58 BIOLOGICAL LECTURES. 



animals like Cerianthus. It may suffice to suggest the possi- 

 bility 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 migra- 

 tion or arrangement in every or in several directions is 

 possible. 



4. The ovum 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 communi- 

 cate 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 ovum is not deter- 

 mined by the preformation of germ regions in the protoplasm, 

 or nucleus, but by the geometrical shape of the ovum and 

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

 circumstances determine the number of blastulae. In my 

 experiments, I got double or triple embryos when the ovum 

 formed a double or triple sphere, as every sphere determines 

 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 formation of the blastula, 

 of course it is not possible to get the later stages which 

 are determined by the blastula. 



