The view, according to which the determiners in the chromosomes only tend to give special characters to the embryo or to the adult while the cytoplasm of the egg may be considered the real embryo, receives some support from the fact that the first development of the egg is purely maternal, even if the egg nucleus has been replaced by sperm of a different species. If an egg of a sea urchin be cut into two pieces, one with and one without a nucleus, and the enucleated piece be fertilized with the sperm of a different species of sea urchin, the blastula and gastrula stages are purely maternal and only the skeleton of the pluteus stage begins to betray the influence of the foreign sperm inasmuch as this skeleton is purely paternal, according to Boveri. In all experiments on hybridization it has been found that the rate of cell division of the egg is a purely maternal character. Thus when fish eggs of a species, in which the rate of first segmentation of the egg is about eight hours, are fertilized with sperm of a species for which the same process requires about thirty minutes or less at the same temperature, the rate of segmentation is again about eight hours. There is then no chromosome influence noticeable in the early development. When two forms of sea urchins, _Strongylocentrotus_ _franciscanus_ and _purpuratus_,[209] are crossed, certain features of the skeleton of the embryo, _e. g._, the so-called cross-bars, are a dominant, inasmuch as they are found in _purpuratus_ and both the crosses, while they are absent in _franciscanus_. The development prior to the formation of the skeleton is purely maternal. These observations again lend support to the idea that the Mendelian factors of heredity must have the embryo to work on and that the organism is not to be considered a mere mosaic of Mendelian factors. This is further supported by the idea that the species specificity resides in the proteins of the unfertilized egg (see Chapter III), and it is quite likely that this species specificity decides which type of animal should arise from an egg. [209] Loeb, J., King, W. O. R., and Moore, A. R., _Arch. f. Entwcklngsmech._, 1910, xxix., 354. These experiments have been repeated at different seasons of the year and in different years and have been found to be constant.