THE BODY-CAVITIES 347 
not continued into these meshes. Thus free embryonic cells 
of the primordium of the spleen enter the venous circulation 
directly, and become transformed into blood-corpuscles. 
On account of the intimate relation between the pancreas and spleen 
in early embryonic stages, certain authors (see esp. Woit) have asserted 
a genetic connection, deriving the spleen from the pancreas. There 
is, however, no good evidence that the relation is other than that of 
propinquity. 
Fig. 197. — Photograph of transverse section through a chick embryo of 
8 days. 
A.o. m., Omphalomesenteric artery. Du., Duodenum. Giz., Gizzard. 
Gon., Gonad. IIL., Ilium. M.D., Miillerian duct. Pe., Pancreas. V. umb., 
Umbilical vein. 
It should also be noted that the absence of rotation of the 
chick’s stomach (as contrasted with mammals) and the lesser 
development of the great omentum appear to be the causes of 
the more primitive position of the spleen in birds as contrasted 
with mammals. 
