250 Influence of environment on animals 



But granted that such hybridisations were possible, would they 

 have influenced the character of the fauna ? In other words, are the 

 hybrids between sea-urchin and starfish, or better still, between 

 sea-urchin and mollusc, capable of development, and if so, what is 

 their character? The first experiment made it appear doubtful 

 whether these heterogeneous hybrids could live. The sea-urchin 

 eggs which were fertilised in the laboratory by the spermatozoa of 

 the starfish, as a rule, died earlier than those of the pure breeds. 

 But more recent results indicate that this was due merely to 

 deficiencies in the technique of the earlier experiments. The writer 

 has recently obtained hybrid larvae between the sea-urchin egg and 

 the sperm of a mollusc (Chlorostoma) which, in the laboratory, 

 developed as well and lived as long as the pure breeds of the sea- 

 urchin, and there was nothing to indicate any difference in the 

 vitality of the two breeds. 



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

 ments on heterogeneous hybridisation of the egg of the sea-urchin 

 with the sperm of starfish, brittle-stars, crinoids and molluscs, 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 ; while neither the larvae of 

 the starfish nor those of the mollusc form a skeleton at the corre- 

 sponding stage. It was, therefore, a matter of some interest to find 

 out whether or not the larvae produced by the fertilisation of the 

 sea-urchin egg with the sperm of starfish or mollusc 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 hybridisa- 

 tion 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 fertilised with the sperm of a crinoid the fusion of the 

 egg-nucleus and sperm-nucleus takes place in the normal way. It 

 remains for further experiments to decide what the character of the 

 adult hybrids would be. 



(b) 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 



