104 Cell Structure in Relation to Heredity 



gressive differentiation from the mother-nucleus. The division of 

 the cell-body is completed midway between the two daughter-nuclei. 

 In animal cells, which possess no chemically differentiated membrane, 

 separation is effected by simple constriction, while in the case of 

 plant cells provided with a definite wall, the process begins with the 

 formation of a cytoplasmic separating layer. 



The phenomena observed in the course of the division of the 

 nucleus show beyond doubt that an exact halving of its substance is 

 of the greatest importance 1 . Compared with the method of division 

 of the nucleus, that of the cytoplasm appears to be very simple. 

 This led to the conception that the cell-nucleus must be the chief if 

 not the sole carrier of hereditary characters in the organism. It is 

 for this reason that the detailed investigation of fertilisation phe- 

 nomena immediately followed researches into the nucleus. The 

 fundamental discovery of the union of two nuclei in the sexual 

 act was then made 2 and this afforded a new support for the correct 

 conception of the nuclear functions. The minute study of the 

 behaviour of the other constituents of sexual cells during fertilisation 

 led to the result, that the nucleus alone is concerned with handing 

 on hereditary characters 3 from one generation to another. Especially 

 important, from the point of view of this conclusion, is the study of 

 fertilisation in Angiosperms (Flowering plants); in these plants the 

 male sexual cells lose their cell-body in the pollen-tube and the 

 nucleus only — the sperm-nucleus— reaches the egg. The cytoplasm 

 of the male sexual cell is therefore not necessary to ensure a trans- 

 ference of hereditary characters from parents to offspring. I lay stress 

 on the case of the Angiosperms because researches recently repeated 

 with the help of the latest methods failed to obtain different results. 

 As regards the descendants of angiospermous plants, the same laws 

 of heredity hold good as for other sexually differentiated organisms ; 

 we may, therefore, extend to the latter what the Angiosperms so 

 clearly teach us. 



The next advance in the hitherto rapid progress in our know- 

 ledge of nuclear division was delayed, because it was not at once 

 recognised that there are two absolutely different methods of nuclear 

 division. All such nuclear divisions were united under the head of 

 indirect or mitotic divisions ; these were also spoken of as karyo- 

 kinesis, and were distinguished from the direct or amitotic divisions 

 which are characterised by a simple constriction of the nuclear body. 

 So long as the two kinds of indirect nuclear division were not clearly 



1 First shown by W. Roux in 1883. 



2 By 0. Hertwig in 1875. 



3 This was done by 0. Hertwig and the author of this essay simultaneously in 

 1884. 



