AN INTRODUCTION TO THE STUDY OF GROWTH. 4! 



which the contents of the egg are apparently inactive. 

 Cell multiplication and enlargement therefore appear to 

 be distinctly rhythmical processes, but it must be under- 

 stood that the cell, though seemingly quiet, is, in a 

 chemical sense, constantly active, and attention is here 

 directed only to the fact that the evident processes in 

 cell multiplication and enlargement are periodic ; like 

 similar phenomena this periodicity is particularly accen- 

 tuated at some times and in some species, though it 

 probably holds for all stages of growth in all animals, 

 man included. 



On bringing the foregoing facts together we find that 

 the animal body is composed of cells, which are the 

 lineal descendants of the fertilised ovum. Any active 

 cell, having reached a point at which it no longer 

 divides, grows larger, the cytoplasm increasing more 

 rapidly than the nucleus. Variations in structure 

 appear, the shape alters, and accompanying all these is 

 a steady change in chemical constitution. Chemically, 

 therefore, the cells are being continuously modified, 

 even though as structural elements they may have an 

 existence conterminous with that of the entire organism. 

 The complex animal when formed is composed of dif- 

 ferent groups of cells, which, though they are fed by the 

 same nutrient lymph, become structurally modified in 

 such a manner that they are suited to the performance 

 of limited and dissimilar functions, thus placing the 

 several systems of tissues in a relation of mutual depen- 

 dence. The size of the adult animal depends on the 

 number and volume of the constituent cells. In develop- 

 ment the determination of number precedes that of size, 

 but the conditions controlling both processes are mainly 

 resident in the cells themselves. The changes, however, 

 which lead up to the final form of the body do not occur 

 continuously, but are periodic, and the rate at which the 



