670 THE PHYSIOLOGY OF REPRODUCTION 



The various other external factors that influence growth 

 in animals of different kinds are discussed by Morgan in his 

 recent work on Experimental Zoology,^ to which the reader 

 is referred for an account of the literature of the subject. 



Puberty 



Puberty, or the period at which the organism becomes 

 sexually mature, is marked by the occurrence of those con- 

 stitutional changes whereby the two sexes become fully dif- 

 ferentiated. 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 remain within 

 the body cavity, it is at puberty that these organs first descend 

 into the scrotal sacs. The puberty acceleration in growth 

 which takes place in Man has been already referred to. This 

 change is accompanied, as is well known, by alterations in the 

 general proportions, associated with an increase of strength, 

 a deepening of the voice and a growth of hair on the face and 

 other parts of the body. In temperate climates puberty begins 

 in boys at about the fourteenth or fifteenth year ; in tropical 

 countries it is usually a few years earlier. It is at this period 

 that ripe spermatozoa first make their appearance in the seminal 

 fluid, which is henceforward secreted in considerable quantity. 



In women puberty occurs at a slightly earlier age than in 

 the male sex. The constitutional changes characterising this 

 period take place more suddenly in the female, the girl almost 

 at once becoming a woman, whereas the boy is several years before 

 he develops into a man. Moreover, the onset of puberty in the 

 girl is marked more precisely by the coming of menstruation, 

 which may make its appearance in temperate climates in the 

 thirteenth year. At about the same time the pelvis widens, 

 and the other characteristic anatomical changes take place ; 

 the subcutaneous layer of fat, the development of which assists 

 so largely in giving the body its graceful contour, is deposited ; 



^ Morgan, Experimental Zoology, New York, 1907. 



^ Disselhorst, " Gewichts- und Volumszunahme der manulichen Keim- 

 driisen," &o., Anat. Anz., vol. xxxii., 1908. 



