THE DEVELOPMENT OP THE EMBRYO ITSELF. 107 



pighian corpnacles; g. e, genninal epithelium; d, alimentary canal; M, commeno 

 mg mesentery; S. O, somatopleore ; 8Pj splanclinopleare ; F, blood - veesels ;■ 

 pPf pleuroperitoneal cavity. 



Fio. 111.— Diagram of portion of digestive tract of chick on fourth day (after GOtte). 

 The blackline represents hypobJaet; the shaded portion, mesoblaat; to, lung di- 

 verticulum, expanding at bases into primary lung vesicle; sit stomach; ^, liver; 

 p, pancreas. 



Fig. 118.— Head of chick of third day, viewed sidewise as a transparent object (Hux- 

 ley), /«, cerebral hemispheres; 7*, vesicle of third ventricle; II, mid-brain; III, 

 hmd-bram; o, optic vesicle; 9, nasal pit; S, otic vesicle; o, inf undibnlum ; e, 

 pineal body ; A, notochord ; V, fifth nerve ; VII, seventh nerve ; VIII, united 

 glossopharyngeal and pnenmogastric nerves. 1, 2, 3, 4, 5, the five visceral folds. 



bellitm and medulla oblongata ; tlie nerves, both cranial and 

 spinal, bud out from the nervous centers. The alimentary ca- 

 nal enlarges, a fore-gut and hind-gut being formed, the former 

 being divided into oesophagus, stomach, and duodenum ; the 

 latter into the large intestine and the cloaca. The lungs arise 

 from the ahmentary canal in front of the stomach ; from simi- 

 lar diverticula from the duodenum, the liver and pancreas orig- 

 inate. Changes in the protovertebrse and muscle-plates con- 

 tinue, while the Wolffian bodies are formed and the Wolffian 

 duct modified. 



Up to the third day the embryo lies mouth downward, but 

 now it comes to lie on its left side. See Fig. 105 with the ac- 

 companying description, it being borne in mind that the view is 

 from below, so that the right in the cut is the left in the em- 



/ 



Pro. tl3.— Head of chick of fourth day, vievfed from below as an opaque ob/ect (Fos- 

 ter and Balfour). The neck Is cut across between third and fourth visceral, folds. 

 C. ff, cerebral hemispheres; F. B, vesicle of third ventricle: Op, eyebill; nf, 

 naso-frontal process; M, cavity of mouth; 8. M, superior maxillarfr piocess of F. 

 1, the first visceral fold (mandibular arch); F. 2, F. 3, second and third visceral 

 arches; N, nasal pit. 



bryo itself. Tig. 110 gives appearances furnished by a vertical 

 transverse section. The relations of the parts of the digestive 

 tract and the mode of origin of the lungs may be learned from 

 Fig. 111. 



