[macallum] cellular OSMOSIS AND HEREDITY 161 



In Darwin's theory of heredity, eveiy cell of the body gives off 

 typifying particles which he called gemmules and which collect in the 

 /ova and spermatozoa to reproduce in the offspring all the features of both 

 parents. This would account for the inheritance of immediately acquired 

 chanacters, but the impossibility of such inheritance is now generally ad- 

 mitted. The main objection, however, to the theory is that it is impos- 

 sible to conceive of such myriads of gemmules being accommodated in 

 (an ovum or spermatozoon. 



The theory of heredity which has to-day received the widest assent 

 is that of Weismann, whiich postulates that the inheritance of ancestral 

 ^characters is due to transmission in the genn cells from generation to 

 generation of a substance, the carrier of heredity, called the germ plasma, 

 which is not affected by metiabolic and other changes in the somatic 

 tissues of the individual, but which may spontaneously vary and thus give 

 rise to varieties in the offspring of the species. According to this theory 

 no immediately acquired characters are transmitted to the offspring. 



What provides for the protection of this germ plasma which is 

 handed down from generation to generation unchanged or changed only 

 in the fasliion tliat suggest the spontaneous origin of "sport'" modifica- 

 ?tions of forms, Weismann does not explain. 



It seems to me that the true explanation of heredity lies between the 

 theory of Darwin and that of Weismann. 



A germ plasma in the sense implied by Weismann may exist, but 

 on the view here advanced, its continuity is one of type rather than of 

 identical molecules, for the nuclear membranes of the germ cells sort 

 out or select from all the iron-holding nucleoproteins from the various 

 portions of the body that reach such germ cells those of a certain definite 

 fixed composition and any other nucleoproteins that may be present are 

 excluded from the nuclei of the ova and spermatid cells. Such selected 

 or sorted out iron-holding nucleoproteins may in a manner represent the 

 gemmules of Darwin's theory. Such compounds transmit the inherited 

 parental characters and, to continue tlie transmission in the offspring of 

 such characters, provide for the maintenance of the same type of nuclear 

 membrane in the germ cells of the offspring. 



Slight changes in the nuclear membrane of the germ cells would 

 provide for the variants or " sports'," as they are called, in the offspring, 

 but the nuclear membrane may itself be supposed to remain constant, 

 although this constancy does not demand that the iron-holding nucleo- 

 proteins which pass through it are of an absolutely unifonn type. Many 

 isomers may be supposed to occur amongst these compounds and yet 



Sec. IV., 1908. 11. 



