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ROBERT CHAMBERS. 



tearing its surface, the fluid contents escape, leaving behind a col- 

 lapsed membrane which disappears within 10 to 15 seconds. 



Fig. 5 shows the effect of cutting the mature egg nucleus of the 

 starfish egg. By pushing the nucleus against the inner surface of 



a b d 



O o CP O 



F-'g- -5 



FIG. 5. Effect of cutting mature nucleus of a starfish (Asterias) or sea- 

 urchin (Arbacia) egg. a, intact egg nucleus ; b, nucleus in process of being 

 cut in two. The nucleus was pushed against the periphery of the egg as it 

 was being cut by a vertical needle ; c, the separated fragments of the nucleus ; 

 d, reunion of the fragments ; e, reconstituted nucleus. 



the egg it is possible to pinch it into two pieces. Each piece 

 rounds up but, if the two are allowed to come into contact, they 

 will fuse into a single nucleus again. The same result obtains in 

 the sand-dollar and sea-urchin eggs. If, however, the nuclear 

 membrane be torn, a disintegration of the cytoplasm results 

 analogous to that produced on rupturing the germinal vesicle. 

 The extent of disintegration is much more limited, owing doubtless 

 to the much smaller amount of nuclear material present. Similar 

 results were obtained on tearing the nucleus of the Arbacia egg. 



It was found possible to destroy the cytoplasm of one egg by 

 injecting into it nuclear material obtained from another egg. This 

 experiment has to be performed very rapidly, for if the nuclear 

 material be allowed to remain longer than ten seconds within the 

 pipette it has no effect whatever when injected into the cytoplasm 

 of an egg. If it be injected within that time the destructive effect 

 is very pronounced. 



If an egg be allowed to undergo normal maturation, the ger- 

 minal vesicle disappears except for a small remnant which be- 

 comes the definite egg nucleus. This egg nucleus moves to the 

 surface of the egg, where it gives off the two polar bodies. It then 

 constitutes the female pronucleus, which remains quiescent until 

 fertilization occurs. The disappearance of the germinal vesicle is 

 a well-known phenomenon. In order, however, to locate definite 

 stages selected for my operations I introduce the following sum- 



