CYTOKINESIS. 97 



the appearance of this clear zone the cell body begins to constrict in this plane, and 

 tliere can be little doubt that the equatorial constriction is in part the result of the 

 structure of this zone. 



The equatorial constriction is, therefore, the result of at least three factors : 

 (1) the decrease of surface tension at the poles and the consequent increase of sur- 

 face tension at the equator; (2) the vortical How of cell substance from the per- 

 iphery of the cell into the spindle axis in the equatorial plane ; (3) the structure of 

 the cytoplasm in the equatorial plane, which is here composed of large alveoles 

 with relatively large amount of enchylemma and small amount of hyaloplasm. 

 It is probable that these different factors are all the expression of some common 

 cause, which may possibly be found in the movements of the cytoplasm. 



Comparisons. 



It is interesting to note how recent Avork on the movements of cell contents 

 was anticipated b\' some of the earliest writers on the subject of cell division — I 

 refer especially to Auerbach ('74), Butschli ('75, '76), 0. Hertwig ('75, '77), Stras- 

 burger ('75), Fol ('75, '79), Whitman ('78, '87), Mark ('81), ct al. All of these did 

 their work, at least in part, on living cells, and it is instructive to contrast the plas- 

 tic, kinetic conception of the cell which they all hold with the rigid, static one 

 which has grown up in recent years, with the development of mici'oscopical tech- 

 nique and the exclusive study of fixed material. Space will not here permit a 

 review of the work of these founders of cytology on the subject of cytokinesis. 

 References to several of these older works are found elsewhere in this paper and 

 an excellent critical review of them ma}' be found in Mark ('81). 



To Butschli, more than to any one else, we owe not only the conception of the 

 structure of protoplasm which is here maintained, but also a mechanical theory of 

 cell division which is built upon the movements of the protoplasm. As long ago as 

 1876 he considered the asters as diffusion centers [cf. views of Auerbach and Biit- 

 schli, p. 49), which could increase the surface tension at the equator, while reduc- 

 ing it at the poles; and although now and since 1892 he maintains that the asters 

 exert an attractive influence on the cytoplasm he considers that this attraction 

 {Zugwirkiing) is still a factor in the constriction of the cell body. 



Of all the observations which have heretofore been made on movements during 

 metakinesis those of v. Erlanger ('97^) most resemble my own. In the living eggs 

 of some small nematodes he observed strong movements in the cytoplasm in the 

 maturation, fertilization and cleavage. The egg nucleus moves to the center of the 

 egg, apparently by means of plasma streaming, and the approach of the germ nuclei 

 is accomplished by such streaming. During the first and second cleavages the egg 

 plasma shows decided streaming which sets the spindles into slow oscillating motion. 

 The direction of this streaming is from the spindle poles toward the equator on 

 the surface of the egg; at the equator it turns in at the furrow and returns to 

 the poles through the interior of the egg. At the same time the astral rays are 

 bent toward the equator, which still further confirms the existence of superficial 



1.3 JOUEN. A. N. S. PHILA., VOL. XII. 



