GENERAL CHARACTERISTICS OF GROWTH-PROCESS 473 



instead of being interrupted when half-completed by birth, as it is in 

 human beings. Corresponding to this we find that the guinea-pig is 

 born at a much more advanced stage of development than man or the 

 rat or mouse; their eyes are open, they have a full coat of hair, are able 

 to choose and eat their own food and may be weaned altogether within 

 a few days after delivery. The very general occurrence of three growth 

 cycles in mammalian development renders very inviting the supposi- 

 tion that they are referable to the existence of three Embryonic Layers, 

 from one or other of which all the tissues of the adult are ultimately 

 derived, but for this hypothesis there are as yet lacking the necessary 

 experimental and anatomical proofs. 



In the accompanying figure (Fig. 31) are compared the growth- 

 diagrams of human males of British birth and parentage and of male 



60 KILOGRAMS 



WEEKS 



FIG. 31. Growth of human males. (Con- 

 structed from the data obtained by the British 

 Anthropometric Committee.) 



Growth of male white mice. 



white mice. The resemblance between the two curves, allowing for 

 the difference of the time-units employed, is of a very striking character. 

 The only notable difference lies in the relatively marked delay of the 

 third, or adolescent growth-cycle in man as cpmpared with the mouse, 

 the possible origin of which will be discussed subsequently. 



These Growth-cycles, so definitely situated in the curve of growth, 

 and so invariable in their occurrence that they may be clearly recog- 

 nised in the growth of mice no less than in the growth of man, must 

 have some very definite physiological significance, and since, as we have 

 seen, growth is essentially a chemical process resulting in the synthesis 

 of living tissue from inanimate materials, these growth-cycles must 

 have a chemical, no less than a physiological significance. The general 

 similarity of the fundamental phenomena of growth in all living forms 



