UNION OF THE GERM-CELLS 



147 



as in echinoderms, mammals, many annelids, etc., while in others 

 several may enter (insects, elasmobranchs, reptiles, the earthworm, 

 Petromyzon, etc.). In the former case more than one spermatozoon 

 may accidentally enter (pathological polyspermy), but development 

 is then always abnormal. In such cases each sperm-centrosome 

 gives rise to an amphiaster, and the asters may then unite to form 

 the most complex polyasters, the nodes of which are formed by the 



Fig- 75- Pathological polyspermy. 



A. Polyspermy in the egg of Ascaris ; below, the egg-nucleus ; above, three entire spermatozoa 

 within the egg. [SALA.] 



D. Polyspermy in sea-urchin egg treated with 0.005% nicotine-solution; ten sperm-nuclei 

 shown, three of which have conjugated with the egg-nucleus. C. Later stage of an egg similarly 

 treated, showing polyasters formed by union of the sperm-amphiasters. [O. and R. HERTW1G.] 



centrosomes (Fig. 75). Such eggs either do not divide at all or 

 undergo an irregular multiple cleavage and soon perish. If, how- 

 ever, only two spermatozoa enter, the egg may develop for a time. 

 Thus Driesch has determined the interesting fact, which I have con- 

 firmed, that sea-urchin eggs into which two spermatozoa have acci- 

 dentally entered undergo a double cleavage, dividing into four at the 

 first cleavage, and forming eight instead of four micromeres at the 



