400 THE EVOLUTION OF MAN. 



Figs. 123, It, 124, u), which cany blood from the placenta to 

 the heart, open, at fii'st, into the united yelk-veins. These 

 last afterwards disappear, and the right navel-vein simul- 

 taneously disappears entirely, so that a single great vein, 

 the left navel-vein, alone remains, which carries all the 

 nutritive blood from the placenta into the heart of the 

 embryo. The two arteries of the allantois, or the navel- 

 arteries {arterice umhilicales, Figs. 123, n, 124, n), are merely 

 the last, posterior extremities of the two primitive aortas, 

 which are afterwards greatly develoj)ed. It is not until the 

 end of the nine months of embryonic life, when the human 

 embryo is born and enters the world as an independent 

 j)hysiological individual, that the navel circulation loses its 

 significance. The navel cord (Fig. 138, as), in which these 

 larger blood-vessels pass from the embryo to the placenta, 

 is removed with the latter at the so-called " after-birth," 

 and an entn-ely new circulation of the blood, limited to the 

 body of the child, comes into operation simultaneously with 

 pulmonary respiration.^"'' 



Now, if, in conclusion, we briefly review the germ- 

 history of Man as far as we have traced it, and endeavour 

 to comprehend the whole subject in one connected view, it 

 seems desirable to divide it into several main sections, or 

 periods, and these into subordinate stages, or steps. With 

 reference to the })hylogenetic significance of this history, 

 which we shall next consider more closely, it seems to me 

 most appropriate to make the four main divisions and ten 

 sub-divisions as distinguished in the following pages, which 

 correspond to the most important i:)hylogcnetic stages of 

 development of onr animal ancestors. (Cf Talkie XXV. 

 at the end of the nineteenth chapter.) This will again 



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