CYTOKINESIS. 115 



only difference in cells which I can positively associate with this differential distri- 

 bution of the spheres is the more or less hyaline character of the cytoplasm. 



This differential distribution of the spheres, like the distribution of the cyto- 

 plasm and yolk, is the result of the polarity of the cell contents ; the mechanism of 

 this distribution is found in the cell movements during every telophase. 



It follows from these conclusions on differential cell division that the various 

 forms of cleavage such as radial, spiral, bilateral, equal, unequal, homogeneous, 

 heterogeneous, etc., are expressions of the activity and structure of the cytoplasm 

 rather than of the nucleus or centrosome, and since the cytoplasm is almost exclu- 

 sively derived from the egg cell Avhile very little of it comes from the sperm we 

 should expect that the early cleavages would be little influenced by the latter. 

 This is just what Boveri ('92) found to be the case in eggs of Sphtsrechmtis which 

 were fertilized by Echinus sperm. From this cross a larval form developed which 

 was intermediate in character between the two genera, but the cleavage was purely 

 maternal in character, thus indicating that it was not influenced by the sperm. 

 Driesch ("98), also, in many crosses between different species of echinids has shown 

 that the rythm of division, the vacuolization of cells of the blastula, the configura- 

 tion of the larval stages, the color, manner of swimming and the number of mes- 

 enchyme cells of the larva^ depend upon the egg cell and not upon the sperm, and 

 therefore, in all probabilities, upon the cytoplasm and not upon the nucleus or 

 centrosome. 



On first thought such conclusions seem to be at variance with the usuallv 

 accepted view that the nucleus is the bearer of inheritance and that it, together with 

 the centrosome, are the prime movers in all formative processes. They do not, how- 

 ever, do more than show that in the early development inherited characteristics, 

 like material substance, are chietiy derived from the mother. But although differ- 

 entiations and inherited characteristics first appear in the cytoplasm there is good 

 reason to believe that the structure of the latter is influenced b}^ the nucleus through 

 the large amount of nuclear material which escapes into the cytoplasm at every 

 mitosis. Certainly many features of later development are derived from the father 

 and the conclusions as to the part which the nucleus has in hereditary transmission, 

 founded as they are upon the remarkable apparatus for such transmission afforded 

 by the nuclei, cannot be lightly cast aside. 



I have attempted to show in what manner the cytoplasm is responsible for some 

 of the earl}- differentiations of development; how many imjjortant features of 

 polarity and differential cell division are caused by movements of the cytoplasm ; 

 how these movements are perhaps caused by chemotropic attractions between unlike 

 substances ; but if we go fixrther and inquire what directs and co-ordinates these 

 cytoplasmic movements we cannot at present find any satisfactory answer. It may 

 of course be said that this is due to the "structure of the cytoplasm", but this is no 

 more than a convenient phrase to include a whole series of more or less unknown 

 phenomena which must still be anahzed. 



