CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 329 



egg, but that the spermatozoon which enters does not cause any 

 portion of the egg to develop except the cell in which it lies. 

 Although the spermatozoon enters the egg of Crepidula about 

 three hours before the formation of the second polar body the 

 influence of the spermatozoon on the egg protoplasm during 

 this time is not sufficient to start development in the second polar 

 body even though it may contain the greater part of the egg 

 substance. This indicates that the second factor concerned in 

 the process of normal fertilization is not to be found in the diffu- 

 sion through the egg of some chemical substance carried in by the 

 spermatozoon, but rather in some non-diffusable substance, prob- 

 ably an organic structure. 



Long ago Boveri ('87) showed that under certain circumstances 

 the egg of Ascaris may divid^ at the first cleavage so that half 

 of the egg nucleus passes into each daughter cell while the sperm 

 nucleus does not divide, but goes entire into one of the first two 

 cells. Such a condition he called 'partial fertilization,' and in 

 such cases' he found that both halves of the egg develop, thus 

 showing that the activating influence of the spermatozoon has 

 affected both halves. Since in this case the centrosome is the 

 only structure derived from the spermatozoon which is known 

 to go into both cleavage cells he reached his well known conclu- 

 sion that the essential thing in fertilization is the addition of a 

 centrosome to the egg cell. 



It is possible of course that other as yet unrecognized struc- 

 tures are introduced by the spermatozoon and serve to activate 

 the egg. Meves ('11) found that the spermatozoon of Ascaris 

 introduces into the egg a number of coarse granules, the 'plasto- 

 chondria,' which he thinks unite with similar granules in the 

 egg and are then distributed to the cleavage cells. However, 

 in one of the Echinids he finds that the large granule or 'plasto- 

 some' which is derived from the middle-piece of the spermato- 

 zoon goes into one only of the first two cleavage cells and yet 

 both develop. I have found that the granules in the eggs of 

 gastropods and ascidians which are presumably identical with 

 'plastosomes' or 'mitochondria' may be distributed very un- 

 equally to the first two cleavage cells without interfering with 



