48 



THE STUDY OF CHICK EMBRYOS 



On either side of the pharynx a pair of large vessels are seen; the ventral pair are the ventral 

 aortce. Two sections cephalad their cavities open into those of the dorsal pair, the descending 

 aorta. The section is thus just caudad of the point where the ventral aortae bend dorsad and 

 caudad to form the descending aortae. The section passes through the caudal end of the 



Ectoderm 



Notochord 



Descending aorta 

 Pericardial cavity 

 Somatic mesoderm 



Endothelium 

 of ventricle 



Hind-brain 



Ant. cardinal vein 

 Auditory placode 



Fore-gut 



Ectoderm 



Entoderm 



Endothelium of hulbus 

 'Myocardium 



Fig. 45. — Transverse section through the hind-brain and auditory placodes of a thirty-eight-hour chick 



embryo. X 75. 



mesencephalon which is'^here thick walled with an oval cavity. Note the large amount of 

 undifferentiated mesenchyme in the section. The structure of the blastoderm is complicated 

 by the presence of collapsed blood vessels. 



Transverse Section through the Hind-brain and Auditory Placodes (Fig. 45).— 

 Besides the auditory placodes already described as the anlages of the internal ear, this sec- 



Ectoderm 

 Mesodermal segment 



Anterior cardinal vein- 

 Somatic mesoderm Cwlom 



Hind-brain 



Pore-gut 



Vitelline vein 



Entoderm. 

 Splanchnic mesoderm 



Heart 



Myocardiiih 



Fig. 46.— Transverse section through the caudal end of the heart of a thirty-eight-hour chick embryo 



X 75. 



tion is characterized by (1) the large hind-brain, somewhat flattened dorsad; (2) the broad 

 dorso-ventrally flattened pharynx, above which on each side lie the descending aortce; (3) the 

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



