CELL DIVISION — SPINDLE IN SEA-UECHIN EGGS 221 



An observation of Spek ('18) on Nematode eggs is also of in- 

 terest. In these eggs, as is well known, the cytoplasm near the 

 surface is in continual movement. But at the moment when the 

 spindle becomes visible all this movement ceases completely. 

 Spek finds "alle amoboide Bewegungen an der Oberfiache der 

 Eizelle .... ganz plotzlich — in dem Augenblick, wo 

 der Spindel sichtbar wird — voUig aufhoren." Probably this ces- 

 sation of movement is due to a cytoplasmic stiffening. This 

 stiffening, however, does not extend to the center of the egg, for 

 in the central region the spindle oscillates from side to side. 



In sea-urchin eggs Albrecht ('98) found that there was an in- 

 crease in viscosity after fertilization. This was more conclu- 

 sively shown by myself in 1915. 



But the most remarkable work is that of Chambers ('17). By 

 microdissection he was able to demonstrate in the dividing eggs 

 first a gelation and later a return to a more fluid condition. My 

 findings agree absolutely with his. Moreover, Chambers was 

 able to study the morphological aspect of the gelation. In the 

 astral radiations only the granular material is solidified. The 

 hyaline rays which lie between the granular radiations are fluid. 



The' solidified materials of the cytoplasm apparently extend 

 out to the cortex of the egg and are attached there. This is in- 

 dicated by the shape of the egg in the centrifuge tests. When 

 they are centrifuged soon after fertilization, the contour of the 

 egg remains spherical. However, as soon as gelation begins, the 

 eggs assume irregular shapes after centrifugal treatment. This 

 distortion of the eggs always occurs at this time and is quite 

 striking. Oftentimes various regions of the eggs are flattened, 

 and frequently parts of the egg surface are indented, indicating 

 a strong pull on certain regions. In the later stages of mitosis, 

 when the cytoplasm is again more fluid, the eggs nevertheless 

 assume irregular shapes when centrifuged. Apparently, there 

 are still some gelatinous strands, perhaps more slender, which 

 retain their connection to the egg surface, and pull upon it when 

 the egg is centrifuged. This view is supported by the fact that 

 when such eggs are centrifuged more vigorously, there is less 

 tendency for irregularity in contour. In this case, very probably 



