THE 



AMERICAN NATURALIST 



Vol. XLIN May, 1915 No. 581 



ON THE NATURE OF THE CONDITIONS WHICH 

 DETERMINE OR PREVENT THE ENTRANCE 

 OF THE SPERMATOZOON INTO THE EGG- 

 JACQUES LOEB 

 The Rockefeller Institute for Medical Research, New York 



I 



The well-known fact that a spermatozoon can no longer 

 enter an egg after it is once fertilized raises the question 

 whether this is due to the changes necessarily connected 

 with development ; or whether development of an egg can 

 take place without the existence of such a block. We are 

 in possession of facts speaking in favor of the second 

 view. Thus the writer has shown that if the eggs of 

 Strongylocentrotus purpuratus or Arbacia are induced to 

 develop by the methods of artificial parthenogenesis a 

 spermatozoon can enter the egg or an individual blasto- 

 mere of a segmenting egg, while the latter is in the full 

 process of development. This leaves no doubt that the 

 block caused by the entrance of a spermatozoon into an 

 egg for the entrance of further spermatozoa must be due 

 to a change not necessarily identical with that inducing 

 the development of the egg. 



A second group of observations made by the author 

 deals with the phenomena of specificity and these prove 

 that the block which an egg offers to heterogeneous spe*m 

 is rapidly reversible and confined to the surface of the egfc 

 or the spermatozoon or both. In the case of the egg of\ 

 purpuratus and sperm of Asterias (and many similar in- 

 257 



