362 EDWIN G. CONKLIN 



living cell consists, according to him, of a gelatinous substance 

 ''surprisingly more rigid than the cytoplasm in which it lies." 

 Mitochondria are not persistent structures but "they disappear 

 and reappear and must be merely changes in the physical states 

 of the colloids which compose the cytoplasm." 



My own observations and experiments on resting and dividing 

 cells, most of which were made before the publication of the work 

 of Kite and Chambers, lead to essentially similar conclusions. 

 The cytoplasm of the eggs of gasteropods and ascidians is com- 

 posed of a viscid, elastic, contractile gel in which are included 

 water, oil, yolk, pigment, microsomes, etc. This gel is more 

 rigid at certain phases of the cell cycle than at others depending 

 probably upon its water content. During the resting stage the 

 nuclear contents are in a state of gel, but in the beginning of the 

 prophase the achromatin becomes more fluid. It is quite evi- 

 dent that the nucleus grows by absorbing substance from the 

 cytoplasm. If the nuclear membrane is really a membrane, 

 and there is much evidence that it is, such absorbed substance 

 must enter as a fluid, though once within the nuclear membrane 

 it is converted into a gel. On the other hand when the nucleus 

 reaches the prophase of mitosis much of its contents becomes 

 more fluid and flows out toward the centrosomes where it again 

 gels in the form of astral rays and spindle fibers (Conklin, '02, 

 '10, '12 a, b). 



3. Protoplasmic flowing and intracellular movements 



Another general phenomenon which is involved in these con- 

 clusions is that regarding the nature of protoplasmic flowing 

 and intracellular movements. In the eggs of Crepidula more or 

 less extensive movements of the cell substance take place, as 

 shown in the movements of the maturation spindles to the ani- 

 mal pole, the migration of the sperm nucleus and aster through 

 the egg, the segregation of cytoplasm at the animal pole and of 

 yolk at the vegetal pole, and the movements of metakinesis and 

 telokinesis during cleavage. If the protoplasm is a viscid, elas- 

 tic, contractile gel how can such movements be explained? 



