108 The Mechanistic Conception of Life 



that in polarized animals the tissues or cells may have such a 

 peculiar structure as to allow the specific formative substances 

 to migrate or arrange themselves only in one direction, while in 

 cases of heteromorphosis migration or arrangement in every 

 direction or in several directions is possible. 



4. The egg of a sea-urchin under normal conditions gives 

 rise to but one embryo. This circumstance is due simply to 

 the geometrical shape of the protoplasm, which, under normal 

 conditions, is that of a sphere. When we make the eggs burst, 

 the protoplasm outside the egg membrane and that which 

 remains within it assume spherical forms, by reason of the 

 surface tension of the protoplasm. When this happens, as a 

 rule, we get twins, if two separate segmentation cavities are 

 formed, and only one embryo, if both cavities communicate 

 with one another. Whether the first or the second case will 

 happen depends upon the molecular condition of the part of 

 the protoplasm connecting the two drops. Therefore, the 

 number of embryos which come from one egg is not determined 

 by the preformation of germ-regions in the protoplasm, or 

 nucleus, but by the geometrical shape of the egg and the 

 molecular condition of the protoplasm, in so far as these circum- 

 stances determine the number of blastulae. In my experiments, 

 I got double or triple embryos when the egg formed two or 

 three droplets or spheres, as every sphere gives rise to a blastula. 

 In Driesch's experiments, one single cell of the four-cell stage 

 necessarily formed a whole embryo after it had been isolated, 

 as it assumed the shape of a single sphere or ellipsoid. Of 

 course, there must be a limit to the number of embryos that can 

 arise from one egg; but the limit is not due to any preformation, 

 but to other circumstances, the chief one being that with too 

 small an amount of protoplasm the formation of a blastula — 

 from merely geometrical reasons, as there must be a minimum 

 size for the cleavage-cells — ^becomes impossible.^ Without the 



1 I stated that the minimal size is about one-eighth of the mass of the sea- 

 urchin's egg and I do not think that this is very far off the limit. 



