96 THE EVOLUTION OF SEX. 



SUMMARY. 



The progressive analysis through organism, organs, tissues, and cells, to 

 the living matter itself. 



1. The Ovum-theory. — Every organism, reproduced in the ordinary 

 way, arises from a fertilised egg-cell, and development proceeds by cell- 

 division. 



2. Epigenesis and Evolution. — Plistory of the different views taken of 

 the development of the organism ; ancient speculations. The scientific 

 renaissance, {a) Harvey's prevision of the ovum-theory, and emphasis upon 

 "epigenesis." {/>) Observations of Malpighi and others, mostly against 

 Harvey's view, {c) The theory of preformation, — of a nest of miniature 

 models within the egg, only requiring to be unfolded in successive genera- 

 tions ; Ovists z'^ri'/^J'Animalculists. (</) VVolfiCsreassertion of "epigenesis," 

 the foundation of modern embryology ; his exaggeration of the simplicity 

 of the germ. (1?) Wolff's successors. 



3. The Cell-Theory. — All organisms are made up of cells, and start from 

 cells. 



4. A protoplasmic basis now being laid. The "germ-plasma" more 

 important than the egg-cell. All to be explained in terms of protoplasmic 

 changes. 



5. The contrast between Protozoa and Metazoa. — The making of a 

 " body " as distinct from reproductive cells. 



6. General origin of the sex-cells, indefinite in sponges, variable in 

 cuelenterates, generally from the mesoderm in higher animals. 



7. Early separation of the reproductive cells to be seen in a minority of 

 cases. 



8. The contrast between somatic and reproductive cells, and the con- 

 tinuity of the latter ; Owen, Hreckel, Rauber, Brooks, Jager, Gallon, 

 Nussbaum. 



9. Weismann's theory of the continuity of the ger\w-/>/as///a (a specific 

 nuclear matter), as opposed to continuity by a chain of undiflerentiated 

 cells, which is known to occur only in a minority of organisms. 



LITERATURE. 



For relevant literature and further details, consult the Text-books of 



Balfour, lladdon, and Ilertwig ; also, 

 Geddks, p. — EncyclopiTcdia Britannica articles already referred to; also 



MORPIIOI.OC.Y, ibid. 

 IlrcNSKN, V. — Op. fit. 



M'Kenm)i;ici<, J. G. — Text-book of Physiology. Lond., 1888. 

 Thomson, J. A. — Arts. Cell and Embryology, new Edition of 



Chambers's Encyclopredia. 



History and Theory of Heredity. Proc. Roy. Soc. Edin.. 1888. 



Wai.deykr, W. — Die Karyokinese, &c. Arch. Mikr. Anat., 1888. 



Wkismann. — 0pp. cit. 



Zoological Record, General Subjects: Cell, Oogenesis, &c., since 1886. 



