62 THE STUDY OF CHICK EMBRYOS 



The anterior cardinal veins pass between the otocysts and the wall of the hind-brain. 

 Ventral to the pharynx, the bulbus cordis is sectioned obliquely where it leaves the heart, 

 and at this level gives off laterad the second pair of aortic arches which connect dorsad 

 with the descending aortee. Surrounding the bulbus cordis is the large pericardial cavity. 

 The student should note that in the sections of this stage so far studied, the mesenchyme 

 of the head is undifferentiated, the tissues peculiar to the adult not yet having been 

 formed. 



Section through the Second Pharyngeal Pouches and Thyreoid Anlage (Fig. 61). 

 As this section is taken at a level between the second and third aortic arches, the de- 

 scending aortffi and heart are unconnected. Tangential shavings have been cut from the 

 walls of the otocysts. Extending laterally from the pharynx are the second pair of pharyn- 

 geal pouches which have already come in contact with the ectoderm to form closing plates. 



Hind-brain 

 Entoderm 



Ectoderm 

 Ant. cardinal vein 



Descending aorta 



Closing plate 



Pharynx 



Thyreoid anlage 



Mesodcrm 



Epi-myocardium of bulbus 

 Endothelium of bulbus 



FIG. 61. Transverse section through the second pharyngeal pouches and thyreoid anlage of a 



fifty-hour chick embryo. X 50. 



A pocket-like depression in the mid-ventral floor of the pharynx represe'nts the thyreoid 

 anlage; later it becomes saccular and loses its connection with the pharyngeal entoderm. 

 The splanchnic mesodermal wall of the heart is destined to give rise later to the epi- and 

 myocardium. 



Section through the Sinus Venosus and Common Cardinal Veins (Fig. 62). At 

 this level, the common trunk formed by the anterior and posterior cardinal veins opens into 

 the thin-walled sinus venosus. The sinus receives all of the blood passing to the heart and 

 is separated only by a slight constriction from the larger atrium. The muscle plates of the 

 first mesodermal segments are seen, and the descending aortce have united to form a single 

 dorsal vessel. On either side of the pharynx are subdivisions of the cceloni which will 

 form the pleural cavities. These cavities are separated from the pericardial cavity by the 



