HEREDITY 195 



are attached like berries on one side to a tissue which is ramified like 

 the branches of a tree. That side which is attached to the tissue is 

 free from pigment, while the other part is pigmented. I consider it 

 quite possible that the difference in structure observed by Boveri in 

 the egg of the sea urchin, as stated above, is of a purely physical 

 character; namely, that it consists in the fact that different phases 

 are represented in the egg. Two liquids separated by a layer of a 

 more viscous substance might give rise to a differentiation, as noticed 

 by Boveri. If two eggs fuse, the complete union of the corresponding 

 layers in both eggs must be possible, in order to produce single 

 embryos, as would be the case if the contents of two vessels were 

 mixed, each of which contained oil and water. 



That the organs which originate later in the larva, e.g. the skeleton, 

 are not preformed in the egg from the beginning is obvious, and re- 

 quires no further discussion. 



The idea that any structure which may exist in the undivided 

 egg is of the simplest physical kind is also corroborated by the ob- 

 servations made by Chun, Driesch, Morgan,* and Fischel, f on the 

 eggs of Ctenophores. The Ctenophores possess a simple bell-shaped 

 body, the outside of which contains eight ribs or rows of cilia in sym- 

 metrical distribution. The above-named authors found that if the 

 first two cleavage cells of the Ctenophore's egg are isolated, a Cteno- 

 phore originates from each cell; that such a Ctenophore, however, 

 possesses only four ribs. Fischel found that if the egg of a Ctenophore 

 is cut into several pieces, and each piece gives rise to an embryo, the 

 total number of ribs possessed by these larvae never exceeds eight. The 

 body of a Ctenophore consists mainly of a jelly, or gel, and, in all prob- 

 ability, this gel already exists in the egg, and determines its shape 

 and the symmetry relations of the future embryo, inasmuch as the ribs 

 arrange themselves symmetrically on its surface. It is obvious that 

 the degree of preformed structure in this case need not exceed the sym- 

 metry relations due to simple physical conditions. The difference 

 between the Ctenophore's egg and the sea urchin's egg deserves some 

 special mention. An isolated blastomere of the two-cell stage of the 

 sea urchin's egg- gives rise to a whole embryo, while an isolated blas- 

 tomere of the same stage in a Ctenophore's egg gives rise to but a half 

 embryo, as far as the ribs are concerned. This difference is probably 

 due to a difference in the viscosity of the contents of the two eggs, the 

 fluid contents of the sea urchin's egg assuming a spherical shape again 

 after isolation ; while the blastomere of the egg of a Ctenophore, being 



* Driesch und Morgan, Arckiv fur Entwickelungsmechanik, Vol. 2, 1895. 

 t Fischel, Archiv fiir Entwickelungsmechanik, Vol. 6, 1897. 



