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. 



' Gon. 



A.o.fn. 



Fig. 197. — Photograph of transverse section through a chick embryo of 

 8 days. 

 A. o. m., Omphalomesenteric artery. Du., Duodenum. Giz., Gizzard. 

 Gon., Gonad. II., Ihum. M. D., Miillerian duct. Pc, 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. 



