Influence of Environment on Animals 201 



butyric, etc. Carbon dioxide is also very efficient in this 

 direction. It was also found that the higher acids are more 

 efficient than the lower ones, and it is possible that the sperma- 

 tozoon induces membrane formation by carrying into the egg a 

 higher fatty acid, namely oleic acid or one of its salts or esters. 



The physico-chemical process which underlies the formation 

 of the membrane seems to be the cause of the development of the 

 egg. In all cases in which the unfertilized egg has been treated 

 in such a way as to cause it to form a membrane it begins to 

 develop. For the eggs of certain animals membrane formation 

 is all that is required to induce a complete development of the 

 unfertilized egg, e.g., in the star-fish and certain annelids. 

 For the eggs of other animals a second treatment is necessary. 

 Thus the unfertilized eggs of the sea-urchin Strongylocentrotus 

 purpuratus of the Californian coast begin to develop when 

 membrane formation has been induced by treatment ^vith a 

 fatty acid, e.g., butyric acid; but the development soon ceases 

 and the eggs perish in the early stages of segmentation, or after 

 the first nuclear division. But if we treat the same eggs after 

 membrane formation, for from thirty-five to fifty-five minutes 

 (at 15° C.) with sea-water the concentration (osmotic pressure) 

 of which has been raised through the addition of a definite 

 amount of some salt or sugar, the eggs will segment and develop 

 normally, when transferred back to normal sea-water. If care 

 is taken, practically all the eggs can be caused to develop into 

 plutei, the majority of which may be perfectly normal and may 

 live as long as larvae produced from eggs fertilized with sperm. 



It is possible that the sea-urchin egg is injured in the process 

 of membrane formation. The nature of this injury became 

 clear when it was discovered that all the agencies which cause 

 hemolysis, i.e., the destruction of the red blood corpuscles, also 

 cause membrane formation in unfertilized eggs, e.g., fatty acids 

 or ether, alcohols or chloroform, etc., or saponin, solanin, 

 digitalin, bile salts, and alkali. It thus happens that the 



