THE AMNION. 387 



sheath (Fig. 139, 2 ks) ; at the back, it also arches upward 

 and forms the tail-cap, or tail-sheath (Fig. 139, 2 ss) ; on the 

 right and left sides, the fold is at first lower, and is here 

 called the side-caps, or side-sheaths (Fig. 140; Figs. 95, 96, af, 

 p. 317). All these caps or sheaths are only parts of a con- 



FiG. 140. — Transverse section through an embryonic Chick (a little 

 behind the anterior opening of the intestine), at the end of the fii'st day of 

 incubation. The medullary furrow above and the intestinal furrow below 

 are still wide open. At each side, the rudiment of the body-cavity [coeloma) 

 can be seen between the skin-fibrous layer and the intestinal-fibrous layer. 

 On the right and left of it, at the outside, the side-caps of the amnion are 

 beginning to rise. (After Remak.) 



nected ring-like fold, which passes round the embryo. This 

 grows higher and higher, rises like a great encircling wall, and 

 finally arches over the body of the embryo, so as to form a 

 cavern-like covering over the latter. The edges of the ring- 

 like fold meet and coalesce (Figs. 141, 142). The embryo, 

 thus, at last lies in a thin membranous sac, filled with the 

 amnion-fluid (Fig. 139, 4, 5 ah). 



When the sac is completely closed, the inner layer of the 

 fold, which forms the real wall of the sac, withdraws com- 

 pletely from the outer layer. The latter attaches itself to 

 the inside of the outer egg-membrane (" prochorion "). It 

 supplants this prochorion, and forms the permanent tufted 

 membrane, the true " chorion." This arises solely from 

 the horn-plate (Fig. 139, 4 sh). The thin wall of the 

 amnion-sac, on the other hand, consists of two strata : of an 

 inner stratum, the horn-plate, and of an outer stratum, the 



