74 THE SOCIAL LIFE OF ANIMALS 



cleave more rapidly than associated but isolated 

 fellows. 



Under the conditions tested, the stimulating effect 

 of crowding could be detected when sixty-five or 

 more eggs were present in the more crowded drop 

 and twenty-four or fewer eggs made up the accom- 

 panying sparse population. 



Within twenty-four hours, under favorable condi- 

 tions, one finds one's cultures full of free-swimming 

 larvae with characteristic arms which are known as 

 plutei. When all our available data collected the 

 first day after fertilization are compared there is 

 again no doubt but that the more crowded cultures 

 usually develop more rapidly than accompanying but 

 sparser populations. However, it must be recorded 

 that throughout the whole series there were occa- 

 sional isolated eggs that developed as rapidly as the 

 best of the accompanying denser populations. Such 

 eggs and embryos were exceptional in our experi- 

 ence; the fact that they exist indicates clearly that 

 under the conditions of our experiments crowding, 

 while usually stimulating, was not absolutely neces- 

 sary for rapid cleavage and early growth. 



In this connection it is interesting to note that 

 others have prepared an extract from sea-urchin eggs 

 and larvae which is growth-promoting, (91) and one 

 which is growth-inhibiting. As has also been found 



