Influence of Environment on Animals 203 



develop into normal larvae (plutei). Some substance contained 

 in the blood causes, presumably, a superficial cytolysis of the 

 egg and thus starts its development. 



We can also cause the development of the sea-urchin egg 

 without membrane formation. The early experiments of the 

 writer were done in this way and many experimenters still use 

 such methods. It is probable that in this case the mechanism 

 of fertilization is essentially the same as in the case where the 

 membrane formation is brought about, with this difference 

 only, that the cytolytic effect is less when no fertilization 

 membrane is formed. This inference is corroborated by 

 observations on the fertilization of the sea-urchin egg with ox 

 blood. It very frequently happens that not all of the eggs form 

 membranes in this process. Those eggs which form membranes 

 begin to develop, but perish if they are not treated ^vith h^'per- 

 tonic sea-water. Some of the other eggs, however, which do not 

 form membranes, develop directly into normal larvae without 

 any treatment with hypertonic sea-water, provided they are 

 exposed to the blood for only a few minutes. Presumably some 

 blood enters the eggs and causes the cj'tolytic effects in a less 

 degree than is necessary for membrane formation, but in a 

 sufficient degree to cause their development. The slightness 

 of the cytolytic effect allows the egg to develop without treat- 

 ment with hypertonic sea-water. 



Since the entrance of the spermatozoon causes that degree 

 of cytolysis which leads to membrane formation, it is probable 

 that, in addition to the cytolytic or membrane-forming sub- 

 stance (presumably a higher fatty acid), it carries another 

 substance into the egg which counteracts the deleterious effects 

 underh'ing or following membrane formation. 



The question may be raised whether the larvae produced 

 by artificial parthenogenesis can reach the mature stage. This 

 question may be answered in the affirmative, since Delage has 

 succeeded in raising several parthenogenetic sea-urchin larvae 



