260 THE AMERICAN NATURALIST [Vol. XLIX 



ently of its neighbors suggests that there is no protoplas- 

 mic connection between the neighboring blastomeres ; 

 otherwise the entrance of a spermatozoon into one should 

 cause its neighbors also to form a fertilization membrane, 

 which does not happen. 



All these facts show that the changes underlying devel- 

 opment do not necessarily prevent the entrance of a sper- 

 matozoon into an egg fertilized by sperm. 



3. Development can be initiated in an unfertilized egg 

 by causing a membrane formation by a fatty acid. Eggs 

 after such an artificial membrane formation perish as a 

 rule rapidly at room temperature (if no second treatment 

 is given them) but they may segment if kept at a low tem- 

 perature. The eggs are usually put after treatment with 

 the butyric acid into normal sea water in which they form 

 a membrane. This membrane is different in the eggs of 

 different species of sea urchins. In the egg of 8. pur- 

 puratus the membrane is tough and entirely impermeable 

 to the spermatozoon. When we add sperm to such eggs 

 with a butyric acid membrane they behave exactly as if no 

 sperm had been added, they all perish rapidly (at room 

 temperature). The question arose, if a spermatozoon 

 could still enter the egg of purpuratus after membrane 

 formation, provided the membrane could be destroyed. 

 This can be done in a certain percentage of the eggs of 

 purpuratus by shaking them after artificial membrane 

 formation; the number of eggs whose membrane is torn 

 varies in different experiments owing probably to dif- 

 ferences in the thickness and toughness of the membrane. 

 Even if the membrane is torn the edges may come close 

 together again so that the opening often is closed 

 again and no spermatozoon can go through. Kupelwieser 

 and the writer performed this experiment on the eggs of 

 purpuratus and it was found that such eggs with torn 

 membranes were fertilized upon the addition of sperm 

 and developed normally; while the eggs whose mem- 

 branes were intact all perished. 5 



»Loeb, "Artificial Parthenogenesis and Fertilization," p. 234. 



