MICRODISSECTION STUDIES. 319 



immature egg is a hyaline sphere containing a sharply differentiated 

 nucleolus and occupying about one fifth the volume of the egg. 

 With the microdissection needle the vesicle may be moved 

 about in the fluid cytoplasm without injury to the egg. With 

 the needle one may considerably indent the surface of the 

 vesicle. On removal of the needle the vesicle reverts again to the 

 spherical shape (Fig. i). The vesicle possesses a morphologically 

 definite surface membrane inclosing an optically homogeneous 

 liquid (cf. Chambers, 'i8 b ). Within this liquid lies a visible body, 

 the nucleolus. By agitating the vesicle the nucleolus may be made 

 to occupy any position within the nuclear fluid. The nuclear mem- 

 brane is very easily injured. If, however, a microneedle be care- 

 fully inserted into the nucleus, the membrane about the puncture 

 adheres to the body of the needle and the tip of the needle may 

 push the nucleolus about with no apparent injury. The existence 

 of considerable tension in the nuclear membrane is shown in the 

 following experiment. An egg was cut into three fragments in 

 such a way that the surface film forming over the cut surfaces of 

 the middle fragment pressed upon the nucleus, deforming it con- 

 siderably (Fig. 2). The attempt of the nucleus to return to a 

 spherical shape bulged out one end of the egg fragment until it 

 was constricted off from the remainder of the fragment (Fig. 



2b-f). 



Tearing the nuclear membrane in most cases results in a de- 

 struction of the nucleus. In a few cases it was possible to produce 

 a slight rupture with no noticeable injurious effects. Such a case 

 is recorded in Fig. 3. At 10:44 A.M. undue pressure on the 

 germinal vesicle when cutting an immature egg in two resulted in 

 its rupture followed by a lobular extrusion bounded by a very 

 delicate film. During the following ten minutes the vesicle began 

 slowly to revert to its original shape (Fig. 3^ and c). Before that 

 was attained the maturation process began and, at 10:55, the out- 

 line of the vesicle had disappeared. The nucleated egg fragment 

 maturated normally and five hours and a half after insemination it 

 had segmented in two. At 8:40 P.M. it had developed into a 

 swimming blastula. 



The cytoplasm of the egg allows of considerable tearing without 



