68 ■ AllTHUR DENDY. 



thill layer of inesoblast derived directly from the original lower 

 layer of the blastoderm, from the underlying yolk-sac (cf. figs, 

 29—32, 47, 50, 55, 68—74). It is doubtful if the sepa- 

 ration of the serous envelope from the true amnion is ever 

 quite complete, for at the latest stage at which it was possible 

 to examine the foetal membranes (Stage H) the two are still 

 connected above the shoulders over a small area. A similar 

 connection has been described by Hirota (17) in the case of 

 the chick ; while in Chelonians, according to Mitsukuri (15), 

 this " sero-amniotic connection ^' remains much more extensive, 

 and separates the " extra-embryonic cavities of the two halves 

 of the amnion " over the dorsal region of the embryo to the 

 end of development. 



The Yolk-sac. — The yolk-sac is formed primarily from 

 what is left of the original lower layer of the blastoderm after 

 the serous envelope has split off from it. It appears, however, 

 that a large portion of the mesoblast derived from the primitive 

 streak spreads into or over it, and gives rise to, at any rate, the 

 commencement of the vitelline vessels (cf. figs. 15 — 17, 20, 

 30, 31, L. W.). The vitelline circulation (figs. 58, 93, 100, 

 101) closely resembles that of the chick, though perhaps the 

 absorbent vessels which dip down into the yolk are more 

 strongly developed, and the arrangement of the yolk-spheres 

 around them, like onions on a string, is certainly very striking 

 (fig. 106). 



The splanchnopleuric portion of the pro-amnion must also 

 doubtless be regarded as forming part of the yolk-sac, but this 

 has been sufficiently described already. 



The Allantois. — The allantois originates at Stage L in a 

 perfectly normal manner as an outgrowth of the ventral wall 

 of the alimentary canal close to the posterior end (figs. 83 — 

 89). As development goes on it extends upwards on the right 

 side of the embryo, and comes to lie beneath the serous en- 

 velope (fig. 100). Its walls become vascular, and it is very 

 greatly distended by the accumulation within it of a clear, 

 semi-gelatinous liquid. Thus the outer wall of the allantois 

 becomes closely pressed against the serous envelope, and the 



