358 EDWIN G. CONKLIN 



fluid medium which has no filar, reticular or alveolar structure." 

 Lillie further concludes that ''microsomes are the primitive 

 formed elements of the cytoplasm" and that in point of origin they 

 are chromatin particles. The appearance of a reticulum in 

 fixed eggs he holds to be an artifact due to coagulation of a 

 colloidal solution. 



It is unquestionably an extremely difficult task to determine 

 with certainty the 'ultimate structure' of a substance which is 

 so changeable in appearance as living protoplasm. There is, 

 however, good reason for believing, as I have attempted to 

 show, that it is composed of a more fluid and a more viscid 

 portion and that while the former may be moved readily by 

 centrifugal force the latter is not so readily moved ; also that this 

 more viscid part of the protoplasm holds nuclei and centro- 

 spheres in a definite relation to the periphery of the cell and 

 brings parts back to their normal positions when once they have 

 been displaced; in short that the polarity and morphogenetic 

 organization of the egg reside in this more viscid substance, and 

 not in the more fluid medium as Lillie maintains. 



Whatever the ultimate structure of this denser portion of the 

 protoplasm of the egg of Crepidula may be, it is certainly not a 

 true fluid, nor is it the more fluid portion of the protoplasm. It 

 does take the form of fibers or strands in the amphiaster of the 

 living egg and these strands anchor the amphiaster to the periph- 

 eral layer; in resting stages similar fibers are present between 

 nucleus and centrosphere and between the latter and the periph- 

 ery of the cell. I suspect that the microsomes of which Lillie 

 speaks are not a part of this denser protoplasm, but that they 

 lie in the more fluid substance, which would account for their free- 

 dom of movement and would also explain the fact that the 

 microsomes aggregate to so large an extent in the middle zone 

 of centrifuged eggs. Mitochondria also, in this respect at least, 

 behave as microsomes ; in the eggs of moUusks and ascidians they 

 move through the cell under the influence of centrifugal force 

 almost as freely as do yoLk spherules and oil drops. 



It is customary to explain the polarity or other differentiations 

 of an egg as the result of its organization. But 'organization' 



