xv THE PHILOSOPHY OF ZOOLOGY 661 



brought forward which have been supposed to afford evidence of 

 the transmission of mutilations from parent to offspring ; but such 

 a transmission must, from the nature of the case, always be 

 extremely difficult to prove, and the majority, at least, of such 

 cases are found, on a careful analysis, to be capable of other inter- 

 pretations. On the other hand, though well-established cases of 

 the inheritance of mutilations would greatly support the doctrine 

 that acquired characters are transmissible, the negative results 

 that have attended certain experiments on mutilations are of little 

 value in the direction of proving that extrinsic variations cannot 

 be transmitted, since, as has already been pointed out, such 

 experiments in mutilation cannot be said to reproduce the con- 

 ditions under which an extrinsic variation might be supposed to 

 be transmitted ; the mutilation is instantaneous ; the variation 

 must be supposed to be the result of long-continued action, which 

 it might be expected, would have a sufficiently profound effect to 

 engraft it permanently on the organism. 



It should be pointed out here that there is no absolutely hard 

 and fast line to be drawn between the intrinsic and extrinsic 

 variations, since changes in the sexual cells may very well be due, 

 directly or indirectly, to influences exerted from without. The 

 material from which reproductive cells may subsequently be 

 fashioned is, in plants and in many animals, in such close and 

 intimate union so far as can be seen with the other proto- 

 plasmic elements of the organism, that it seems highly probable 

 that influences affecting the latter may in many cases affect also 

 the former. 



Another question that presents itself in connection with 

 heredity is : Can any special part of the germ-cell be fixed upon 

 as the part specially concerned in hereditary transmission ? Is it 

 by means of the nucleus alone that transmission takes place, or 

 does the cytoplasm take a share in the process ? The complicated 

 changes which the nucleus undergoes during mitotic division (Vol. I. 

 p. 16), with the definite form and (for each species) constant 

 number of the chromosomes, and their precise halving during the 

 process, tell strongly in favour of the view that the nucleus is the 

 vehicle of transmission rather than the, apparently, less highly 

 differentiated cytoplasm. But such evidence is far from amounting 

 to positive proof. 



To determine this point many series of ernbryological experi- 

 ments have been carried out. Should it prove possible to fertilise 

 by means of a sperm an ovum from which the nucleus had 

 previously been removed, and as a result to obtain an embryo, it 

 might be possible, by a process of exclusion, to get some light on 

 the influence exerted on normal development by the nucleus of the 

 ovum. For such experiments sperms and ova of the same species 

 are not well adapted, since the differences between the individuals 



