712 THE PHYSIOLOGY OF REPRODUCTION 



The various other external factors that influence growth in 

 ; animals of different kinds are discussed by Morgan in his work on 

 "Experimental Zoology," ^ to which the reader is referred for an 

 account of the literature of the subject. 



The internal factors or conditions which- control growth have 

 been recently investigated by Eobertson and Eay,^ who show that 

 early growth in mice is due to over-increase of the cellular elements 

 or parenchymatous tissue, as distinguished from the sclerous or 

 connective tissues. They state that early overgrowth is correlated 

 with longer life, the individuails displaying it being highly resistant 

 to external disturbing factors and tending towards a relative paucity 

 of tissue accretion late in life. Short-lived individuals, on the 

 contrary, are relatively unstable and sensitive to external influences, 

 and tend to display a relatively deficient early growth but rapid and 

 unstable accretions of connective tissue elements in late life. Such 

 accumulations are notoriously characteristic of old age, and their 

 presence throws a strain on the metabolism of the cellular elements 

 which support these accretions, and consequently tend to shorten 

 life. The administration of tethelin or cholesterol, which was found 

 to prolong life, led to an increased anabolism of the distinctively 

 cellular elements to the disadvantage, in the competition for 

 nutriment, of the connective tissue. If tethelin was given to mice 

 only so long as they were ^immature, the cessation of this stimulus was 

 followed by a great increase in size. Such increase was due to the 

 growth of the sclerous tissue, and added to the already highly 

 developed cellular tissue (hitherto not noticeable) resulted in 

 gigantic individuals. It was shown also that brain tissue from 

 which the cholesterol had been extracted was without effect upon 

 growth, whereas nervous tissues not so treated stimulated anabolism. 



Puberty 



Puberty, or the period at which the organism becomes sexually 

 mature, is marked by the occurrence of those constitutional changes 

 whereby the two sexes become fully differentiated. It is at this 

 period that the secondary sexual characters first become conspicuous, 

 and the essential organs of reproduction undergo a great increase in 

 size,' while in those animals in which during immaturity the testicles 



^ Morgan, Experimental Zoology, New York, 1907. See also Bobertson and 

 Ray, " Experimental Studies on Growth," Jour, of Biol. Chem., vol. xxiv., vol. 

 XXV., 1916, and vol. xxxvii., 1919. 



^ Robertson and Ray, "Experimental Studies of Growth," XV. and XVI., 

 Jour, of Biol. Chem , vols. xlii. and xliv., 1920. For Child's views on growth, 

 see above, p. 225. 



' Disselhorst, " Gewichts und Volumszunahme der mannliohen Keimdriisen, 

 etc.," Anat. Am., vol. xxxii., 1908. 



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