506 GENERAL PHYSIOLOGY 



cannot be distinguished from the protoplasm of the ovum after it 

 has appeared in the latter, while the characteristic and profound 

 changes caused by fertilisation appear in the nucleus alone. To 

 the more critical minds, the weakness of the arguments, upon which 

 rested the theory of the dominance of the nucleus in heredity, was 

 painfully evident, and they sought after unequivocal proofs of the 

 theory. 



The fundamental fact, which Nussbaum ('84, '86) established in 

 Infusoria, that non-nucleated pieces of a cell after some time 

 invariably die, while nucleated pieces are regenerated into com- 

 plete cells and continue to reproduce by cell-division, was 

 experimentally confirmed in other Infusoria by Gruber ('85), and 

 brought forward as a direct proof of the dominance of the nucleus. 

 Gruber ('86, 1) says : " By a purely empirical method we are here 

 placed before the undeniable fact that the nucleus is the most 

 important, the species-maintaining, constituent of the cell, and that 

 to it is rightly ascribed the highest significance in the processes of 

 fertilisation and hereditary transmission." But Gruber forgets 

 that, in order that the nucleus may be established as the sole 

 species-maintaining constituent of the cell, the reverse experiment 

 must also be made, namely, the investigation of the nucleus with- 

 out the protoplasm. If the nucleus then continues to live, if it 

 regenerates a new protoplasmic body and forms a complete 

 individual, the experiment would be, in fact, undeniable proof 

 of the all-important significance of the nucleus. But if it perishes 

 without regeneration, like the protoplasm deprived of its nucleus, 

 no reason then exists for ascribing more to the nucleus than to the 

 protoplasm ; with equal right the protoplasm could then be spoken 

 of as the species-maintaining constituent of the cell. Such an 

 experiment has been performed and shows that the nucleus 

 deprived of its protoplasm perishes like non-nucleated proto- 

 plasm. In the large radiolarian Thalassicolla (Fig. 171, p. 380) the 

 nucleus, which is visible to the naked eye, can by a skilful operation 

 with delicate instruments be removed uninjured from the 

 protoplasm of the central capsule, and be observed isolated. 

 The result is that, even when it is protected from all injury, after 

 some time it invariably dies without a trace of regenerative pheno- 

 mena being seen. 1 The same may be observed in Infusoria. 

 Such a result breaks the force of Gruber's argument. 



Another experiment, which is claimed to support the theory of 

 the dominance of the nucleus, was performed by Boveri ('89) upon 

 the eggs of the sea-urchin. In connection with the fact observed 

 by the brothers Hertwig ('87), that non-nucleated pieces of 

 protoplasm of the ova of sea-urchins are capable of being fertilised 

 by spermatozoa, Boveri found that these fertilised pieces develop 

 into dwarf larval forms, which, apart from their small size, are 



1 CJ. Verworn ('91), 



