304 OUTLINES OF CHORDATE DEVELOPMENT 



vt 



b a a - 



oo. 



vt 



am 



FIG. 123. Sagittal sections through the head of the chick. In A the heart 

 is shown in optical section. After Lillie. A. Of an embryo with twenty-two 

 or -three pairs of somites (about forty-four hours). B. Of an embryo with thirty- 

 nine pairs of somites (end of the fourth day), a, Auricle; am, amnion; ao, dorsal 

 aorta; ba, bulbus arteriosus; cf, cranial flexure; D, diencephalon; dv, ductus 

 venosus; e, epiphysis; h, hypophysis; i, infundibulum; ip, anterior intestinal 

 portal; is, isthmus; I, rudiment of lung; li, liver; m, mandibular arch; Ms, mesen- 

 cephalon; Mt, metencephalon; My, myelencephalon; n, notochord (degenerating) ; 

 o, oral membrane (oral plate); oe, oesophagus; p.pharynx; P, parencephalon; 

 r, optic recess; S, Seessel's pocket (preoral gut); st, stomach; sv, sinus venosus; 

 Sy, synencephalon; T, telencephalon; th, rudiment of thyroid body; tp, tuber- 

 culum posterius; v, ventricle; vt, velum transversum. 



