3/2 E. Newton Harvey 



less in the lot which had stood. This is also the case when eggs 

 are fertilized with their own sperm. I tried this experiment with 

 Hipponoe with the same result. 



Apparently the mcrease m surface tension on standing is not 

 connected with an mcrease of permeability as I believe the increase 

 after fertilization to be. The increase in surface tension may 

 account for the entering of starfish sperm which could not enter 

 immediately after shedding of the eggs, for Loeb^^ has expressed 

 the opinion that the surface tension of the egg and sperm are the 

 determining factors in the entrance of the spermatozoon. As 

 the sea-urchin egg stands a decreasing per cent of its own sperm 

 becomes capable of entrance while an increasing per cent of foreign 

 sperm may enter. 



VII. ACTION OF DEVELOPMENT-STARTING SUBSTANCES IN GENERAL 



Throughout this paper a momentary increase in permeability 

 of the egg membranes has been mentioned as the fundamental 

 change underhing membrane formation and the initiation 

 of development. Kggs in which no membranes are formed are 

 excited to development by the same means as eggs which do 

 secrete a membrane, so that this increase of permeability 

 is probably a change occurring after the entrance of a 

 spermatozoon in all eggs. A consideration of the various par- 

 thenogenetic agents bears this out. 



A classification of the present known means of causing eggs to 

 develop is as follows: 



1 Hypertonic solutions (with an OH ion concentration lo-^). liaising the os- 



motic pressure of the medium by electrolytes or non-electrolytes or 

 evaporated sea-water. The most universal method for Fxhinoderms, 

 Annelids, Molluscs and Vertebrates. 



2 Hypotonic solutions and distilled water (Asterias and Arbacia), Shucking. 



3 Mechanical agitation (Annelids and Star-fish). 



4 1 emperaturc changes (Kchinoderms). 



^'''Loeb.: Dynamics of Living Matter, p. 163. 



