THE ULTIMATE SEX-ELEMENTS. 91 



SUMMARY. 



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

 living matter itself. 



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

 arises from a fertilized egg-cell, and development proceeds by cell-division. 



II. Epigenesis and Evolution.— History 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." 



(b) Observations of Malpighi and others, mostly against Harvey's view. (V) 

 The theory of preformation, — of a nest of miniature models within the egg, 

 only requiring to be unfolded in successive generations ; Ovists versus 

 Animalculists. (d) Wolff's reassertion of "epigenesis," the foundation of 

 modern embryology ; his exaggeration of the simplicity of the germ, (e) Wolffs 

 successors. 



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

 cells. 



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

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

 changes. 



V. The contrast between Protozoa and Metazoa. — The making of a " body " 

 as distinct from reproductive cells. 



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

 ccelenterates, generally from the mesoderm in higher animals. 



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

 cases. 



VIII. The contrast between somatic and reproductive cells, and the 

 continuity of the latter ; Owen, Haeckel, Rauber, Brooks, Jiiger, Galton, 

 Nussbaum. 



IX. Weismann's theory of the continuity of the germ-p/asma (a specific 

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

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



LITERATURE. 



For relevant literature and further details, consult the textbooks of Balfour, 



Haddon, and Hertwig ; also, 

 Geddes, P. — Encyclopaedia Britannica articles already referred to ; also 



"Morphology," ibid. 

 Hensen, V. — Op. cit. 



M'Kendrick, J. G.-- Textbook of Physiology. Lond., 1888. 

 Thomson, J. A.— Arts. " Cell " and " Embryology," new Edition of Chambers's 



Encyclopaedia. 



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



Waldever, W.— Die Karyokinese, &c. Arch. Mikr. Anat., iSSS. 



Weismann. — Opp. cit. 



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



