THE EMBRYO. 



1197 



In consequence of the atrophy of the Wolffian bodies the cardinal veins 

 diminish in size ; the primitive jugular veins, on the other hand, become enlarged, 

 owing to the rapid development of the head and brain. They are further aug- 

 mented by receiving the vein (subclavian) from the upper extremity, and so come 

 to form the chief veins of the Cuvierian ducts ; these ducts gradually assume an 



FIG 756. Diagrammatic outline of a longitudinal vertical section of the chick on the fourth day. ep. 

 Epiblast, sm. Somatic mesoblast hy. Hypoblast. vm. Visceral mesoblast. of. Cephalic fold, of. Caudal fold. 

 am. Cavity of true amnion. ys. Yolk-sac f. Intestine . Foregut. a. 'Future anus, still closed, m. The 

 mouth, me. The mesentery, at.. The allantoic vesicle, pp. Space between inner and outer folds of amnion. 

 (From Quain's Anatomy, Allen Thomson.) 



almost vertical position in consequence of the descent of the heart into the 

 thorax. The right and left Cuvierian ducts are originally of the same diameter, 

 and are frequently termed the right and left superior vence cavce. By the 

 development of a transverse branch (the future left innominate vein) between the 



mnio n 



-Allct-ntois 

 Hind-gut 



Fore-gut' / Heart 



Chorda Dorsalis 



FIG. 757. Diagram of a longitudinal section of a mammalian embryo. Very early. (After Quain.) 



two ducts the blood is carried from the left duct into the right, which thus 

 becomes much enlarged and forms the permanent superior vena cava, and into 

 which the vena azygos major opens. The left duct atrophies ; its upper part 

 remains pervious as a small vein, which receives the left superior intercostal vein ; 

 its intermediate portion is represented by the vestigial fold of Marshall ; its 

 lower part persists as a small vein, the oblique vein of Marshall, which runs 

 downward across the back of the left auricle to join the coronary sinus ; this 



