VTII.] THE ALLANTOIS. 203 



appendages by a narrow somatic umbilicus, in which run 

 the stalk of the allantois and the solid cord suspending 

 the yolk-sac. 



The cleavage of the rnesoblast still progressing, the yolk 

 is completely invested by the (splanchnopleuric) yolk-sac 

 except at the pole opposite to the embryo, where for some 

 little time a small portion remains unenclosed ; at this spot 

 the diminished white of the egg adheres as a dense viscid 

 plug, 



The allantois meanwhile spreads out rapidly, and lies 

 over the embryo close under the shell, being separated from 

 the shell membrane by nothing more than an attenuated 

 membrane the chorion, formed out of the outer primitive 

 fold of the amnion and the remains of the vitelline mem- 

 brane. With this chorion the allantois partially coalesces, 

 and in opening an egg at the later stages of incubation, 

 unless care be taken the allantois is in danger of being torn 

 in the removal of the shell membrane. As the allantois in- 

 creases in size and importance, the umbilical (or allantoic) 

 vessels are correspondingly developed. They are very con- 

 spicuous when the egg is opened, the pulsations of the 

 umbilical arteries at once attracting attention. 



5. On about the sixteenth day, the white having en- 

 tirely disappeared, the cleavage of the mesoblast is carried 

 right over the pole of the yolk opposite the embryo, and 

 is thus completed (Fig. 8). The yolk-sac now, like the 

 allantois which closely wraps it all round, lies loose in a 

 space bounded outside the body by the chorion, and con- 

 tinuous with the pleuroperitoneal cavity of the body of the 

 embryo. Deposits of urates now become abundant in the 

 allantoic fluid. 



The loose and flaccid walls of the abdomen enclose a 

 space which the empty intestines are far from filling, and on 

 the nineteenth day the yolk-sac, diminished greatly in bulk 

 but still of some considerable size, is withdrawn through the 

 somatic stalk into the abdominal cavity, which it largely 

 distends. Outside the embryo there remains nothing now 

 but the highly vascular allantois and the practically blood- 

 less chorion and amnion. The amnion, whose fluid during 

 the later days of incubation rapidly diminishes, is continuous 

 at the umbilicus with the body-walls of the embryo. The 



