98 CYTOKINESIS. 



currents from the poles to the equator. These movements in the cytoplasm, though 

 slow and weak, were actually observed by Erlanger in the living eggs. Biitschli 

 (1900) suggests that the cause of this slowness of movement is the viscosity of the 

 cytoplasm, "though powerful, turbulent vortical movements can have no part in 

 normal cell division." The cause of these movements Biitschli finds either in 

 increased surface tension at the equator, or in decreased tension at the poles. 



A similar view as to the mechanics of cytodieresis was briefly expressed by 

 Loeb ('95). He suggested that a mechanical explanation of the division of an egg 

 or embryo was to be found in diffusion and vortex movements of the protoplasm, 

 similar to those observed by Quincke in an emulsion of oil and soda solution. " I 

 conceive," says he "that on the surface of the egg, possibly in the meridian or 

 circle whose plane separates from one another the two radiating systems of the 

 centrosomes, diffusion phenomena occur as soon as the nuclear division has physi- 

 cally ended. These lead to the formation of vortical movements, symmetrical in 

 relation to this plane." If these movements are violent they lead to the complete 

 separation of the daughter cells; if not, ordinary cleavage results. It will be 

 observed that in two respects this view of Loeb's differs from Biitschli's and 

 Erlanger's and from my own observations, viz. : (1) the diffusion phenomena are not 

 limited to the equatorial circle, and (2) they occur before the nuclear division is ended. 

 Later, Loeb ('95) observed in the segmenting eggs of Ctenolabrus droplets over the 

 surface of the egg which collected in the plane of the next succeeding cleavage ; 

 this phenomenon he considered a confirmation of this theory. 



The movements which occur during karyokinesis in Crepidula and other gas- 

 teropods entirely confirm the theories of Biitschli and Erlanger as to the mechanics 

 of cell division. These theories also find confirmation in many other observations 

 on a large number of animals. Among these may be mentioned the following : 



Morgan ('93) observed that the reddish pigment granules found over the surface 

 of the eggs of Arbacia move entirely away from the micromere pole of the egg- 

 before the micromeres are formed. In some eggs this movement begins in the two- 

 cell stage, and is carried on until the micromeres are formed at the sixteen-cell 

 stage. Nusbaum ('93) observed in the division of entoderm and mesoderm cells of 

 young embryos of Rana temporaria that the brown-black pigment collected in a 

 ring around the equator of the dividing cell, and as the division advanced the ring- 

 became narrower and deeper until it formed a true cell plate between the daughter 

 cells. Van Bambeke ('96) has observed a similar phenomenon in the cleavage of 

 the toad's egg. Gardiner ('95) observed in the eggs of Polychcerus and Aphano- 

 stoma a reddish yellow pigment, which, because of its form and peculiar movements, 

 he supposed might be some form of alga. After the egg is laid it migrates from the 

 center toward the periphery, and forms a girdle around the ovum in the plane of 

 the first cleavage. A similar line of pigment marks out the division plane of every 

 succeeding cleavage up to the ten-cell stage. He also observed that these pigment 

 granules migrated from one pole of the egg to the other, though they never passed 

 from one cell to the other. These movements greatly impressed Gardiner with the 



