PRELIMIXARY 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 1875, that 



Fig. Qij. — Fertilization of the egg of Nereis, from sections. ( x 400.) 

 A. Soon after the entrance of the spermatozoon, showing the minute sperm-nucleus at <3 , 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 (c?) advancing, a minute amphiaster in front of if, first polar mitotic figure established; 

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

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

 disappeared, leaving only the sperm-amphiaster {cf. F"ig. 60). 



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

 single aster that first appears near the sperm-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 Astcnas and 

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



