CYTOKINESIS AND THE CELL WALL 



169 



worms, the daughter cells (blastomeres) round up and become more or 

 less spherical, whereas in larger eggs, such as those of frogs, a narrow 

 cleavage furrow appears at one pole and develops through the egg without 

 altering greatly the shape of the latter, so that the first two blastomeres 

 have the form of hemispheres. In many animals, notably birds, the 

 cleavage is superficial, not extending entirely through the yolk-laden egg. 



So far as known, no animal cells are divided by cell-plates of the type 

 found so commonly in plants. As noted previously, there is often a 

 slight differentiation (the "mid-body") at the equator of the achromatic 

 figure in the telophase, but it plays no part in cytokinesis. In the 

 spermatogonia of certain insects Janssens (1924) finds a cell-plate-like 

 differentiation which appears to represent a region of protoplasmic 

 continuity rather than a structure concerned in cytokinesis. 



Experiments with dividing eggs have led to the identification of some 

 of the factors involved in the development of cleavage furrows. Many 



Fig. 97. — Diagram of streaming and furrowing in the egg of Rhabditis pellio (A) and an oil 



droplet (S). {After Spek, 1918a.) 



years ago Biitschli (1876) advanced the view that cytoplasmic currents 

 flowing toward the centrosomes lead to the production of a relatively 

 high surface tension at the equator of the cell, this in turn bringing about 

 furrowing through this region. Evidence favoring this interpretation 

 has been contributed by several later investigators.^ Spek imitated 

 furrowing and division with oil and mercury droplets in water and showed 

 that by lowering the surface tension at two poles of the droplet the rela- 

 tively high surface tension at the equatorial region could be made to 

 bring about the constriction and fission of the droplet. In both droplet 

 and dividing nematode egg he found streamings such as Erlanger (1897) 

 had described in the egg: an inner movement poleward to the region of 

 low surface tension and a superficial streaming toward the equatorial 

 region of higher surface tension, the streams turning inward at the furrow 

 (Fig. 97). Belaf (1927) has observed such cytoplasmic streams in the 

 furrowing spermatocyte of Chorthippus (Fig. 59). 



Closely associated with such streaming and surface-tension phe- 

 nomena are certain periodic alterations in the viscosity of the egg sub- 



sMcClendon (1910, 1913), Spek (1918a, 19206), Just (1922), Cannon (1923), 

 Bancroft and Gurchot (1927). 



