CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 315 



There is good evidence, which will be presented in the de- 

 scriptive part of this paper, that this is indeed the case and that, 

 while the greater part of the cytoplasm is free to move under the 

 influence of centrifugal force, there is in these eggs a denser, 

 more viscid portion of the protoplasm which forms a framework 

 running through the cell and connecting the nucleus and centro- 

 some, or centrosphere, with a peripheral layer which surrounds 

 the entire egg. This framework may be stretched or distorted 

 and yet may be able to 'bring back dislocated parts to their 

 normal positions unless partition walls have been formed in 

 the meantime which prevent this return. This framework is the 

 seat of the polarity and pattern of organization of the cell; it 

 holds the cell organs, especially the centrosphere and the nucleus, 

 in a definite relation to one another and to the cell axis, and it 

 prevents the complete stratification of cell substances into 

 sharply marked zones according to their specific weights. The 

 substance of this framework is probably identical with the 

 'ground substance' of Lillie, though in Crepidula it constitutes 

 a relatively small part of the cell contents, and in my opinion it 

 does have a ''filar, reticular or alveolar structure." Further- 

 more this substance is affected by centrifuging; it is stretched 

 and distorted if centrifuging is strong enough, but is capable of 

 recovering its normal form afterward. 



It seems evident that the term 'ground substance' is not an 

 appropriate one for this denser protoplasm, which constitutes the 

 achromatic substance of the mitotic figures and of the resting 

 nucleus as well as the astral radiations and strands which con- 

 nect these with the peripheral layer of the cell; this denser pro- 

 toplasm is much less abundant than the more fluid protoplasm 

 which forms the chief part of the middle zone of centrifuged 

 eggs; it is not uniformly distributed throughout the cell, but 

 exists in astral radiations and fine strands which run through 

 the more fluid protoplasm as well as through the yolk. It is 

 probable, however, that it is identical with the ' spongioplasm' 

 of Leydig, the 'kinoplasm' of Strasburger and in many respects 

 it corresponds to the ' interalveolar substance' of Biitschh, and 

 to the 'archoplasm' of Boveri. 



