144 PROBLEMS OF FERTILIZATION 



a well-defined optimum exposure, and the temperature 

 coefiicient is over loo. ''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 (igi^b) 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. 



