THE FERTILIZATION-REACTION 



this: they represent extruded endoplasm escaping tlirough 

 the ruptured surface-layer of the egg, and occur when this 

 layer has been so altered that it loses the elasticity so char- 

 acteristic of it on the normal unfertilized egg. The resi- 

 dence in concentrated sea-water brings about abstraction of 

 water from the eggs as revealed by their shrinkage and the 

 closer apposition of their cytoplasmic inclusions. The 

 surface-layer also loses water; its greater visibility is owing 

 to its altered structure: on the normal unfertilized egg it 

 appears homogeneous, on the treated egg it appears as a 

 system of fine cytoplasmic prolongations attached to the 

 vitelline membrane. When these treated eggs are brought 

 suddenly into normal sea-water, this altered ectoplasmic 

 surface ruptures; and the endoplasm flows out as a cohering 

 bud. This outflow is checked as the eggs come into equi- 

 librium with their normal medium. 



Every egg with a protrusion, then, is made up of two com- 

 ponents, one possessed of the altered surface-layer, the 

 other, an endoplasmic bud, devoid of it. Following insemi- 

 nation, that portion of every egg enclosed by ectoplasm 

 separates a membrane, goes through cleavage and develops 

 into a swimming larva. The endoplasmic bud undergoes 

 no change; it can not be fertilized. As an Qgg with a bud 

 of endoplasm develops, the bud persists as a mass of undif- 

 ferentiated intact material which finally disintegrates. 



The size of protruded endoplasm is without significance; 

 it may be extremely minute, of the size of a polar body, 

 or it may contain most of the egg-substance. In the latter 

 case only the minute component with ectoplasm separates 

 the membrane and cleaves, the cleavage resembling the 

 so-called disocidal cleavage which is normal for the eggs of 

 many animals; but such eggs never form the typical inver- 

 sion, invagination, of cells by which the egg becomes a cup- 

 like swimming form, composed of two cell-layers. It is 

 interesting to note that this invagination (a form of gas- 



^93 



