476 The University Science Bulletin. 



and that this metabolic rate increases markedly in lower forms after 

 starvation or when they are regenerating lost parts. 



The question of growth regulation may, therefore, be temporarily 

 set aside for a study of the conditions which regulate metabolism in 

 general in the cell. As the tables for growth clearly show, the 

 metabolic rate in the organism is never constant (Child and Thomp- 

 son).^ In early embryonic life the increment is high, then it gradu- 

 ally decreases, with irregularities, up to the death of the individual. 

 In the early stages of development the blastomeres are simple, ex- 

 panding, growing cells. Later they differentiate and assume a func- 

 tional state. That this differentiation and the development of func- 

 tion is brought about by the continuous changing environment ef- 

 fected by the growth and filling up of cells in the growing individual 

 has been clearly indicated by many already well-known studies of 

 experimental embryology.^ What we have not known are (1) the 

 nature of those conditions which bring about these changes, and (2) 

 the importance of these changes for the life of the organism itself. 



In later life we have learned to recognize the fact, however, that 

 all tissues do not become actively functioning cells. A part of the 

 mesenchyme changes to muscle and glands, while another part re- 

 mains in what appears to be a more primitive state, such as the cells 

 of the interstitial tissue, bone and cartilage. The same is true for 

 epithelium. A part is used for covering and protection and another 

 part becomes functioning glands. For the sake of simplicity, I shall 

 call the cells w^iich are active in the glands, in the muscle, etc., as 

 functioning cells, in contradistinction to those of the connective tis- 

 sue, bone, cartilage and the epithelial coverings, the nonfunctioning 

 cells. This is not a strict use of the terms, but the latter resemble 

 more closely in their behavior the undifferentiated cells of early life. 



There was a time when it was thought that all life processes cen- 

 tered within the cell. At the present time there is some question 

 whether this is true. The primary changes in early development ap- 

 pear to be a rapid swelling of the mass, a rapid inhibition of water 

 and oxygen, a rapid elimination of CO2, and a secondary splitting 

 of the whole into cells. Cellular growth, division and differentiation 

 are not primary activities, but apparently secondary to other more 

 formative forces or stimuli. Life does not manifest itself the same 

 throughout the life of the animal. In the beginning it is recognized 

 chiefly by the rapid expansion of the whole, the rapid division of 

 cells and the careful building of its various parts. This period of 

 building is completed in man, as is well known, within ten days after 



