GROWTH AND REPRODUCTION. 223 



cells is certain ; an analysis of their precise nature — the final 

 problem of histology — is still far in the distance. AVe cannot 

 get within miles of it. The problem has always loomed before 

 embryologists and histologists, — the historians and mechanicians 

 of the organism. Pander, in the first quarter of this century, 

 was inquiring into the mechanics of development, and Lotze 

 followed him with some luminous suggestions. The task has 

 been continued by His and Rauber; while the experimental 

 investigations of O. Hertwig, Fol, Pfliiger, Born, Roux, Schultze, 

 Gerlach, and others, have added further stepping-stones. 

 Observers such as Van Beneden and Boveri, in their masterly 

 accounts of the morphological facts, have not left the pro- 

 blem of the actual dynamics unessayed; while the title of 

 Berthold's book on " Protoplasmic Mechanics," shows how 

 the biologist persistently seeks the aid of the student of 

 physics in his endeavour to explain the architecture of the living 

 organism. 



§ 4. Proioplas7nic Restatement. — In the above helpful sugges- 

 tion, Spencer has emphasised the reasonableness and general 

 necessity of cell-division at the limit of growth, refraining from 

 the deeper question of the actual mechanism involved. In 

 truth such cautious reserve must still be maintained, but Spencer's 

 analysis admits of being expressed in lower and more definite 

 terms. The early growth of the cell, the increasing bulk of 

 contained protoplasm, the accumulation of nutritive material, 

 correspond to a predominance of protoplasmic processes, which 

 are constructive or anabolic. The growing disproportion between 

 mass and surface must however imply a relative decrease of 

 anabolism. Yet the life, or general metabolism, continues, and 

 this entails a gradually increasing preponderance of destructive 

 processes, or katabolism. As long as growth continues, the 

 algebraic sum of the protoplasmic processes must of course be 

 plus on the side of anabolism, and growth may be now more 

 precisely defined as the outcome of the preponderance of 

 an anabolic tendency, rhythm, or bias. The limit of growth, 

 when waste has overtaken and is beginning to exceed the 

 income or repair, corresponds in the same way to the maximum 

 of katabolic preponderance consistent with life. The limit of 

 growth is the end of the race between anabolism and katabolism, 

 the latter being the winner. Thus cell-division occurs especi- 

 ally at night, when nutrition is at a standstill, and when there 

 is therefore a relative katabolic preponderance ; and so explorers 



