ioi6 A MANUAL OF ANATOMY 



source, but those portions of the left lumbar veins, which cross behind the 

 abdominal aorta to reach the inferior vena cava, represent the persistent 

 transverse retro-aortic posterior cardinal anastomotic vessels. 



Development of Azygos System ol Veins. — The right vena azygos represents 

 the, upper part of the right posterior cardinal vein, above the level of the 

 future right renal vein. The lower and upper left azygos veins are developed 

 from the thoracic portion of the left posterior cardinal vein. The lower and 

 upper transverse azygos veins are persistent transverse retro-aortic posterior 

 cardinal anastomotic vessels. 



Development of Spermatic and Ovarian Veins. — Each of these veins is 

 originally a segmental vein of the corresponding intermediate cell mass, in 

 which the sexual glands are developed. The vessels of the right side have 

 no other source, but the terminal part, of each left vein is formed by that 

 portion of the left posterior cardinal vein which extends between the point 

 of entrance of the left spermatic (or ovarian) vein and the future left renal 

 vein. 



Development of the Lymphatic System. 



The lymphatic system consists of lymphatic vessels and lymphatic glands 

 or nodes. There is also lymphoid or adenoid tissue, as in the thymus body, 

 lymphoid follicles and villi of the intestinal mucous membrane, Malpighian 

 corpuscles of the spleen, palatine tonsils, and pharyngeal tonsil. There are 

 two stages in the development of the lymphatic system — primary and second- 

 ary. The primary stage is concerned in the formation of lymph-sacs, and 

 the secondary stage consists in the formation of lymphatic vessels and lymph- 

 glands. 



Lymph Sacs. — There are two pairs of lymph-sacs and two single sacs, eis 

 follows : 



1. Cervical or jugular (2). 



2. Retro -peritoneal or pre-aortic (i), 



3. Receptaculum or cisterna chyli (i). 



4. Caudal or sciatic (2), 



Opinions differ as to the development of these lymph-sacs. The view of 

 Florence R. Sabin will be stated first. According to this lady the sacs are 

 developed as sproutings from the endothelial lining of veins, and this constitutes 

 the primary stage in the development of the lymphatic system. 



Jugular Sacs. — The jugular lymph-sacs, right and left, are the first to 

 appear. Each is situated on the outer side of the lower part of the primitive 

 anterior cardinal vein, which becomes the internal jugular vein. It is formed 

 from part of a capillary venous plexus, connected in early life with the anterior 

 cardinal vein. A large part of this plexus disappears, whilst the connection 

 of the remainder with the anterior cardinal vein is severed. There thus 

 results a detached collection of capillaries, lined with endothelium and in 

 close proximity to the outer side of the lower part of the anterior cardinal 

 vein. These capillaries become dilated, and subsequently join to form the 

 jugular lymph-sac of each side, which is lined with endothelium, and which 

 establishes a fresh connection with the lower part of the anterior cardinal 

 vein, where it is joined by the subclavian vein, a valve being formed at the 

 venous junction by a protrusion of part of the lymph-sac at the place of 

 junction. The jugular sac of each side is connected externally with per- 

 ipheral lymphatic vessels which extend to the head, neck, and fore-limb bud 

 of the same side. The caudal end of the left sac becomes connected with the 

 thoracic duct, whilst the caudal end of the right sac becomes connected with 

 the right lymphatic duct. The dorsal part of the sac becomes converted into 

 a plexus of lymphatic vessels, from which chains of lymph-glands are de- 

 veloped. 



Retro-Peritoneal Sac. — The retro-peritoneal or pre-aortic lymph-sac is 

 single, and is formed from a capillary venous plexus in the root of the mesentery, 

 which plexus is connected with the great pre-aortic transverse anastomotic 

 vessel which extends between the two subcardinal veins. The capillary 



