Io8 THE EVOLUTION OF SEX. 



SUMMARY. 



1. 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. 



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

 increase of nutrition and size. 



3. The yolk 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. 



4. 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 

 neighbours. This hardly aff'ects its unicellular character. 



5. Egg-envelopes are produced from the ovum itself (>?.,;?"., vitelline 

 membrane), or from surrounding cells (follicular sheath), or from special 

 glands (the outside shell). 



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



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

 mixture of complex, unstable, highly nutritive substances. 



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

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

 parthenogenetic ova only one seems to occur. 



9. This polar globule formation has been interpreted variously : — {a) 

 As an extrusion of male elements (Minot, Balfour, Van Beneden); (/') as an 

 atavistic occurrence of cell-division (Biitschli, Whitman, Hertwig, &c.); 

 (c) by Weismann's more complex hypothesis. It seems to be a case of cell- 

 division at the limit of growth. 



LITERATURE. 



Balfour, F. M. — 0/>. cit. 



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



IV., 1883. 

 Carnoy. — La Cellule II., 1886, &c. 

 Geddes, p. — Op. cit. 

 Haddon, a. C. — Op. cit. * 

 Hensen, V. — Op. cit. 

 Hertwig, O. — Op. cit. 

 Haiche'it Jackson. — Introduction to his edition of Rolleston's Forms 



of Animal Life. 

 M'Kendrick, J. G. — On the Modern Cell Theory, 6cc. Proc. Phil. Soc. 



Glasgow, XIX., 1888. 

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

 Thomson, J. A. — Recent Researches on Oogenesis. Quarterly Journ. 



Micr. Sci., XXVI., 1886. 



Art. Embryology, Chambers's Encyclopaxlia. 



Weismann, a. — Die Continuitiit des Keimplasmas. Jena, 1885. 



Die Bedcutung der sexuellen Fortpflanzung. Jena, 1886. 



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



