58 



THE STUDY OF CHICK EMBRYOS 



mesenchyma in the section. The structure of the germinal area is complicated by the presence 

 of collapsed blood-vessels. 



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

 43). — Besides the auditory placodes already described as the anlages of the internal ear, this 

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

 broad dorso-ventrally flattened pharynx, above which on each side lie the dorsal aorta; (3) by the 



Ectoderm. 



Mesendyi 



Descending 

 aorta. 



Neural tube 



Ndtochord 



FbrequT 



Pharyngeal 

 membrane 



Splanchnopleure 



Fig. 42. — Transverse section through the pharyngeal membrane of a thirty-six-hour chick embryo. 



X 75- 



Ectode 



Notochord 

 Descending Qorta 



Pericardial 

 cavity 



Somatic trie 



Neural tube 



Ant Cardinal Vein 

 Auditory placode 



Forec/ut 



Ectoderm 



Endolheliurn 

 of heart \\ 



Entoderm 



Endothelium of ventral aorta 

 Myocardium 



Fig. 43. — Transverse section through the hind-brain and auditory placodes of a thirty-six-hour chick 



embryo. X 75- 



presence of the ventral aorta and bulbar portion of the heart. The descending aortas are located 

 on each side dorsal to the pharynx. The ventral aorta is suspended dorsally by the mesoderm, 

 which here forms the dorsal mesocardium. The bulbus of the heart lies to the left in the figure 

 (right of embryo) and a few sections caudad in the series is continuous with the ventral aorta 

 (see Fig. 40). Between the somatic and splanchnic mesoderm is the large pericardial cavity. 

 It surrounds the heart in this section. 



