[Illustration: FIG. 2. Five-days-old larvæ of Strongylocentrotus purpuratus produced by artificial parthenogenesis. (Side view.) The larvæ in Figs. 1 and 2 are identical in appearance, proving that heterogeneous hybridization leads to a larva with purely maternal characters.] If such heterogeneous hybridizations are carried out, two striking results are obtained. The one is that the resulting larva has only maternal characteristics (Figs. 1 and 2), as if the sperm had contributed no hereditary material to the developing embryo. This result could not have been predicted, for if we fertilize the egg of the same Californian sea urchin, _Strongylocentrotus purpuratus_, with the sperm of a very closely related sea urchin, _S. franciscanus_, the hereditary effect of the spermatozoön is seen very distinctly in the primitive skeleton formed by the larva.[36] (Fig. 3.) In the case of the heterogeneous hybridization the spermatozoön acts practically only as an activating agency upon the egg and not as a transmitter of paternal qualities. [36] Loeb, J., King, W. O. R., and Moore, A. R., _Arch. f. Entwicklngsmech._, 1910, xxix., 354. [Illustration: FIG. 3. Five-days-old larvæ of two closely related forms of sea urchins (S. purpuratus ♀ and S. franciscanus ♂). In this case the larva has also paternal characters as shown by the skeleton.] The second striking fact is that while the sea-urchin eggs fertilized with starfish sperm develop at first perfectly normally they begin to die in large numbers on the second and third day of their development, and only a very small number live long enough to form a skeleton; and these are usually sickly and form the skeleton considerably later than the pure breed. It is not quite certain whether the sickliness of these heterogeneous hybrids begins or assumes a severe character with the development of a certain type of wandering cells, the mesenchyme cells; it would perhaps be worth while to investigate this possibility. The writer was under the impression that this sickliness might have been brought about by a poison gradually formed in the heterogeneous larvæ.