262 OUTLINES OF CHORDATE DEVELOPMENT 



faces of the head, but otherwise the lateral limits of the embryo 

 proper are not definitely marked until twenty-two to twenty- 

 eight hours later, i.e.j forty-two to fifty hours after fertilization. 

 We may proceed, therefore, to describe the development of the 

 essential structures of the embryo, during the period from the 

 formation of the head-fold up to the time when the embryo 

 begins to be completely marked off from the extra-embryonic 



FIG. 99. Median sagittal section through the head end of a chick with eighteen 

 pairs of somites (about forty hours). From Lillie (Development of the Chick). 

 a.i.p., Anterior intestinal portal; Ao., dorsal aorta; Au., auricle; E.E.B.C., 

 exocoelom (extra-embryonic body cavity); F.B., fore-brain; H.B., hind-brain; 

 H.F.Am., head-fold of amnion; Inf., infundibulum; Isth., isthmus; M.B., mid- 

 brain; N'ch., notochord; or.pl., oral plate (oral membrane); P.O., pericardial 

 cavity; Ph., pharynx; Pr'a., proamnion; pr'o.g., preoral gut; Rec.opt., optic 

 recess; S.V., sinus venosus; Tr.A., truncus arteriosus; Ven., ventricle. 



region of the blastoderm. We shall describe the history of the 

 mesoderm first, not only because this seems the easiest method 

 of approach, but because the differentiations within the meso- 

 derm afford valuable landmarks in describing other structures. 

 Thus the age of the chick embryo is usually indicated by 

 reference to the number of pairs of mesodermal somites formed. 



2. Mesoderm 



While the head-fold is becoming well established (Fig. 100), 

 the mesoderm is in the form of a pair of sheets extending from 

 the sides of the primitive streak (peristomial) and head process 

 (gastral), across the whole extent of the area pellucida to the 



