374 



COMPARATIVE ANATOMY 



the heart to the aortic arches. The liulbus and truncus are primarily a 

 single vessel. During the second month of foetal life, however, a spiral 



BULB 



INTERNAL CAROTID A 



■A3RTIC ARCH I 

 ILL POUCH I 

 •AORTIC ARCH H 

 POUCH 2 



DORSAL AORTA. 

 ATRIUM 



ARCH I 



POUCH 4+5-^ 



INT CAROTID A. 

 POUCH I 

 ARCH n 

 ARCH HI 

 ^^POUCH 3 



ARCH W+ 

 PULMONARY 



DORS. AORTA 



INT. CAROTID A; 



POUCH 2 



POUCH 3 

 POUCH 4+5- 

 PULM. ARCH 

 PULMONARY A: 

 LUNG BUD 



VENTRICLE 



INT CAROTID A. 



POUCH 



POUCH 2 



ARCH ni 

 POUCH 3 



ARCH W 



POUCH 4+5 PULMONARY 



PULM. ARCH VI 



LUNG 

 ES0PHA3US 



INT CAROTID A. 



ARCH 17= 

 AORTIC ARCH 



DORS. 

 "AORTA 



ARCH 



PULMONARY A" 



DUCTUS 

 ARTERIOSUS 



LUNG 



'^LUNGS \ PULMONARY A. 



D. E. F 



Fig. 312. — A~F show successive stages in the development of the aortic arches in 

 human embryos. The arteries are shown in black, the pharyn.x and lung anlagen are 

 stippled. A is an embryo of four weeks, B of four and a half weeks, C of five weeks, D of 

 six weeks, E of six and a half weeks, and F at birth. (Redrawn from Romer, after 

 Streeter.) 



LUNG ANLAGt 

 PHARYNGEAL POUCHES 

 MYELENCEPHAlON ^ ^<^\' 



ECTODERM 



SPINAL CORD 



TOMACH 



NOTOCHORO 



DORSAL AORTA 



OPTIC VESICLE 



Fig. 313. — A diagram of a 4.2 mm. human embryo showing five aortic arches. 

 (Redrawn after W. His.) 



septum is formed, dividing them lengthwise into two trunks, the aorta 

 and the pulmonary artery. At the time when this occurs, four thickenings 

 of the wall of the bulbus are converted into the six semilunar valves, three 



