346 THE WORLD OF LIFE 



functions are performed ; the periodicity which occurs with the 

 greatest regularity under constant conditions of environment ; nor, 

 above all, the fact that the power of discharging all the operations 

 requisite for growth, nutrition, renovation, and multiplication is 

 liable to be lost. We call the loss of this power the death of the 

 protoplasm" (vol. i. p. 51). 



Growth by Cell-Division ; What it Implies 



As the account now given of the most recent discoveries 

 as to what actually takes place in the living cell preparatory 

 to its division and subdivision, which are the very first steps 

 in the growth or building up of the highly complex and 

 perfect animal or plant, is very technical, and will be perhaps 

 unintelligible to some of my readers, I will now give- a very 

 short statement of the process with a few illustrations, and 

 remarks as to what it all really means, and how alone, in my 

 opinion, it can possibly be explained. 



The egg is a single cell with a special central point or 

 organ, called the nucleus, and it is this nucleus which makes the 

 cell a germ-cell. That this is so has been proved in many 

 ways, — in plants by grafting or budding, where the flower-bud 

 which contains a germ-cell, when inserted in the bark of a 

 different variety, and sometimes a different species of plant, 

 reproduces the exact kind of flower or fruit that characterised 

 the tree or bush the bud was taken from, not that of the 

 plant of which it now forms a part, and whose sap forms its 

 nourishment. 



Again, Professor Boveri deprived an egg of a species of 

 sea-urchin (Echinus microtuberculatus) of its nucleus, and then 

 fertilised the egg with the spermatozoa of another species 

 (SphcErcchinus granularis). The egg so treated developed 

 larvae with the true characters of the latter species only, so 

 that the main substance of the egg provided nutriment for 

 the offspring, but did not transmit to it any of its parental 

 characters. A similar illustration, at a later period of life, is 

 that of an infant which from birth is fed on cow's milk, yet, 

 if it lives, possesses only human characteristics. 



This nucleus, therefore, which, when fertilised, has such 

 marvellous powers and properties, is the seat of heredity and 

 development. What is it that gives it this power? What 



