EMBRYO OF SEVENTEEN SEGMENTS 



49 



Section through the Hind -brain and Auditory Placodes (Fig. 45). Besides the audi- 

 tory placodes already described as the anlages of the internal ear, this section is character- 

 ized by: (i) the large hind-brain, somewhat flattened dorsad; (2) the broad, dorso-ventrally 



Ectoderm 



Mesenchyme 



Descending aorta 

 Ventral aorta 



Somali pleure 



Mid-brain 

 Notocnora 



Fore-gut 



Pharyngeal membrane 



Spianchno pleure 



FIG. 44. Transverse section through the pharyngeal membrane of a thirty-eight-hour chick 



embryo. X 75. 



flattened pharynx, above which on each side lie the descending aortas; (3) the presence of the 

 bulbar and ventricular portions of the heart. The bulbus is suspended dorsally by the 

 mesoderm, which here forms the dorsal mesocardium. The ventricle lies on the right side 



Ectoderm 



Notochord 



Descending aorta 

 Pericardial cavity 

 Somatic mesoderm 



Endotheliu 

 of ventricle 



Hind-brain 



Ant. cardinal vein 



Entoderm 



Endothelium of bidbus 

 Myocardium 



FIG. 45. Transverse section through the hind-brain and auditory placodes of a thirty-eight- 

 hour chick embryo. X 75- 



of the embryo; a few sections caudad in the series it is continuous with the ventral aorta 

 (cf. Fig. 41). Between the somatic and splanchnic mesoderm is the large pericardial cavity. 

 It surrounds the heart in this section. Dorsal to the aortae are the anterior cardinal veins, 

 which return blood from the head region. 



