THE MECHANISM OF LIFE 507 



wholly like normal larvas. Boveri employed this fact in experi- 

 ments upon the cross-fertilisation of non-nucleated pieces of ova of 

 one species of sea-urchin with spermatozoa of another species, 

 especially non-nucleated pieces of ova of Sphcenchinus granularis 

 with spermatozoa of Echinus tnicrotuberculatus. He shook a 

 number of eggs of Sphcer echinus in a test-tube, by which treatment 

 non-nucleated pieces of protoplasm were broken off, and fertilised 

 the shaken liquid with sperm of Uchinus. It was not possible to 

 fertilise isolated non-nucleated pieces, since cross-fertilisation 

 between the two forms takes place relatively very seldom. Among 

 the larvae obtained by fertilisation were the following : 



a. Bastards, such as were always obtained by the crossing of the 

 two species. 



b. Dwarf bastards, obtained by the fertilisation of nucleated 

 pieces. 



c. Dwarfs possessing genuine Echinus characters, obtained by the 

 fertilisation of non-nucleated pieces. 



According to Boveri, the occurrence of the last-mentioned 

 larval forms is a direct proof of the theory of nuclear dominance ; 

 for, since from one species of sea-urchin only non-nucleated pro- 

 toplasm from the ovum was transmitted, while from the other the 

 nucleus of the spermatozoon, the result, namely, larvee of the 

 paternal form, proves that the nucleus alone can be the bearer 

 of the hereditary substances. Critical examination, however, 

 shows that this experiment, which thus far has been considered 

 by many as the strongest support of the theory of dominance, 

 appears as such inadequate in more than one respect. In the 

 first place, the derivation of dwarf larvae of the type of the paternal 

 species can be doubted. Since the fertilisation of non-nucleated 

 pieces of ova of one species with spermatozoa of the other species 

 was not carried on isolated, it is very questionable whether the 

 larvae in question were really derived from such a fertilisation. 

 It is conceivable that larvae pre-eminently of the paternal form can 

 develop from the fertilisation of nucleated pieces of ova or whole 

 ova of one form with spermatozoa of the other ; we see that very 

 frequently the characteristics of either the father or the mother 

 are transmitted pre-eminently to the offspring. But the various 

 larval forms that Boveri obtained possess in the developmental 

 stage in question so few distinguishing marks, that from their 

 presence conclusions ought not to be drawn with certainty regard- 

 ing the one-sided derivation. Yet, even if the explanation that 

 Boveri gives of the derivation of the larvae is to be accepted, the 

 experiment is far from conclusive. It will be conclusive only 

 when the nucleus alone, not the whole spermatozoon, unites with 

 the non-nucleated protoplasm of the ovum. If in this case larvse 

 of the character of the father appear, we shall be obliged to ac- 

 knowledge that the nucleus alone is the bearer of hereditary 



