593 THE UMBILICAL VESICLE. 



tion from the simple plan of development described in the preceding 

 chapter. Here, the whole of the vitellus is not directly converted 

 into the body of the embryo ; but while a part of it is taken, as 

 usual, into the abdominal cavity, and used immediately for the 

 purposes of nutrition, a part is left outside the abdomen, in the 

 umbilical vesicle, a kind of secondary organ or appendage of the 

 foetus. The contents of the umbilical vesicle, however, are after- 

 ward absorbed, and so appropriated, finally, to the nourishment of 

 the newly-formed tissues. 



