624 ZOOLOGY SECT. 



deprived of its nucleus, and a sperm of another species be intro- 

 duced, the ovum develops into an embryo which has the special 

 peculiarities of the embryo of the second species, and not of that 

 of the first ; it being thus proved that the cytoplasm of the ovum 

 is not active in the transmission of special characters, while the 

 sperm is. In this case the transmission of the special characters 

 of the second species must obviously be due to the sperm ; and 

 since the cytoplasm of the ovum seems to be inactive in this 

 direction, it is likely that the same is true of the cytoplasm 

 of the sperm, and thus the conclusion is arrived at that it is 

 nucleus, and not cytoplasm, through which the transmission of 

 specific characters takes place. But during ordinary impregnation 

 it has been shown that the penetration of the sperm into the 

 ovum involves the entry, not only of a nucleus (the male pro- 

 nucleus), but of a centrosome ; and it is quite possible that the 

 latter may take a share in transmission. If, as appears probable, 

 the nucleus is the chief vehicle in this process, the chromatin 

 loops are the parts of the nucleus which, on account of their 

 regularity of form and the regular nature of the changes which 

 they undergo during cell-division, are usually set down as contain- 

 ing the germinal matter. Accordingly, certain theories of an 

 ideal character, which there is not space to touch upon here, 

 have been put forward, aiming at enabling us to picture to our- 

 selves the way in which the hereditable characters are stored 

 up, combined, and transmitted. It is well to bear in mind, 

 however, that there are other structures in the nucleus besides 

 the chromatin loops structures which, though less conspicuous 

 optically, may have as much to do with transmission as the 

 chromatin. 



Though the experiment referred to above, of substituting a 

 sperm of one species for the nucleus of an ovum of another, and 

 obtaining as a result the development of a larva with the special 

 characters of the larva of the former species, shows that the 

 nucleus of the sperm bears some of the more superficial features 

 by which one species is. distinguished from another, it by no 

 means proves that the cytoplasm of the ovum does not bear many 

 of the more general characters ; the cytoplasm of the ovum of an 

 Echinus may be the bearer of the more fundamental characters 

 by which an Echinid differs from other Echinoderms and by 

 which Echinoderms differ from members of other phyla. In short, 

 so far as these experiments go, the cytoplasm of the oosperm may 

 be the bearer of all the more ancient features the nucleus only 

 of those which are more recently acquired. 



It has been urged in connection with the question of heredity, 

 that what is transmitted from generation to generation is not so 

 much matter as energy. The quantity of matter is always 

 relatively small; the important fact appears to be that this 



