386 THE EVOLUTION OF MAN. 



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

 side. In Fig. 1, the prochorion (d), studded with tufts (d 1 ), surrounds the 

 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-layer (mesoderma, 

 m) 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, Tcs, behind as the tail-sheath, 

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

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

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

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

 out. In Fig. 4, the allantois (al) is bigger ; the yelk-sac (ds) 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 ; m, middle germ-layer ; 

 t, inner germ-layer ; am, amnion ; (ks, head-sheath ; ss, tail-sheath) ; all, 

 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; chz, tufts of the 

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

 and chorion. (After Kolliker.) (Cf. PL 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- 



