THE GERM-CELLS 47 



conceived the embryo lying literally ready made in the 

 germ-cell they respectively favoured, though in a very 

 minute form. All that was supposed to take place was 

 an unfolding of the preformed embryo (evolutio), as a bud 

 unfolds itself into a flower. This theory is known, there- 

 fore, by the name of Preformation or Evolution theory 

 (Evolution not to be confused with Evolution in the 

 modern and wider sense of progressive development), 

 also Scatulation theory, because the miniatures of the 

 successive generations were imagined to lie, like nests of 

 boxes, one within the other, in ever-increasing minute- 

 ness. 



It is evident from what has been learnt already that this 

 crude theory is altogether baseless and unsubstantiated 



FIG. 32. GASTRULATION OF A CORAL (MONOXEMIA DARWINII). 

 (From E. Haeckel, "Evolution of Man.") 



A and B, impregnated ovum (in A immediately after fertilization 

 the nucleus is invisible) ; C, two segmentation cells ; D, four 

 segmentation cells ; E } morula ; F, blastula ; G, transverse section 

 of F ', H, hollowed blastula (transverse section) ; K, gastrula ; 

 I, longitudinal section of K. 



by facts. The truth had, indeed, been vaguely guessed 

 at here and there (Aristotle, Harvey), but was only 

 finally established by the splendid observations of Caspar 

 Friedrich Wolff, who in his Theoria Generationis (Theory of 

 Generation) (1759), demonstrated conclusively that the 

 chick gradually assumed its complex organization from a 

 comparatively simple and homogeneous matrix. The 

 organs were not preformed, " but could actually be seen 

 being formed." Later Von Baer first formulated the funda- 

 mental laws of this development by showing how the 

 embryonic cells arranged themselves first into the germinal 

 layers, then into the different tissues, organs, etc. 



Just to indicate in outline the first stages of embryo- 

 genesis, we must remember that we distinguished, according 



