PHYSIOLOGY OF SEX AND REPRODUCTION. 243 



SUMMARY. 



I. Sexual maturity generally occurs toward the limit of growth, is marked 

 by liberation of reproductive elements and by secondary characteristics, due to 

 the reaction of the reproductive function on the general system. Precocious 

 maturity may be due to constitutional or environmental conditions, and has 

 been of much importance in the evolution of flowering plants. 



II. Menstruation is interpreted as a means of getting rid of the anabolic 

 surplus of the female in absence of its foetal consumption. 



III. Sexual union, at first very passive and random, becomes active and 

 definite with the gradual evolution of sex and secondary sexual organs. 



IV. Birth is at first accomplished by rupture, but becomes a definite process 

 usually effected through special ducts. Oviparous and viviparous birth only 

 differ in degree. 



V. Early nutrition is usually an absorption of the yelk, but in mammals is 

 accomplished by osmotic transfusion from the blood of the mother to that of the 

 foetus. 



VI. Lactation is interpreted as an anabolic overflow. 



VII. Besides milk, there are other secretions associated with the nutrition 

 and sheltering of the young. Pigeon's milk, edible birds' nests, and the mucous 

 threads of sticklebacks, are illustrations. 



VIII. Incubation, reaching a climax in birds, is paralleled in many other 

 classes. 



IX. Reproduction and death both represent katabolic crises. Primitively, 

 they are nearly akin. Reproduction may ward off death from the Protozoon,. 

 but in the simplest RIetazoa it probably caused it. 



X. The Protozoa come nearer immortality than other organisms. The fact 

 of germinal continuity involves an organic immortality. 



LITERATURE. 



For the special physiology of sex and reproduction consult standard text- 

 books such as those of Foster, Landois and Stirling, and especially Hensen's 

 work already often cited. 



On the continuity of the germ-plasma, consult recent translation of Weis- 

 mann's papers — "Heredity," Oxford, 18S9; while a full bibliography will be 

 found in " History and Theory of Heredity," by J. A. Thomson, Proc. Roy. 

 Soc. Edin., 1888; and, since 1886, in the Zoological Record. 



On the nemesis of reproduction, and on organic immortality, see A. Goette, 

 "Uber den Ursprung des Todes," Hamburg and Leipzig, 1883 ; and A. YYeis- 

 mann, " Ueber die Dauer des Lebens," Jena, 1882 ; " Ueber Leben und Tod," 

 Jena, 1884; E. Maupas, " Archives de Zoologie exp£rimentale, " 1888. 



