CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 367 



nuclei, centrospheres and mitotic figures in a definite relation 

 to the cell axes, and which is so elastic that when it is distorted 

 by pressure or centrifugal force it tends to bring parts back to 

 their normal positions. But such an explanation does not ex- 

 plain the thing which we most wish to know, namely what de- 

 termines the definite, orderly succession of orientations in de- 

 velopment. For example, why do the two maturation spindles 

 usually have the same orientation, while all succeeding divisions 

 of the egg alternate in direction? Why are the first and second 

 cleavages in Crepidula equal while subsequent ones are unequal? 

 Why does every cleavage take place normally in a perfectly 

 definite way, which differs from every other cleavage, and give 

 rise to perfectly definite blastomeres which differ from all other 

 blastomeres? If the orientations of development depend upon 

 the spongioplasm does the structure of this spongioplasm change 

 in a definite way from cleavage to cleavage? These are ques- 

 tions which for the present must be left unanswered. 



V. SUMMARY AND INDEX 



1. If the fertilized but unsegmented eggs of Crepidula plana are 

 subjected to a centrifugal force of approximately 600 times 

 gravity yolk is thrown to the centrifugal pole, oil and other fight 

 substances to the centripetal pole, while nucleus, centrospbere 

 and most of the cytoplasm occupy the middle zone between the 

 other two. The relative volumes of these three zones is about 

 49 : 1 : 14, or in other words the yolk occupies a little more than 

 f and the protoplasm a little less than j of the volume of the 

 entire egg. This relatively large quantity of heavy yolk makes 

 it possible to displace nuclei and cytoplasm in any direction and 

 to study the effects of this on later development (p. 328). 



2. While the greater portion of the cytoplasm may be displaced 

 by the yolk a small residual portion of viscid spongioplasm is left 

 between the yolk spheres and in a peripheral layer around the 

 egg; this spongioplasm also forms a framework throughout the 

 entire cell and connects nucleus and centrosphere of resting 

 stages, or mitotic figure of dividing ones, to the peripheral layer. 



