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, 8zc., 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 Embryolog}'. 2 vols. 



London, iSSi. 

 Bell, F. Jeffrey. — Comparative Anatomy and Phvsiolog}-. London, 



1S85. 

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



2 vols. London, 1885. 

 Geddes, p. — Op. cit. 

 Gegexbaur, C. — Elements of Comparative Anatomy, trans, by Prof. 



Jeffrey Bell. London, 187S. 

 Haddon, A. C. — An Introduction to the Studv of Embryologv. London, 



1887. 

 Hensex, V. — Op. cit. 

 Hertwig, O. — Lehrbuch der Entwicklungsgeschichte des Menschen und 



der ^Virbelthiere. Jena, iSSS. 

 Hatchett Jackson's (\V.) Edition of Rolleston's Forms of Animal Life. 



Oxford, 1S88. 

 Huxley, T. H. — Anatomy of Yertebrate and Invertebrate Animals. 



London, 1 871 and 1877. 

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



Oxford, 1882. And similar works. 

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



Ward. Cambridge, 1887. 

 Yixes, S. H. — Yegetable Reproduction (Ency. Brit.). Lectures on the 



Physiolog}' of Plants. Cambridge, 1886. 

 WiEDERSHElM, R. — Elements of the Comparative Anatomy of Yerte- 



brates, trans, by Prof. AY. X. Parker. London, 1SS6. Also un- 



abridired work. 



