144 PROBLEMS OF FERTILIZATION 



a well-defined optimum exposure, and the temperature 

 coefficient is over 100. "The responsiveness of the eggs 

 to this form of treatment was found to depend on the 

 stage of maturation; warming before the dissolution 

 of the germinal vesicle had begun was ineffective, and 

 in fact inhibited maturation entirely; the most favor- 

 able period lay between the breakdown of the germinal 

 vesicle and the separation of the first polar body; after 

 both polar bodies had separated development was 

 imperfect and never proceeded far, even membrane 

 formation then failed in many eggs." 



Thus while the eggs of the starfish are still in a 

 condition of unimpaired vitality, as is shown by contin- 

 uation of the maturation divisions, they lose capacity 

 both for fertilization and parthenogenesis. 



The inference that the failure to respond to parthe- 

 nogenetic agents is due to loss of some substance in 

 sea- water was strongly supported by Just (1915^) in 

 a study of heat parthenogenesis in Nereis. He found 

 that eggs of Nereis which were first washed in sea-water 

 could not be induced to develop by exposure to warmed 

 sea-water, or only an exceedingly small percentage are 

 affected; if, however, the eggs are exposed, without 

 previous contact with sea-water, to a favorable temper- 

 ature for an optimum time in a small quantity of sea- 

 water, all of them may segment, and as many as 20 per 

 cent may develop into trochophores. Just found that 

 capacity for heat parthenogenesis is lost much more 

 rapidly in sea-water than capacity for fertilization, but 

 the latter is also affected, though at a lesser rate. These 

 capacities he found to run parallel to loss of sperm- 

 agglutinating substance by the eggs. 



