CYTOKINESIS. 99 



wonderfully active and powerful forces within the egg. When the living egg is 

 seen under an immersion lens, he says, "the surface fairly scintillates with the move- 

 ments of the protoplasm and these pigment granules." 



Fischel ('99) has observed a regular and orderly movement of granules, which 

 stain with neutral red, in the living eggs of echinoderms. These granules, wdiich 

 are uniformly distributed throughout the cell during the rest, stream in toward the 

 nucleus at the beginning of division and surround the division figure ; after the 

 division they are again distributed throughout the cell. In this case there is no 

 accumulation of these granules in the plane of the cleavage ; on the contrary, thej- 

 move away from this plane. Rhumbler ('96, '99) has studied the movement of the 

 pigment granules of amphibian eggs during division and finds that during the 

 growth of the nucleus they collect around it; in the cytodieresis they are found in 

 the plane of the division wall, and in all cases aggregations of the pigment are found 

 only in thickenings of the hyaloplasm. Van der Stricht ('99) has observed in 

 Thysanozoon that at the moment when the nuclear membrane disappears, fatty 

 granules which were scattered through the cell accumulate around the achromatic 

 figure, most of them being found at the equator of the spindle. 



Rhumbler ('96, '97) has developed an extensive theory as to the mechanics of 

 cell division. He holds that the spheres do not enlarge by the reception of nuclear 

 substances (as Auerbach, Biitschli, Ziegler and I maintain), but that the nuclear 

 sap is pressed out of the nucleus into the equatorial region of the cell, and that this 

 nuclear sap goes to form the new cell membrane, the amount of membrane formed 

 being proportional to the quantity of sap which escapes ('97, p. 697). I have else- 

 where (p. 48) shown reason for believing that the nuclear sap first escapes at the 

 poles of the nucleus and while a considerable portion of the sap may later escape in 

 in the equatorial region, the assumption that the new cell membrane is formed by 

 this sap and that the amount of membrane formed is proportional to the quantity 

 of sap which escapes has little in its favor. Even though some of the sap may 

 escape in the equatorial region it does not always lie where the division wall will 

 form. In the first maturation division the new cell wall forms a considerable dis- 

 tance from the place where the nuclear sap escapes ; and in the first and second 

 cleavages the division wall appears all around the equatorial circle, though the sap 

 escapes from the nucleus only in the cytoplasmic area near the animal pole. There 

 is absolutely no reason for believing that in these divisions the nuclear sap collects 

 all around the cell in the equatorial plane as it must do if the new division wall is 

 formed from it. Moreover, the quantity of sap which escapes is not always propor- 

 tional to the amount of membrane formed. In the case of the formation of the first 

 polar body a larger amount of nuclear sap escapes than at any other mitosis in the 

 whole course of development, yet the increase in the membrane is here perhaps less 

 than in any other cell division, except the second maturation. Therefore, while 

 recognizing the great value and suggestiveness of many of Rhumbler's conclusions, I 

 cannot accept his views as to the formation of the division wall. In egg cells this 

 wall is, as I have maintained elsewhere (p. 96), principally composed of old cell wall 

 infolded at the equator, while the new wall is chiefly formed at the poles. 



