518 CELL DIVISION IN EGGS OF CREPIDULA. 



in the cell. (4) Mitotic movements of the cell contents, which cause the spindle 

 and the surrounding plasma to move into certain axes and positions in the cell. 



The quality of a cell, that is, the character of the cell substance which it 

 contains, appears to be due in the main to the relation between the spindle axis 

 and the stratification or localization of cell substance at the time of its formation, 

 i. e., to the way in which the cleavages cut through the different cell substances. 

 The morphogenetic value of a blastomere appears to depend primarily upon the kind 

 of protoplasm, apart from mere inclusions, such as oil and yolk, contained in the cell, 

 though it is known that the nuclear substance may influence the prospective potency 

 of a blastomere in cases of regulation or restitution. 



In the ascidians, as I have pointed out elsewhere (1905) "the prospective 

 significance of a blastomere is a function of its position" (Driesch) only because 

 of its relation to the organization of the egg as a whole — because of the particular 

 morphogenetic substances which go into it — and not because of its position 

 relative to other blastomeres. The experiments described above, on the effects 

 of pressure and of centrifugal force, on the cleavage of the eggs of Crepidula, 

 indicate that the same principles are involved in this case also, though the 

 cytoplasm of this egg contains a larger amount of inclusions or non-morphogenetic 

 materials and is more capable of regulation than is the case with the ascidian egg. 



So far as the nucleus is concerned these pressure experiments indicate that 

 the divisions are non-qualitative. When the third cleavage spindle is forced 

 into an equatorial plane and the cells divide meridionally instead of latitudinally, 

 as in normal eggs, the subsequent development of the extra macromeres thus 

 formed may be quite normal, each giving rise to three ectomeres, just as if the 

 normal number of macromeres were present. Therefore the organization of the 

 nucleus is not such that each of its divisions has a specific morphogenetic value; 

 on the contrary the nuclear divisions are non-differential and in this respect, 

 non-morphogenetic. Of course the nucleus has a definite structure, polarity, sym- 

 metry, etc., but its division by mitosis is such that the daughter nuclei are alike. 



IV. Effects of Electkic Cukrent. 

 (Plates XLVIII, XLIX. Exps. 995-999, 1100-1123, 1140-1142.) 

 Many authors beginning with Fol (1873) have emphasized the resemblance 

 between the mitotic figure and the lines of force in a magnetic field. Neverthe- 

 less attempts to identify the forces at work in the two cases have largely failed. 

 Errera (1890) was unable to find any effect of electricity or magnetism upon 

 cell division in Tradescantia. Nevertheless Ziegler (1895) and Gallardo (1896) 

 maintained that the mitotic figure was the expression of a bipolar force, and the 

 latter held that this force was probably electrical in nature. Hartog (1902, 

 1905, 1907) also held that the mitotic figure is an expression of a bipolar force 

 — his mitokinetic force — which he likens to, but does not identify with, elec- 

 tricity. Hartog, as well as Gallardo in his earlier work, held that the poles of 

 the spindle were heteropolar, bearing opposite signs, though Gallardo has since 



