THE AMNION. 313 



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

 the left side. In Fig. 1, the tufted (d) chorion encloses the germ. vesicle, 

 ihe wall of which consists of the two primary germ-layers. Between the 

 outer (a) and inner (i) germ-layers, the middle germ-layer (m) has developed, 

 co-extensively with the germ-area. In Fig. 2, the embryo (e) is beginning to 

 separate from the germ -vesicle (ds), while the wall of the amnion-fold is 

 developing round it (in front as the head-sheath, Is, in rear as tail-sheath, 

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

 the embryo and thus form the amnion -cavity (ah) j while the embryo (e) 

 separates still more from the germ-vesicle, the intestinal canal (dd) is 

 developed, and from the posterior end of this, the allantois (al) grows out. 

 In Fig. 4, the allantois (aZ) becomes larger ; the yelk-sac (ds) smaller. In 

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

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

 figures e signifies 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 umbilical cord; Tch^ds, intestinal 

 germ-vesicle; ds, yelk-sac (navel-vesicle) ; dg, yelk-duct; df, intestinal- 

 fibi"ous layer; dd, intestinal-glandular layer; al, allantois; vl=hh, region 

 of the heart ; d, yelk-membrane (prochorion) ; d', tufts on prochorion ; sh, 

 serous membrane ; sz, tufts of the foregoing ; ch, tufted membrane or 

 chorion; r, the space between the amnion and chorion, filled with fluid. 

 (According to Kolliker.) Compare Table V. Fig. 14 and 15. 



raise itself higher and higher around the entire embryo, and 

 at last coalesces above it (Fig. 94, 2> 3> 4^ 5^ <^'^)- To 

 keep up the simile of a fortress imagine that the sur- 

 rounding wall of the fortress becomes extraordinarily high, 

 and towers far above the fortress. Its edges arch like 

 the crests of a jutting cliff which is about to enclose the 

 fortress ; they form a deep cavern, and at last grow together 

 above. At last the fortress lies entirelv within the cavern 

 forme by the concrescence of the edges of this mighty 

 wall. (Cf Figs. 95-98, p. 319, and Plate V. Fig. 14.) 



These two outer strata of the germ-area, rising in this 

 way in the form of folds around the embryo and coalescing 

 above it, at last form a spacious sac-like envelope around 

 it. This envelope bears the name of germ-membrane, 



