334 EDWIN G. CONKLIN 



in a cytoplasmic field, while the sperm nucleus and centrosphere 

 approach the animal pole through the yolk in the vegetal hemi- 

 sphere of the egg. Other things being equal, the sizes of these 

 germ nuclei and centrospheres depend upon the volume of cyto- 

 plasm in which they lie; consequently in normal eggs the egg 

 nucleus and centrosphere are larger than those of the sperm 

 (fig. 28). If eggs are centrifuged in the anaphase or telophase 

 of the second maturation division, both egg and sperm nuclei 

 remain unmoved while yolk may be driven to the animal pole 

 and cytoplasm to the vegetal pole. Under these circumstances 

 the egg nucleus comes to lie in a field of yolk and remains cor- 

 respondingly small, while the sperm nucleus lies in the cyto- 

 plasm and grows large (figs. 23 to 25, 29, 30). In the earlier 

 phases of their growth both egg and sperm nuclei are moved but 

 little by centrifugal force (fig. 59); in later stages both nuclei 

 move more freely (figs. 60 to 62). This is probably due to the 

 fact that in the earlier phases mitotic fibers still bind the nuclei 

 to the surface layer, whereas in later stages these relax. 



Even after the germ nuclei have become quite large they may 

 occasionally be seen to be held by a cytoplasmic framework, 

 which connects the egg nucleus to the animal pole, as in figure 60, 

 and which prevents the free movement of the nuclei so that the 

 latter become stretched and distorted under centrifugal force 

 (figs. 60, 61). Sometimes strands of this framework may be 

 seen running through all the zones of the centrifuged egg (fig. 

 60). On the other hand, if centrifuging continues for a long 

 time the strands become less evident and the nuclei again as- 

 sume a spherical form (fig. 62), as is also the case when centri- 

 fuging ceases. In all these cases nuclei and cytoplasm come 

 back again to the animal pole after centrifuging. This move- 

 ment takes place especially during mitosis, which probably in- 

 dicates that this orienting framework is stronger or more active 

 during mitosis than during resting stages. 



