or quite 

 180°, so that the pointed end is directed outward and the middle- 

 piece is turned inward (Fig. 94, A-F)} During or shortly after the 

 rotation appears a minute aster centring in or very near the middle- 

 piece. As it enlarges, the middle-piece itself is thrown to one side 

 (Fig. 12), where it soon degenerates, while in the centre of the aster 

 a minute intensely staining centrosome may be seen. Both sperm- 

 nucleus and aster now rapidly advance toward the centre of the egg, 

 the aster leading the way and its rays extending far out into the 

 cytoplasm and finally traversing nearly an entire hemisphere. The 

 central mass of the aster comes in contact wdth the egg-nucleus, 

 divides into two, and the daughter-asters pass to opposite poles of 

 the egg-nucleus, while the sperm-nucleus flattens against the latter 

 and assumes the form of a biconvex lens (Fig. 95). The nuclei now 

 fuse to form the cleavage-nucleus. Shortly afterward the nuclear 

 membrane fades away, a spindle is developed between the asters, and 

 a group of chromosomes arises from the cleavage-nucleus. These 

 are 36 or 38 in number ; and although their relation to the paternal 

 and maternal chromatin cannot in this case be accurately traced, 

 owing to the apparent fusion of the nuclei, there can be no doubt on 

 general grounds that one-half have been derived from each germ- 

 nucleus. The ^g^ then divides into two, four, etc., by ordinary 

 mitosis (Figs. 4, 52). 



In the type of fertilization just described, the polar bodies are 

 formed long before the entrance of the spermatozoon and the germ- 

 nuclei conjugate immediately upon entrance of the spermatozoon, 

 fusing to form a true cleavage-nucleus. In a second and more 

 frequent type {Ascaris, Fig. 90; Physa, Fig. 89; Nereis, Fig. 97; 

 Cyclops, Fig. 98) the sperm-nucleus penetrates for a certain distance, 

 often to the centre of the ^gg, and then pauses while the polar 

 bodies are formed. It then conjugates with the re-formed egg- 

 nucleus. In this case the sperm-aster always divides to form an 

 amphiaster before conjugation of the nuclei, while in the first case 

 the aster may be still undivided at the time of union. This 

 difference is doubtless due merely to a difference in the time 

 elapsing between entrance of the spermatozoon and conjugation 

 of the nuclei, the amphiaster having, in the second case, time to 



1 The first, as far as I know, to observe the rotation of the sperm-head was Flemming 

 in the echinoderm-egg ('8i, pp. 17-19). It has since been clearly observed in several other 

 cases, and is probably a phenomenon of very general occurrence. 



