UNION OF THE GERM-CELLS 



199 



"merocytes" or accessory nuclei. The fate of the latter is still in 

 doubt ; but they certainly take no part in fertilization. 



It is an interesting question how the entrance of supernumerary 

 spermatozoa is prevented in normal monospermic fertilization. In 

 the case of echinoderm-eggs Fol advanced the view that this is 

 mechanically effected by means of the vitelline membrane formed 

 instantly after the first spermatozoon touches the egg. This is indi- 

 cated by the following facts. Immature eggs, before the formation 



Fig. 101. Pathological polyspermy. 



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

 within the egg. [SALA.] 



B. 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. HERTWIG.] 



of the polar bodies, have no power to form a vitelline membrane,, 

 and the spermatozoa always enter them in considerable numbers. 

 Polyspermy also takes place, as O. and R. Hertwig's beautiful ex- 

 periments showed ('87), in ripe eggs whose vitality has been dimin- 

 ished by the action of dilute poisons, su.ch as nicotine, strychnine, 

 and morphine, or by subjection to an abnormally high temperature 



