Influence of Environment on Animals 199 



and if so, what is their character ? In all cases of heterogeneous 

 hybridization the vitality of the egg or the embryo seems 

 weakened and it is still doubtful whether any heterogeneous 

 hybrid can reach maturity. The number of experiments is still 

 limited and this statement is therefore not yet final. 



So far as the question of heredity is concerned, all the 

 experiments on heterogeneous hybridization of the egg of 

 the sea-urchin with the sperm of star-fish, brittle-stars, crinoids, 

 and mollusks have led to the same result, namely, that the larvae 

 have purely maternal characteristics and differ in no way from 

 the pure breed of the form from which the egg is taken. By way 

 of illustration it may be said that the larvae of the sea-urchin 

 reach on the third day or earlier (according to species and 

 temperature) the so-called pluteus stage, in which they possess 

 a typical skeleton (Fig. 10, p. 11); while neither the larvae 

 of the star-fish nor those of the moUusk form a skeleton at the 

 corresponding stage. It was, therefore, a matter of some 

 interest to find out whether or not the larvae produced by the 

 fertilization of the sea-urchin egg with the sperm of star-fish 

 or moUusk would form the normal and typical pluteus skeleton. 

 This was invariably the case in the experiments of Godlewski, 

 Kupelwieser, Hagedoorn, and the writer. These hybrid larvae 

 were exclusively maternal in character. 



It might be argued that in the case of heterogeneous hybridi- 

 zation the sperm nucleus does not fuse with the egg nucleus, and 

 that, therefore, the spermatozoon cannot transmit its hereditary 

 substances to the larvae. But these objections are refuted 

 by Godlewski's experiments, in which he showed definitely that 

 if the egg of the sea-urchin is fertilized with the sperm of a 

 crinoid the fusion of the egg nucleus and sperm nucleus takes 

 place in the normal way. 



h) Artificial parthenogenesis. — Possibly in no other field of 

 biology has our ability to control life phenomena by outside 

 conditions been proved to such an extent as in the domain of 



