HEREDITY 



377 



are inherited from the parent protoplasm and nucleus respectively. In the 

 chromatophores, however, we meet with difficulties at the very outset. So far 

 as we know, only the egg possesses these chromatophores, the sperm-cell has 

 none. Should a more thorough examination lead to another result — which is 

 quite possible — we should expect the chromatophores to fuse during fertilization 

 just as the nucleus and protoplasm do. According to the present point of view, 

 however, either the peculiarities of the chromatophores of the father are incapable 

 of being inherited, or they must be passed on by another portion of the cell, e. g. 

 the protoplasm or nucleus or both. The assumption that the inheritance of a 

 character by part of a cell which was not itself the natural carrier of this feature 

 cannot be avoided, since all the peculiarities of the cell are not associated with 

 those organs of the cell which are capable of division and which increase 

 by that means only. The wall of the vacuole with its contents and the external 

 protoplasmic layer with its product, the cell-wall, are, for example, parts of the 

 cell which are not directly transmitted from generation to generation. The 

 capacity of forming such parts, which undoubtedly is possessed by the eg§,, is 

 what is meant when we say that such a cell contains the initials of such organs. 

 We may now inquire whether there is any evidence for the definite localization 

 of these ' initials '. Although we do not really know what the ' initials ' are, we 

 must nevertheless assume that they are inherent in the protoplasm. It is not 

 probable that any selected part of the protoplasm functions as the bearer of the 

 initials, and yet since the time of Nageli (1884) it has been customary to 

 designate as ' idioplasm ' that part of the protoplasm which contained the 

 initials and induces the rest to go through a certain development, and to 

 describe the remainder of the protoplasm as trophoplasm. Nageli considered 

 the idioplasm as a network which extended throughout the entire cell, although 

 in recent times greater localization has been given to it. 



On the zoological side (Hackel, 1866) the nucleus is very generally claimed 

 to be the bearer of hereditary characters, and of the idioplasm (compare 

 Hacker, 1902), and on the botanical side, investigators, such as Strasburger 

 (1884) and De Vries (1889), have supported this view or endeavoured to prove 

 it independently. It may be pointed out, in the first place, that in the process 

 of fertilization the male element consists of a nucleus with little or no protoplasm. 

 That the idioplasm is localized in the nucleus has not satisfied every one, and 

 other hypotheses have been advanced, such as, for example, that the chromosomes 

 of the nucleus are the agents specially concerned in heredity. In this connexion 

 the changes preceding division of cells and fertilization must be kept in view. In 

 every normal cell division the characters of the mother-cell are equally divided 

 between the two resulting daughter-cells, and so also the units which act as 

 the bearers of the initials must be divided into exactly similar halves. This 

 tallies with the well-known division of the chromosomes, where the division 

 allots a longitudinal half to each daughter-cell. The bearers of individual initials 

 are believed to be serially arranged in the chromosome, and to correspond to 

 a certain degree with the chromatin-granules, which in certain cases may be 

 seen with the microscope, and are separated from each other by linin-threads. 

 These chromatin-granules, arranged longitudinally in the chromosome, are 

 necessarily present in the resting nucleus, although they cannot be seen 

 in it. The conclusion has been drawn, therefore, that the chromosomes 

 are persistent organs of the nucleus which can only multiply by division, and 

 moreover by longitudinal division, but which can never be created afresh. It 

 is very remarkable that at every division the chromosomes appear in the same 

 number, and their behaviour during fertilization in the animal kingdom is even 

 more remarkable still. There the so-called reduction division (p. 367) comes into 

 operation, ovum and sperm have only half the number of chromosomes found in 

 the somatic cells, and so the doubling of the number of chromosomes at each 

 fertilization is prevented. The fact that the sperm-nucleus contains the same 



