282 THE EVOLUTION OP MAN. 



evolution of the individual, the assumption of which ia 

 justified by the comparative germ-history of Vertebrates, 

 re-occur essentially in all branches of this tribe, though in 

 single cases they are more or less changed, or kenogenetically 

 modified. In their simplest and earliest form, which is 

 certainly mainly palingenetic, we yet find them in the 

 Amphioxus ; in the Round-mouths (Cyclostomi), Fishes, and 

 Amphibia they have already become much changed and 

 vitiated, kenogenetically transformed ; and this is true 

 in a much greater degree of the three higher vertebrate 

 classes, Reptiles, Birds, and Mammals. In these the gradual 

 formation of a very large nutritive yelk and of peculiar 

 egg-membranes has introduced so many changes, or 

 secondary kenogenetic modifications, that at first sight it 

 is hardly possible to recognize the primary palingenetic 

 incidents of evolution. 



In these, the kenogenetic relation of the germ to the 

 nutritive yelk is especially prominent, and till quite recently 

 caused an entirely false conception of the first and most 

 important conditions of the germ of the higher Vertebrates, 

 introducing many false views as to the Ontogeny of these. 

 Previously, the germ-history of the higher Vertebrates was 

 universally based on the view that the first rudiment of the 

 germ is a flat layer-shaped disc ; and for this reason the 

 cell strata which compose the germ-disc (also called the 

 germ-area) were called " germ-layers." This flat germ-disc 

 which is at first circular, afterwards oval, and which in the 

 hen's egg we have learned to call the tread (cicatricula), 

 lies at a particular point on the outer surface of the large 

 globular nutritive yelk. When germination begins, the flat 

 germ-disc arches outwards and detaches its outer surface 



