70 



THE STUDY OF CHICK EMBRYOS 



about the cephalad end of the pharynx. Between the ventral wall of the fore-brain and the 

 pharynx is an invagination of the ectoderm, Rathke's pocket. 



Section through the Otocysts and Second Aortic Arch (Fig. 58). — The otic 

 vesicles are sectioned caudal to their apertures and appear as closed sacs lateral to the wall of 

 the hind-brain. The cavity of the pharynx is somewhat triangular and its dorsad wall is thin. 

 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 

 aorta?. Surrounding the bulbus cordis is the large pericardial cavity. Between the first and 

 second aortic arches (Fig. 58) is the first pair of pharyngeal pouches, lateral diverticula of the 

 entoderm. The student should note that in the sections of this stage so far studied, the mesen- 



Aorticarch 



z 



Ventral 

 aorta. 



Endothelium 



of bulbus 



Fig. 58. — Transverse section through the otic vesicles and second aortic arches of a fifty-hour chick 



embryo. X 50. 



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

 formed. 



Section through the Sinus Venosus and Common Cardinal Veins (Fig. 59)- — 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 aorta have united to form a single dorsal 

 vessel. On either side of the pharynx are seen subdivisions of the ccelom which will form the 

 pleural cavities. These cavities are separated from the pericardial cavity by the septum trans- 

 versum in which the common cardinal veins cross to the sinus venosus. 



The folds of the amnion envelop the right side of the embryo and the ectoderm of these 



