THE EARLY DEVELOPMENT OF THE CHICK 283 



ms 



Crfl. 



Mete/ic. 



Myelenc./. -/-^ 

 au.P 



S./O. 



prstr 



FIG. 112. FIG. 113. 



FIG. 112. Chick embryo with twenty pairs of somites (about forty-three 

 hours). Dorsal view. From Lillie (Development of the Chick). A. a.m., 

 Vitelline artery; au.P., auditory pit; Cr.FL, cranial flexure; D.C., ductus Cuvieri; 

 Dienc., diencephalon; Mesenc., mesencephalon; Metenc., metencephalon; Myelenc. 

 1, and 2, anterior and posterior divisions of the myelencephalon; Op.Ves., optic 

 vesicle. Ph., pharynx; pr.str., primitive streak; s.2, s.5, etc., second, fifth, etc., 

 somites; Telenc., telencephalon; Vel.tr., velum transversum; Ven., ventricle. 



FIG. 113. Chick embryo with twenty-seven pairs of somites (about forty- 

 eight hours). Viewed from above. After Lillie. a, Auricle; am, posterior 

 margin of amnionic folds; c, carotid loop; cf, cranial flexure; d, diencephalon; 

 dC, ductus Cuvieri; g\, g*>, gz, first, second and third gill clefts; i, isthmus; 

 I, lens; ma, mandibular arch; ms, mesencephalon; mt, metencephalon; o, otocyst 

 (auditory sac); just to the right of the otocyst is a thickening representing the 

 ganglion of the VII and VIII cranial nerves; r, retinal layer; so, sio, s 2 o, second, 

 tenth, and twentieth somites; t, tail-bud; v, ventricle; va, vitelline artery; vv, 

 vitelline vein; 1, 2, 3, first, second and third aortic arches; V, ganglion of V 

 cranial nerve. 



