IX.J THE ALLANTOIS. 279 



still much looser and less firm than the walls of the 

 chest may be said to be definitely established, and the 

 loops of intestine, which have hitherto been hanging 

 down into the somatic stalk, are henceforward confined 

 within the cavity of the abdomen. The body of the 

 embryo is therefore completed ; but it still remains 

 connected with its various appendages by a narrow 

 somatic umbilicus, in which run the stalk of the aUan- 

 tois and the solid cord suspending the yolk-sac. 



The cleavage of the mesoblast 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 sepa- 

 rated from the shell membrane by nothing more than 

 an attenuated membrane, the serous membrane, formed 

 out of the outer primitive fold of the amnion and the 

 remains of the vitelliae membrane. With this serous 

 membrane 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 

 increases in size and importance, the allantoic vessels 

 are correspondingly developed. They are very con- 

 spicuous when the egg is opened, the pulsations of the 

 allantoic arteries at once attracting attention. 



On about the sixteenth day, the white having 

 entirely disappeared, the cleavage of the mesoblast is 

 carried right over the pole of the yolk opposite the 



