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’. 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? 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® 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 O. Hertwig in 1875. 
® This was done by O. Hertwig and the author of this essay simultaneously in 
1884. 
