SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 263 



SUMMARY. 



1. Sexual maturity generally occurs towards the limit of growth, is 

 marked by liberation of reproductive elements and by secondary charac- 

 teristics, due to the reaction of the reproductive function on the general 

 system. Precocious maturity may be due to constitutional or environ- 

 mental conditions, and has been of much importance in the evolution of 

 flowering plants. 



2. Ivlenstruation is interpreted as a means of getting rid of the anabolic 

 surplus of the female in absence of its foetal consumption. 



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

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



4. 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. 



5. Early nutrition is usually an absorption of the yolk, but in mammals 

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

 that of the foetus. 



6. Lactation is interpreted as an anabolic overflow. 



7. 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. 



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

 classes. 



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

 they are nearly akin. Reproduction may ward off death from the Proto- 

 zoon, but in the simplest Metazoa it probably caused it. 



10. 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 

 Weismann's papers — "Heredity," Oxford, 1889; while a full biblio- 

 graphy 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. Weismann, " Ueber die Dauer des Lebens," Jena, 1882 ; " Ueber 

 Leben und Tod," Jena, 1884; E. Maupas, "Archives de Zoologie 

 experimentale," 1888. 



