86 EDWIN G. CONKLIN 
of differentiation and metabolism within the cell, or a dense nu- 
clear membrane, decreases metabolism and leads to senescence; 
anything which facilitates this interchange increases metabolism 
and leads to rejuvenescence. It is suggestive that in early devel- 
opment increased oxidation is associated with fertilization and 
mitosis (Loeb, Lyon, Warburg) (pp. 57-62). 
Part II. Experiments 
17. By centrifugal force the substance of the eggs and blasto- 
meres of Crepidula may be stratified into a zone of heavy yolk 
at one pole, a zone of lighter oil and water at the other pole, and 
a zone of clear cytoplasm between these two; and since these 
eggs orient but slightly if at all while being centrifuged, the axis 
of centrifuging and of stratification may form any angle with the 
egg axis. In the early development of Crepidula the volume of 
yolk is much greater than the volume of cytoplasm and conse- 
quently the latter may be displaced to any side of the center of 
the egg or blastomere (p. 64). 
18. On the other hand the mitotic figure, after the prophase, 
can be moved only with great difficulty, and owing to this fact 
the substances of a cell can be distributed in very atypical man- 
ner with respect to the poles of the spindle and the resulting daugh- 
ter cells. In this way all the yolk present in a dividing cell may 
be thrown into one of the daughter cells, and almost all of the 
cytoplasm into the other (p. 64). 
19. These experiments show that the spindle is a specific 
structure and not merely a dynamic expression of lines of force. 
It remains in position and functions normally when the substance 
in which it usually lies is completely replaced by other substance. 
The spindle fibers are denser than the general cytoplasm and 
may be stretched, shortened or bent by pressure (p. 65). 
20. If centrifuging occurs during the second maturation divi- 
sion, when the poles of the egg are clearly marked, the yolk may 
be driven to the animal pole and the cytoplasm to the vegetal 
pole, the spindle may be much elongated and a giant polar body 
may be formed (fig. 11). In such cases the sperm nucleus, which 
