PRELIMINARY GENERAL SKETCH 



191 



Hertwig and Fol. The latter {^Ji) early reached the conclusion that 

 the asters represented " centres of attraction " lying outside and 

 independent of the nucleus. Oscar Hertwig showed, in 1875, that 



Fig. 97. — Fertilization of the egg of Nereis, from sections. (X 400.) 



A. Soon after the entrance of the spermatozoon, showing the minute sperm-nucleus at cf, the 

 germinal vesicle disappearing, and the first polar mitotic figure forming. The empty spaces repre- 

 sent deutoplasm-spheres (slightly swollen by the reagents), the firm circles oil-drops. B. Sperm- 

 nucleus (cf) advancing, a minute amphiaster in front of it; first polar mitotic figure established; 

 polar concentration of the protoplasm. C. Later stage ; second polar body forming. D. The 

 polar bodies formed; conjugation of the germ-nuclei; the egg-centrosomes and asters have 

 disappeared, leaving only the sperm-amphiaster (cf. Fig. 60). 



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

 single aster that first appears near the sperrn-nucleus and accompanies 

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

 soon afterward made by Fol ('79) in the eggs of Asterias and 

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



