ON EGGS OF MARINE ANIMALS 13 



condition. Although eggs from the same female may be fertil- 

 ized, their development eventually reveals that the eggs are 

 subnormal. In passing I may say that eggs with separated 

 membranes due to the action of distilled water never develop 

 beyond the monaster stage. This method therefore is not one for 

 inducing experimental parthenogenesis. 



3. A third index for normality of the echinid egg is furnished 

 by the response of the eggs to exposure to distilled water 30 to 

 40 seconds after insemination, that is, during the process of 

 membrane-separation. This response varies with the species of 

 egg. The egg of Echinarachnius thus exposed, if normal, breaks 

 down within sharply 15 seconds. The egg of Arbacia, on the 

 other hand, if in normal condition, shows the eflFect of exposure 

 only in the blastula stage. 



4. Years ago Loeb (1894) found that eggs of Arhacia exposed 

 to dilute sea-water some time after fertilization tend to protrude 

 through the ruptured membranes forming extra-ovates which 

 develop into twins with or without separation of the two egg- 

 fractions. European workers who endeavored to repeat Loeb's 

 observation on European species were not always successful. I 

 have found, however, that one can produce such extra-ovates in 

 various species of sea-urchins. My method is simpler and I 

 think more exact than Loeb's. I expose the eggs, of Arhacia, 

 for example, to distilled water 3 minutes after insemination. 

 If the eggs are in best condition, 90 to 100 per cent, of them 

 protrude through the ruptured membrane. Eggs now returned 

 at once to normal sea-water develop into twins which may 

 remain attached or become separated. This capacity for the 

 formation of extra-ovates is an excellent criterion for the eggs' 

 normality. 



I speak somewhat at length concerning these criteria for 

 normality in the echinid egg, because very recently my experi- 

 ence at RoscofF has shown me again how valuable they are as 

 diagnostic for the eggs' physiological condition. 



In nereid eggs 



In forms like the nereids, that have a heteronereid phase, 

 swimming at the surface of the sea at definite phases of the moon, 

 nature provides us already with an index of normality, for only 



