58 



THE EVOLUTION OF SEX. 



SUMMARY. 



The gradual differentiation of essential sexual organs in animals, — isolated 

 cells, aggregated tissues, definite organs. 



Associated male and female ducts for the liberation of- male elements, fertili- 

 zation, exit of ova, or birth of embryos. 



Yelk-glands, &c, for nourishment and equipment of the ova. Vitellaria have 

 been interpreted as degenerate ovaries. 



Illustrations of organs auxiliary to impregnation. In the male — penis, storing 

 sacs, spermatophore-making organs, " claspers." Curiosities, such as the hecto- 

 cotylus of cuttlefishes, and the Cupid's dart of snails. Adaptations in the female 

 are less frequent, but storing receptacles for the male elements are common. 



Egg-laying organs : — frequency of ovipositors. 



Brood-pouches and the like are widely present in most classes of animals. 



LITERATURE. 



Balfour, F. M.— A Treatise on Comparative Embryology. 2 vols. London, 



1 881. 

 Bell, F. Jeffrey. — Comparative Anatomy and Physiology. London, 1885. 

 Claus, C— Elementary Textbook of Zoology, trans, by A. Sedgwick. 2 vols. 



London, 1885. 

 Geddes, P. — Op. cit. 

 Gegenbaur, C— Elements of Comparative Anatomy, trans, by Prof. Jeffrey 



Bell. London, 1878. 

 Haddon, A. C. — An Introduction to the Study of Embryology. London, 1887. 

 Hensen, V. — Op. cit. 

 Hertwig, O.— Lehrbuch der Entwicklungsgeschichte des Menschen und der 



Wirbeltheire. Jena, 1888. 



