138 



FERTILIZATION OF THE OVUM 



central point a minute intensely staining centrosome may be seen. 1 

 As the sperm-nucleus advances, the aster leads the way, and at the 

 same time rapidly grows, its rays extending far out into the cytoplasm 

 and finally traversing nearly an entire hemisphere of the egg. The 

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

 vides 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. 70). The nuclei now fuse 

 to form the cleavage-nucleus. Shortly afterwards the nuclear mem- 

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



* 







C . 



Fig. 69. Entrance and rotation of the sperm-head and formation of the sperm-aster in the 

 sea-urchin Toxopneustes (A.-F., X 1600; G, //., X 800). 



A. Sperm-head before entrance; , nucleus; m, middle-piece and part of the flajellum. 

 B. C. Immediately after entrance, showing entrance-cone. D.-F. Rotation of the sperm-head, 

 formation of the sperm-aster about the middle-piece (the minute centrosome not shown). 

 G. H. Approach of the germ-nuclei ; growth of the aster. 



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. Throughout these changes no trace of an egg-centrosome 

 is to be discovered. This centrosome, though present in earlier stages, 

 has been lost after the polar bodies were formed by the ovarian egg. 



1 I was unable to find such a centrosome in Toxopneustes, but the observations of Boveri 

 and Hill prove that it is certainly present in other sea-urchins, and I now believe my own 

 account to have been at fault in this respect. 



