64 THE EVOLUTION OF SEX. 



SUMMARY. 



1. The gradual differentiation of essential sexual organs in animals, — 

 isolated cells, aggregated tissues, definite organs. 



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

 fertilisation, exit of ova, or birth of embryos. 



3. Yolk-glands, &c. , for nourishment and equipment of the ova. 

 Vitellaria have been interpreted as degenerate ovaries. 



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

 penis, storing sacs, spermatophore-making organs, " claspers. " Curiosities, 

 such as the hectocotylus of cuttle-fishes, and the Cupid's dart of snails. 

 Adaptations in the female are less frequent, but storing receptacles for the 

 male-elements are common. 



5. Egg-laying organs : — frequency of ovipositors. 



6. 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, 1881. 

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



1885. 

 Claus, C. — Elementary Text-Book of Zoology, trans, by A. Sedgwick. 



2 vols. London, 1885. 

 Geddes, p. — Op. cit. 

 Gegf.nbaur, C. — Elements of Comparative Anatomy, trans, by Prof. 



Jeffrey Bell. London, 1878. 

 H ADDON, A. C. — An Introduction to the Study of Embryology. London, 



1887. 

 Hensen, V. — Op. cit. 

 Hertvvig, O. — Lehrbuch der Entwicklungsgeschichte des Menschen und 



der Wirbelthiere. Jena, 1888. 

 Hatchett Jackson's (W. ) Edition of RoUeston's Forms of Animal Life. 



Oxford, 1888. 

 HuxLEV, T. H. — Anatomy of Vertebrate and Invertebrate Animals. 



London, 1871 and 1877. 

 Sachs, J. — Text-Book of Botany, edited by Prof. Vines. Second edition. 



Oxford, 1882. And similar works. 

 Lectures on the Physiology of Plants, trans, by Prof. Marshall 



Ward. Cambridge, 1887. 

 Vines, S. H. — Vegetable Reproduction (Ency. Brit.). Lectures on the 



Physiology of Plants. Cambridge, 1886. 

 Wiedersheim, R. — Elements of the Comparative Anatomy of Verte- 

 brates, trans. l)y Prof. W. N. Parker. London, 1886. Also un- 

 abridged work. 



