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DE VEL OP ME NT. 



nal, above the point of junction of the vena cava, becomes obliterated, the upper 

 portion, which receives some of the lumbar and the intercostal veins, persisting 

 as the vena azygos major ; while the left cardinal, separating below from the left 

 iliac, sends a branch across the middle line of the body to form a communication 

 with the azygos major and persists as the azygos minor. 



The veins first formed in the upper part of the trunk are, as above stated, the 

 primitive jugular veins. In the greater part of their extent they become the internal 

 jugular vein. Shortly, two small branches may be noticed opening into them near 

 their termination ; these form the subclavian veins. From the point of junction of 

 these veins on the left side, a communicating branch makes its appearance, running 

 obliquely across the neck downward and to the right, to open into the primitive 

 jugular vein of the right side below the point of entrance of the subclavian vein. 

 At the same time, in consequence of the alteration in the position of the heart, 

 and its descent into the thorax, the direction of the ducts of Cuvier becomes 

 altered, and they assume an almost vertical position. From the portion of the 

 primitive jugular veins, above the branch of communication, the internal jugu- 

 lars are formed, except that part of the right one which lies between the point 

 of entrance of the subclavian of this side and the termination of the communi- 

 cating branch, which becomes the right innominate vein. The communicating 

 branch becomes the left innominate vein. The primitive jugular of the right 

 side, below the communicating vein, and the right duct of Cuvier, become the 

 vena cava superior, into which the right cardinal (vena azygos major) enters. 

 The lower part of the left primitive jugular becomes almost entirely oblite- 

 rated, except at its lower end, where it remains as a fibrous band, or sometimes 

 a small vein, and runs obliquely over the posterior surface of the left auricle. 

 The termination of the left duct of Cuvier remains persistent, and forms the 

 coronary sinus (Fig. 100), the left cardinal separating from it and emptying its 

 blood through the transverse connecting branch into the vena azygos major. 

 The fetal circulation is described at a future page. 



Development of the Alimentary Canal. The development of the intestinal 

 cavity is, as shown above (page 109), one of the earliest phenomena of embryonic 

 life. The original intestine consists of an inflection of the hypoblast extending 

 from one end of the embryo to the other, and is situated just below the primitive 

 vertebral column. At either extremity it forms a closed tube, in consequence of 

 the cephalic and caudal flexures (page 109), and this manifestly divides it into three 

 parts ; a front part, enclosed in the cephalic fold, called the fore-gut ; a posterior 

 part, enclosed in the caudal fold, the hind-gut ; and a central part or mid-gut, 



FIG. 101. Diagrammatic outline of a longitudinal vertical section of the chick on the fourth day. en. 

 Epiblast. srre. Somatic mesoblast. hy. Hypoblast. vm. Visceral mesoblast. a/. Cephalic fold. pf. Caudal fold 

 am. Cavity of true amnion. i/.s. Yolk-sac." i. Intestine. s. Stomach and pharynx, a. Future anus, still closed. 

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

 amnion. (From Quain's Anatomy, Allen Thomson.) 



which at this time freely communicates with the umbilical vesicle (Fig. 101). 

 The ends of the fore- and hind-gut do not communicate with the surface of the 

 body, the buccal and anal orifices being subsequently formed by involutions of the 

 t, which later on form communications with the gut. From the fore-gut 



