386 THE EVOLUTION OF MAN. 



separates the right aud left halves ; in Fig. 5 the germ is seen from the left 

 side. In Fig. 1, the prochorion (cZ), studded with tufts (ci'), surrounds Mie 

 germ-vesicle, the wall of which is composed of the two primary germ- 

 layers. Between the outer (a) and the inner (i) germ-layer within the 

 limits of the germ-area (area germinativa) the middle germ-laycr (jnesoclerma, 

 t?j.) has developed. In Fig. 2, the embryo (e) is already beginning to separate 

 from the germ-vesicle (ds), and the wall of amnion-fold is beginning to rise 

 round the embryo (in front as the head-sheath, ks, beliind as the tail-shoath, 

 8s.) In Fig. 3, the edges of the amnion-fold (am) meet over the back of the 

 embryo, thus forming the amnion-cavity (ah) ; in consequence of the further 

 separation of the embryo (e) from the germ-vesicle (<is), the intestinal- 

 caual (dd) originates, and from the hind end of this the allantois (al) grows 

 oat. In Fig. 4, the allantois (al) is bigger; the yelk-sac ((2s) is smaller. 

 In Fig. 5, the embryo already shows the gill-openings and the rudiments of 

 the two pairs of limbs ; the chorion has formed branched tufts. In all five 

 figures, e indicates embryo; a, outer germ-layer; w, middle germ-layer; 

 t, inner germ-layer; am, amnion; {ks, head-sheath; ss, tail-sheath); ak, 

 amnion-cavity ; as, amnion-sheath of the navel-cord ; kh, intestinal germ- 

 vesicle ; ds, yelk-sac; dg, yelk-duct; df, intestinal-fibrous layer; dd, in- 

 testinal-glandular layer ; al, allantois; vl=hh, region of the heart; d, yelk- 

 membrane or prochorion ; d', tufts of the latter ; sh, serous covering ; sz, 

 tufts of the latter ; ch, tufted membrane or chorion ; cht, tufts of the 

 latter; st, terminal vein; r, cavity, filled with liquid, between the amnion 

 aud chorion. (After KoUiker.) (Cf. PI. V. Fig. 14 and 15.) 



higher classes, in Eeptiles, Birds, and Mammals ; and, secondly, 

 because the placenta is developed from the allantois only in 

 the higher Mammals, including' Man, and not in the lower 

 Mammals. The former are therefore called " Placental 

 Animals " (Placentalia). 



Another characteristic common to the three higher classes 

 of Vertebrates alone, is the formation of the third appendage 

 of the embyro, the amnion, which has already been men- 

 tioned. We have already learned something of the amnion 

 in noticing the separation of the embryo from the intestinal 

 germ-vesicle. We found that the walls of the latter rise in 

 a ring-shaped fold round the embryonic body. In front, this 

 fold appears in the form of the so-called head-cap, or head- 



