MICRODISSECTION STUDIES. 



343 



These furrows then disappear, to reappear again after a short 

 interval. This may occur several times until the egg finally re- 

 verts to a spherical shape and remains so. In. stage / the ger- 

 minal vesicle has disappeared except for the definitive egg nu- 

 cleus. Of such eggs any non-nucleated portion down to a cer- 

 tain size is capable of being fertilized and undergoing cleavage. 



The above experiments lead one to infer the existence of a sub- 

 stance in the germinal vesicle which, on dissolution of the nuclear 

 membrane, diffuses throughout the cytoplasm. The fertilizability 

 of any egg fragment apparently depends upon the extent of dif- 

 fusion of this substance. An egg fragment taken when a minimum 

 amount of this substance has diffused into it will allow the sperm 

 nucleus which has entered into it to divide. The presence of a 

 little more of this substance will allow the fragment to undergo 

 abortive segmentation. It is not until a sufficient amount is dis- 

 tributed throughout the egg that any fragment can develop 

 properly. 



Mature eggs were now studied, and it was found that any egg 

 fragment in order to be capable of fertilization must contain a 

 portion of the original cortex. The cortex and interior of mature 

 unfertilized eggs were separated according to the method described 

 under heading 4 (Fig. 25 a and b). The endoplasmic sphere and 

 the cortical remnant were then inseminated. The fragment con- 

 sisting of the cortical remnant is readily fertilizable and undergoes 

 segmentation (Fig. 25 b and c}. The endoplasmic sphere is non- 

 fertilizable, no matter whether it contains the egg nucleus or not. 



That the protoplasm of the endoplasmic spheres has not been 

 irreparably injured in the process of flowing through a small tear 

 in the cortex is shown in the following experiment. Eggs were 

 squashed until the endoplasm protruded as lobate processes, where- 

 upon the pressure on the eggs was lifted and the extrusion allowed 

 to flow back into the egg. Such eggs are fertilizable and are capa- 

 ble of undergoing cleavage. One such case is illustrated in Fig. 26 

 where the cortex was torn in two places on squashing the egg and 

 two exovates were formed. The nucleated exovate was allowed to 

 pinch itself off. The other exovate flowed back into the remainder 

 of the egg upon insemination (Fig. 26 b and r). A fairly com- 



