Ejfect of a Centrifugal Force upon Development and Sex 133 



GENERAL DISCUSSION 



Lyon has found in the eggs of the sea-urchin, Arbacia, that the 

 first cleavage plane is always at right angles to the plane of stratifi- 

 cation of the egg material. 



Lillie has shown that the stratification, caused by centrifuged 

 force, of the material in the egg of Chaetopterus plays no part in 

 determining the position of the polar lobe. When the germinal 

 vesicle is still intact the egg has a well defined polarity, that is, 

 one end is the animal and the other end the vegetative pole. If 

 the germinal vesicle is driven to the vegetative pole the polar 

 spindle which develops from it always migrates to the animal 

 pole and there forms the polar bodies. 



In later work on sea-urchin's eggs Morgan and Lyon show that, 

 "while the cleavage conforms strictly to the induced stratification, 

 the gastrulation does not conform to the symmetrical arrangement 

 of the materials. The exceptional cases show that there is no 

 necessary relation between stratification of the materials as such 

 and the embryonic axes." 



However, in the eggs of Cumingia Morgan has shown that the 

 stratification of the egg material does not influence the position 

 of the first cleavage plane. He says, "This difference is due to 

 the shifting of the nucleus in the egg of the sea-urchin, while the 

 spindle in Cumingia retains its original orientation." 



It must be borne in mind that the above results were obtained 

 from eggs centrifuged at different stages in their maturation. 

 The eggs of Chaetopterus were centrifuged when in the germinal 

 vesicle and maturation spindle stages, while the eggs of arbacia 

 were centrifuged when in the female pronucleus stage. The eggs 

 of Cumingia were also centrifuged when in the maturation spindle 

 stage and those of Hydatina in the germinal vesicle stage as well 

 as in the maturation spindle stage. 



When the eggs of these different animals are centrifuged in the 

 maturation spindle stage the spindle is not usually moved from its 

 original position and consequently the first cleavage take place 

 precisely as it does in normal eggs. 



When the eggs of Chaetopterus and Hydatina are centrifuged 



