1914] Lafayette B. Mendel i73 



equivalent to construct a unit of new substance in young animals. 

 Rubner has also formulated conclusions regarding the length of life 

 of individual species in relation to the number of calories metab- 

 olized during life. Exact mathematical expressions have already 

 been devised for certain features of growth. In the light of our 

 scattered knowledge I can only agree with Friedenthal that in 

 biology it is best to avoid the term "law" and content ourselves 

 with generalizations which have their exceptions and are not abso- 

 lute in their comprehension. We are still far from the stage where 

 laws of growth can successfully be propounded. The path which 

 the natural sciences have successfully pursued in the past has not led 

 from proposed laws to facts, but rather in the reverse direction 

 from the collection of facts to generalized rules. This is likewise 

 the way which the research of the future must follow. 



It is almost impossible to review the salient features of the phys- 

 iology of growth without directing attention to the obvious gaps in 

 our knowledge thereof. Problems await us at every turn, — some 

 of them clearly defined and open to experimental investigation, 

 others obscured in the haze of conflicting data or complicated by the 

 manifold factors which enter into the questions of development. 

 No review of any aspects of a progressive and growing science is 

 complete or illuminating unless it suggests problems as well as 

 answers them. A passing reference to some of them may not be 

 inappropriate here. Statistics of growth being the easiest to ob- 

 tain, have been collected in greatest number. But there are numer- 

 ous Statistical details involving the growth of the individual parts 

 or Organs which are quite as essential for the understanding of 

 what is involved in the correlation between organs incidental to 

 growth. 



More than ever the need of additional studies of the histology 

 of growing tissues in mammals looms up at the present time. They 

 are fundamental to any investigation of the morphological back- 

 ground against which the growth functions are projected. The 

 popularity which histological investigation has enjoyed in embry- 

 ology in recent years should be extended into the postnatal stages 

 of life. In general we are taught that different tissues have unlike 

 power of growth in the sense of cell multiplication. Some, like the 

 testes, multiply their cells throughout life; others, like the muscles 



