io2 THE EVOLUTION OF SEX. 



SUMMARY. 



I. The ovum presents all the essential features of a cell ; its substance 

 and nucleus described. The chromatin-elements of the latter are the essential 

 parts. 



II. The ovum usually grows from an amoeboid to an encysted phase, with 

 increase of nutrition and size. 



III. The yelk is derived from the vascular fluid, or surrounding cells, or 

 special glands, and is present in varying quantity and disposition If little, it 

 is diffuse ; if much, it is polar or central ; and the different modes of egg- 

 division are associated with this. 



IV. In some cases the ovum is surrounded by a number of nutritive cells 

 (composite ova), and often becomes what it is by preying upon its neighbors. 

 This hardly affects its unicellular character. 



V. Egg-envelopes are produced from the ovum itself (for example, viteline 

 membrane), or from surrounding cells (folicular sheath), or from special glands 

 (the outside shell). 



VI. Bird's egg noted as a concrete illustration of facts and problems. 



VII. The egg, so far as its nutritive material is concerned, includes a 

 mixture of complex, unstable, highly nutritive substances. 



VIII. The maturation of the ovum is usually associated with a double cell- 

 division or budding, known as the extrusion of polar globules. In partheno- 

 genetic ova only one seems to occur. 



IX. This polar globule formation has been interpreted variously : — (a) As 

 an extrusion of male elements (Minot, Balfour, Van Beneden); (6) as an atavistic 

 occurrence of cell-division (Biitschli, Whitman, Hertwig, &c); (r) by Weis- 

 mann's more complex hypothesis. It seems to be a case of cell-division at the 

 limit of growth. 



LITERATURE. 

 Balfour, F. M.— Op. cit. 

 Van Beneden, E. — Recherches sur la Fecondation. Arch, de Biologie, 



IV., 1883. 

 Carnoy.— La Cellule II., 1886, &c. 

 Geddes, P. — Op. cit. 

 Haddon, A. C— Op. cit. 

 Hensen, V. — Fp.cit. 

 Hertwig, O. — Op. cit. 

 Hatchett Jackson, — Introduction to his edition of Rolleston's Forms of 



Animal Life. 

 McKendrick, J. G. — On the Modern Cell Theory, &c. Proc. Phil. Soc. 



Glasgow, XIX., 1888. 

 Minot, C. S.— American Naturalist. XIV., 1880. 

 Thomson, J. A. — Recent Researches on Oogenesis. Quarterly Joum. Micr. 



Sic, XXVI., 1886. 



Art Embryology, Chamber's Encyclopaedia. 



Weismann, A. — Die continuitat des Keimplasmas. Jena, 1885. 



Die Bedeutung der sexuellen Fortpflanzung. Jena, 1886. 



And other papers recently translated, " Heredity." Oxford, 1889. 



