CATALYZERS OF GROWTH 509 



while the growth of the entire animal is retarded. Evidently, there- 

 fore, there are in the body certain other and relatively bulky tissues of 

 which the growth is directly or indirectly retarded by tethelin. 



The most probable reason for this retardation lies in the varying 

 Metabolic Rates of the different tissues of the body and their consequent 

 differing success in the competition for nutrients. There are, broadly 

 speaking, two easily distinguishable groups of tissues in the animal 

 body which differ fundamentally in function and metabolism. These 

 are on the one hand the Parenchymatous Tissues, which are essentially 

 cellular, self-maintaining cells derived from the ectoderm and entoderm 

 of the three embryonic layers and on the other hand a variety of tissues 

 which originate mainly but not exclusively from the mesoderm and 

 constitute the Sclerenchyma or tissues of primarily structural or archi- 

 tectural significance. These latter tissues are dependent. They can 

 only arise through the activities of nucleated living cells, of which they 

 constitute outgrowths, secretions, or products of retrogressive change. 

 Of this character, for example, are the various fibrous tissues, the 

 elastic and calcified tissues, and the ligaments, tendons and other 

 structures which bind together and support the tissues of more varied 

 and complex function. The sclerous tissues have a low Metabolic Rate, 

 are among those which lose most heavily in the competition for a sub- 

 normal supply of nutrients and, since they are as a rule devoid of the 

 pow r er of multiplication or even of repair without the intervention and 

 assistance of other cells, we may legitimately infer that they do not 

 produce, as the parenchymatous tissues do, Endogenous Catalyzers 

 which accelerate their synthesis and degradation. In fact since their 

 synthesis is accomplished by other cells there would be no particular 

 purpose served by their doing so. Thus the horny cells of superficial 

 epidermis, which have lost the power of reproduction and growth in the 

 course of the degenerative changes which have resulted in their trans- 

 formation into Keratin, are renewed from time to time by the multi- 

 plication of the cells of the Malpighian layer of the deeper epidermis. 

 Cartilage and bone are similarly formed from cellular tissues and the 

 fibrous tissues are excretions or transformation-products of the Fibro- 

 blasts from which they originate. Even the muscular tissues may in 

 like manner originate from special cells which have retained the 

 potentiality of reproduction. But if these tissues do not produce 

 endogenous catalyzers and in many cases cannot form the material of 

 which they are composed, it is evident that growth-catalyzers from 

 other sources can only affect their development in the indirect fashion 

 of promoting the growth or multiplication of the cells or other tissues 

 from which they arise. 



A catalyzer of growth may accelerate the formation of parenchyma- 

 tous tissues, but its exceptional abundance or potency may actually 

 retard the growth of the tissues which are not directly affected by it, 

 through the deflection of nutrients to the parenchymatous elements. 

 An important proportion of the total increment in weight of an animal 



