CELL DIVISION IN EGGS OF CREPIDULA. 553 



Of the occurrence of amitosis in protozoa I shall not here treat; the condi- 

 tions in these forms are peculiar, and the form of nuclear division may also 

 be peculiar, but it seems to me that there is as yet no satisfactory evidence 

 that amitosis ever occurs in normally differentiating cells of the metazoa. God- 

 lewski concludes (p. 120): "Dass aus der bisherigen Literatur sich kein einzige 

 Aufgabe anfiihren lasst, durch welche ganz positiv bewiesen wurde, dass die 

 Amitose der Karyokinese nicht gleichwertig sein konnte." On the other hand, 

 I think it may be affirmed with equal emphasis that there are numerous evidences 

 against the view that amitosis can take the place of mitosis in normally developing 

 cells of the metazoa, and not a single entirely positive one in favor of that view. 



XL Conclusions and Summary. 

 A. Observations and Experiments. 



1. In the eggs of Crepidula plana abnormal cleavages and mitoses are rela- 

 tively rare in the conditions found in nature; those which occur are due, prob- 

 ably, to pressure, diluted sea water, or increased temperature. (Figs. 1-29; 

 pp. 505-507.) 



2. The cleavage of isolated blastomeres is strictly partial; the only modi- 

 fications as compared with entire eggs being due to the rounding of the isolated 

 cells. All the early cleavages are differential (morphogenetic) ; non-differential 

 cleavages appear only after the formation of ectomeres, mesomeres and ento- 

 meres, and in certain subdivisions of these cells. So far as the cleavage stages 

 are concerned the prospective significance and the prospective potency of early 

 blastomeres are identical. (Figs. 30-44; pp. 508-510.) 



3. In eggs under pressure lobes may be formed, usually opposite the poles of 

 the spindles; spindle axes and cleavage planes may be turned out of their normal 

 positions, so that the blastomeres formed lie in one plane and bear totally atypical 

 relations to one another and to the axes of the unsegmented egg; if the pressure 

 is in the direction of the spindle axis, but is not sufficient to turn the spindle out 

 of that axis, it may lead to suppression of cell division. In all cases the prospec- 

 tive significance and the prospective potency of such blastomeres, during the 

 cleavage stages at least, is determined by the relations of the cleavage planes 

 to the axes of the unsegmented egg. All cells formed by meridional cleavages 

 give rise to three separate ectomeres at the animal pole, and to one entomere at 

 the vegetal pole; all cells formed by equatorial or latitudinal cleavages are 

 ectomeres at the animal pole, entomeres at the vegetal pole; mesomeres arise 

 only from cells lying between these poles on the posterior side of the egg. There 

 is, presumably, a stratification of the hyaloplasm, or "ground substance," of the 

 unsegmented egg, wholly apart from the yolk or other inclusions, which determines 

 the morphogenetic trend of each blastomere. (Figs. 45-67, 76, 77; pp. 510-518.) 



4. When eggs are subjected to a weak electric current, spindles, nuclei and 

 cytoplasm may be displaced as if by pressure and subsequently formed blasto- 





