PRELIMINARY GENERAL SKETCH 



191 



Hertwig and Fol. The latter ('73) early reached the conclusion that 

 the asters represented " centres of attraction " lying outside and 

 independent of the nucleus. Oscar Hertwig showed, in 1^75, that 



Fig 97. — Fertilization of the egg of Nereis, from sections. \ a 400.) 

 A. Soon after the entrance of the spermatozoon, showing the minute '>:^'''^:';^^^^J^:^'^l 

 germinal vesicle disappearing, and the first polar mitotic figure forming ^ '^^^.^^'^ 'P*)^'' '^/'^ 

 fent deutoplasm-spheres (slightly swollen by the reagents), the firm circles o Wrop^ J^,\^^^Z 

 nucleus (J) advancing, a minute amphiaster in front of it; first polar '"'^ .^/.f;^ . "''^'^j ^'^I^ ' 

 polar concentration of the protoplasm. C. Later stage; second polai '^;^'^, '*':" "^^.^.^jllve 

 polar bodies formed; conjugation of the germ-nuclei; the egg-ccn.rosomes and asters have 

 disappeared, leaving only the sperm-amphiaster {cf. tig. 60). 



in the sea-urchin egg, the amphiaster arises by the division of a 

 single aster that first appears near the sperm-nucleus and accom,ianies 

 it in its progress toward the egg-nucleus. A similar observation was 

 soon afterward made by Fol ('79) i" the eggs of Asl.nas and 

 Sagiita, and in the latter case he determined the fact that the astral 



