No. 581] ENTRANCE OF THE SPERMATOZOON 259 



which have been induced to develop into larvae by a hyper- 

 tonic solution can be fertilized with sperm while they are 

 in the process of segmentation. When such eggs are in 

 the two-, four-, eight-, or sixteen-cell (and possibly also 

 later) stages the sperm can enter into one or more blasto- 

 meres of such an egg and this entrance betrays itself by 

 a distinct and clear membrane formation around each 

 blastomere. 3 While the segmenting eggs which were not 

 fertilized with sperm develop into larvae, those into which 

 sperm enters perish very rapidly. This simple and 

 rather striking experiment which can easily be performed 

 in the eggs of Strongylocentrotits, where the membrane 

 formation around a single blastomere can be clearly recog- 

 nized, shows that the process of development in a fer- 

 tilized egg in itself can not be responsible for the block 

 caused by fertilization. It looks as if the entrance of a 

 spermatozoon into the mature egg, independently of the 

 developmental changes it induces in the egg, causes some 

 physical or physico-chemical change (of the surface of 

 the egg?) which renders the subsequent entrance of a 

 spermatozoon impossible. 



2. With the eggs of most females of purpuratus the 

 treatment with a hypertonic solution does not lead to a 

 development into larvae, but only to the first segmentation 

 stages in a limited number of eggs (provided that the 

 eggs have been exposed to the solution the proper period 

 of time). Such blastomeres afterwards go into a resting 

 stage. If one waits long enough, until there is no doubt 

 left that the blastomeres have reached a resting condition 

 and will divide no further, and if one then adds sperm, 

 the individual blastomeres can again be fertilized, which 

 is indicated by a membrane formation around each indi- 

 vidual blastomere and the subsequent development of such 

 blastomeres into swimming larva?. 4 The fact that each 

 individual blastomere in this case is fertilized independ- 



