FERTILIZATION REACTION IN ECHINODERM EGGS. 267 



Consequently the vitelline membrane is distended at fertiliza- 

 tion, and such distension indicates a softening or degelation of 

 the membrane. According to R. S. Lillie ('15) the temperature 

 coefficient of heat activation in the starfish egg indicates that a 

 degelation process is involved. Just's ('22) recent experiments 

 show that the membrane l becomes very much less resistant at 

 the places where it is separating from the egg for such places 

 burst when the egg is exposed to diluted sea-water during eleva- 

 tion. Thus the decreased resistance or softening of the vitelline 

 membrane like other fertilization changes is initiated at the point 

 of contact of sperm and egg and is transmitted from this point 

 over the egg. Within a few minutes after elevation the membrane 

 appears to toughen again. This hardening or toughening of the 

 membrane after elevation has been spoken of by several observers. 

 Thus Herbst ('93), Goldfarb ('13), C. R. Moore ('16), F. R. Lillie 

 ('21) have recorded that the fertilization membrane of urchin 

 eggs is much more easily removed by shaking within a few 

 minutes after fertilization than later. Chambers ('216) also 

 finds that the fertilization membranes of Asterias, Arbacia, and 

 Echinarachnius begin to toughen very soon after they are ele- 

 vated. The extensive hardening of the fertilization membrane 

 in nematode eggs is well known. That the membrane also under- 

 goes chemical alteration on elevation may be inferred from an 

 experiment of Harvey's ('10). He found that the vitelline mem- 

 brane is soluble in concentrated sulphuric acid while the fertiliza- 

 tion membrane is not. 



5. Viscosity Changes at Fertilization. It was pointed out in 

 the first part of this paper that the cytoplasm of the unfertilized 

 egg is a slightly viscous fluid. It can be determined by the same 

 procedure as described there that at fertilization there is a 

 sudden and marked increase in viscosity. In the eggs of Stron- 

 gylocentrotns (both species) this increase in viscosity occurs just 

 before the membrane begins to elevate; this is probably also 

 true of the Patiria egg although it was not determined with 

 certainty. It is present in this egg at least as soon as the mem- 

 brane lifts at the site of sperm entry. The viscosity increase is 



1 Just speaks only of the softening of the cortex but probably includes the 

 membrane in this term. It seems evident that the membrane must also be involved 

 in the softening otherwise it would not yield at the places in question. 



