Growth and Senescence 



others; a genuinely morphogenetic senescence depending upon 

 alteration of cell-responses, and having evolved, or re-evolved, 

 within the phylogeny of vertebrates. The mammalian pattern 

 of ageing, if it differs from that of other vertebrates, would in 

 this case have evolved as a correlate, though not necessarily a 

 consequence, of several fundamentally important physiological 

 processes — homoeothermy, the development of a complex endo- 

 crine regulation centred in the pituitary, the avian-mammalian 

 pattern of determinate growth, which is linked with this 

 development, and the system of immune response and tissue 

 specificity characteristic of higher vertebrates. The possible 

 association between size-limitation and homoeothermy is inter- 

 esting in view of the different relationships between pituitary 

 and thyroid hormones in the determination of growth which 

 have been found in mammals and in amphibians (Evans, 

 Simpson and Pencharz, 1939; Scow and Marx, 1945; Stein- 

 metz, 1954); at some point in vertebrate evolution, a balance- 

 mechanism between thyroid and growth-hormone, which leads 

 to gigantism in the thiouracil-treated tadpole, has become con- 

 verted into a synergism such as normally operates in the rat or 

 in man. Unfortunately for any phylogenetic theory, the pattern 

 in fish appears to resemble that in mammals (Goldsmith et ai, 

 1944; Hopper, 1950). This subject will be further considered in 

 a subsequent chapter. 



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