xv THE PHILOSOPHY OF ZOOLOGY 641 



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. 18), 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 embryological experiments 

 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 of a species, 

 especially in the early stages, are not of a sufficiently strongly 

 marked character to permit of any definite conclusions being 

 arrived at regarding inheritance from one parent as against the 

 other. Recourse has, therefore, been had to crossing between 

 distinct species or distinct genera, or even between the members of 

 distinct classes. In such experiments the Echinoderms have 

 proved capable of affording the most convenient material, and have 

 been very largely employed. It has been found to facilitate suc- 

 cessful crossing between distinct kinds of Echinoderms if the ova 

 experimented with are first treated by certain methods which 

 have been found to prepare the way for the process of artificial 

 parthenogenesis briefly referred to in the Introduction (Vol. I., 

 p. 22). The ova so treated are then shaken violently in a tube. 



