726 Comparative Animal Physiology 



ical reactions or acts only as a biocatalyst. In view of the inherent impos- 

 sibihty of arriving at a clear and restricted definition of a hormone, it is 

 proposed in this chapter to deal only with those chemical coordinators which 

 have come to be considered by the majority of investigators in those fields 

 as endocrines. 



The important point to emphasize for comparative physiological purposes 

 is that in a number of phyla and classes of animals special chemical sub- 

 stances essential to the integrative activities of the body are produced. The 

 point of origin within the organism, the specific chemical nature of the hor- 

 mone, and the routes or forces of transport are secondary in importance to 

 the abilities of these hormones to induce certain characteristic, and often 

 differential, effects within the animal. Furthermore, the nature of the effects 

 produced depends as much on the nature of the reacting tissues as on the 

 chemical properties of the circulating hormone. Hormones spreading random- 

 ly through the body in the body fluids are obviously powerless to produce 

 tissue and organ differentiation, or induce any directed or. organized activi- 

 ties in the absence of an underlying gene-determined differentiation. The 

 activities of hormones are in a sense, therefore, superimposed on the basic pat- 

 tern of the organism, bringing into functional integration the numerous and 

 complex latent differentiations. In a study of endocrine mechanisms the 

 phylogenetic and ontogenetic development of ability to respond to an en- 

 docrine is therefore as important as the appearance of the endocrine itself. 



HORMONES AND GENERAL GROWTH AND DIFFERENTIATION 



Vertebrates. Vertebrates are characterized by a growth and differentia- 

 tion in which the organism increases in size through the accumulation of 

 water, salts, and organic constituents. Not only is there an absolute inerease 

 in the quantities of these substances, but there are also during this period 

 characteristic changes in the ratios of these within the body until growth 

 terminates in the adult. The latter period is one where a balance of con- 

 structive and destructive phases of metabolism is reached and a steady state 

 is maintained. Variations from this steady state are largely the results of fat 

 depositions; the proportions of protein, ash, and water remaining strikingly 

 constant. Hormones are known to operate not only in regulating the growth 

 and differentiation processes but also in the maintenance of the final steady 

 state of the adult. We shall first treat the developmental processes, although 

 there is actually no true separation between the hormonal control of growth, 

 on the one hand, and of general metabolism, on the other. 



The anterior lobe of the pituitary is essential to normal growth in the 

 mammal, its importance being least in early embryonic development, but 

 gradually increasing thereafter. The subject has been well reviewed by 

 Long.^''^ Hypophysectomy results in the rapid cessation of growth in 80-100 

 gm. rats, and injection of crude alkaline pituitary extracts into such animals 

 restores growth. Such extracts also accelerate growth in normal young mam- 

 mals or stimulate further growth in those whose growth-curve has reached 

 a plateau. The additional growth that results resembles quite closely normal 

 growth and gigantism. One of the important sites of action of the growth- 

 promoting pituitary extracts is the epiphyseal cartilage. Histological evidence 

 of an activation of this tissue is an early response to the extract injections. 



