Surface Layer in the Living Egg Cell. 



43 



not uniform can be demonstrated by the following experiment on 

 the starfish egg: If the surface of a mature unfertilized egg be 

 torn while the egg is kept under compression almost all of the 

 internal cytoplasm may be made to flow out to form a spherule of 

 cytoplasm which pinches off from the rest of the egg. What is left 

 behind is a collapsed remnant consisting mainly of protoplasm 

 which originally enveloped the egg. This remnant consisting 

 largely of the more solid ectoplasm tends only slowly to round up. 

 The extruded mass, which is very fluid, immediately assumes a 

 shape approximating that of a sphere. This may be termed an 

 endoplasmic sphere. The remnant containing the original ecto- 

 plasmic substance of the egg is readily fertilizable and undergoes 

 segmentation. The endoplasmic sphere is unfertilizable. If, on 

 the other hand, the endoplasmic sphere remains for some time 

 connected by means of a bridge of protoplasm with the remnant 

 containing the original ectoplasmic substance it is fertilizable. 

 The ability of the endoplasmic sphere to approximate normal 

 conditions of segmentation is a function of the length of time that 

 it remains in organic continuity with the original ectoplasmic 

 mass. Possibly there exists a substance necessary for develop- 

 ment which normally accumulates in the surface layer of an egg. 

 This substance is diffusible and will distribute itself over new 

 protoplasmic surfaces. If a bridge of protoplasm connects the 

 ectoplasmic remnant with the endoplasmic sphere this substance 

 will diffuse into the sphere thereby rendering it fertilizable. 



The nature of the surface film produced by cutting an egg cell 

 differs in an unfertilized egg from one which has been fertilized. 

 Before fertilization the needle may be pushed vertically into the 

 side of the egg and moved through the egg from one side to the 

 other without cutting the egg in two. The cytoplasm close s 

 behind the needle thus obliterating the furrow. Shortly after 

 fertilization, however, such a procedure cuts the egg cleanly in 

 two. The sides of the furrow produced by the needle do not fuse 

 although contiguous. The character of the surface film which 

 forms over a cut is thus changed upon fertilization. This change 

 prepares the egg for the ensuing segmentation process by causing 

 the formation of a type of surface film which prevents contiguous 

 blastomeres from fusing with one another. 



