IDIOPLASM OR GERM-PLASM 29 



follows that the idioplasm can only be located in the nucleus. This 

 conclusion is supported b} T the fundamentally important observation 

 that, in the process of fertilisation, the nuclei of the two gametes fuse 

 with one another in a characteristic manner. Further, when the 

 fertilised egg undergoes its first segmentation, the component parts of 

 the nuclear reticulum, the so-called chromosomes, split longitudinally 

 into halves, which distribute themselves between the two daughter-cells 

 in such a manner that each of the latter receives equal numbers of 

 " male " and " female " half-chromosomes [that is to say, equal chromo- 

 some-halves, derived respectively from the male and from the female 

 parent]. The daughter-nuclei are formed by the combination of these 

 half-chromosomes. Precisely the same apportionment of the chromo- 

 some-halves takes place at every subsequent cell-division. 



This characteristic behaviour on the part of the two sexual nuclei 

 and on that of the product of their union (the " fusion-nucleus " 

 or " cleavage-nucleus ") was first discovered by van Beneden in the 

 Nematode Ascaris megalocephala. Soon after, Strasburger, Hertwig, 

 Kolliker and Weismann, all came to the conclusion, that the nucleus, 

 or more precisely the nuclear reticulum which breaks up into the 

 chromosomes during nuclear division, must be regarded as the vehicle 

 of the hereditary substance. 



This conception of the nucleus as a structure which embodies the 

 hereditary properties of the organism, and as an organ which controls 

 the growth and differentiation of the cell, was promptly subjected to the 

 test of experiment both from the zoological and from the botanical 

 side. While the general relations of the nucleus to the processes of 

 regeneration were studied, particularly in the case of unicellular 

 organisms, special attention was directed to the differences in behaviour 

 exhibited by nucleated and non-nucleated fragments of protoplasts. 

 Thus Klebs showed that if a protoplast of a filamentous Alga, 

 such as Zygncma, Spirogyra, or Oedbgmiium be divided (by plas- 

 molysis with 16-25 per cent, solution of cane-sugar) into nucleated and 

 non-nucleated halves, it is the nucleated portion alone which is able to 

 clothe itself with a new 'cell-wall, to grow in length and generally 

 to resume the condition of a normal cell. The author has made a 

 similar observation with regard to the cells of the hairs of certain 

 CucuitBiTACEAE. In these the protoplast not infrequently becomes 

 separated into two parts owing to the very uneven deposition of thicken- 

 ing layers upon the lateral walls ; if cell-wall formation continues at all 

 after this separation has taken place, it is only the nucleated half of 

 the protoplast that produces new layers of cellulose (Fig. 2). Schmitz 

 and the author have further demonstrated, in the case of various 

 Siphonales, such as Valonia, Siphonocladus and Vaucheria, that an 



