282 OUTLINES OF CHORDATE DEVELOPMENT 



This remains open to the yolk nearly throughout embryonic 

 life. The walls of this open portion of the gut elongate into 

 a tubular stalk known as the yolk stalk. In reality this is 

 double, composed of an outer wall of somatopleure, and an 

 inner tube of splanchnopleure; the former is sometimes known 

 as the somatic stalk, the latter as the splanchnic stalk or yolk 

 stalk proper. The splanchnic stalk is the pathway by which 



FIG. 111. Ventral view of the anterior end of a chick embryo with sixteen 

 pairs of somites (about thirty-eight hours). From Lillie (Development of the 

 Chick), a.i.p., Anterior intestinal portal; au.P., auditory pit; B.a., bulbus 

 arteriosus; F.B., fore-brain; Inf., infundibulum; op.Ves., optic vesicle; Or.pl., 

 oral plate; Pr'am., proamnion; s.4, fourth somite; Tr.a., truncus arteriosus; 

 v.Ao., ventral aorta; Ven., ventricle; V.o.m., omphalo-mesenteric (vitelline) 

 vein; VII VIII, acustico-facialis ganglion. 



the blood-vessels pass between embryonic and extra-embryonic 

 structures. The narrow space between the two stalks is, of 

 course, ccelomic, connecting embryonic or true coelom, and 

 extra-embryonic coelom or exocoelom. 



The inner limbs of the lateral folds of the somatopleure form 

 the body watt of the embryo, of which we may now describe 

 lateral and ventral surfaces, as well as the dorsal, which has 

 hitherto been the only free surface of the embryo, save in the 

 head and tail. 



VI. THE ESTABLISHMENT OF THE EXTERNAL FORM 

 OF THE EMBRYO 



Rapid growth and extensive changes in the external form of 

 the embryo have been proceeding, as the embryo has been 



