2,86 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'), 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-\sbjer (mesoderma, 

 111) 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 (ah) ; 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 ; 

 i, inner germ-layer ; am, amnion ; (fcs, head-sheath ; ss, tail-sheath) ; ah, 

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

 vesicle ; ds, yelk-sac ; dg, yelk-duct ; df, intestinal-fibrous layer j 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. PI. V. Fig. 14 and 15.) 



higher classes, in Reptiles, 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- 



