CHAP. X.] THE DEVELOPMENT OF THE CAT. 349 



into what was the right ductus Cuvieri and what is ultimately the 

 vena cava superior. The left azygos veins (the old left cardinal 

 vein) opens at last, as at first, into what was the right ductus 

 Cuvieri, and what is ultimately that part of the superior intercostal 

 vein which is nearest to the left innominate vein. 



After birth and the cessation of the placental circulation, the 

 ductus venosus diminishes and becomes obliterated, save as that fibrous 

 cord, the round ligament which traverses the liver. The umbilical 

 vein also disappears, save as that part of the round ligament which 

 extends from the umbilicus to the liver. Thenceforth all the blood 

 returning from the stomach and intestines is compelled to circulate 

 through the portal and hepatic veins on its way to the heart. 



Those parts of the umbilical arteries which are within the body — 

 the /i//ogastric arteries — similarly abort where they intervene be- 

 tween the bladder and the umbilicus, becoming merely two fibrous 

 cords. Those parts which intervene between internal iliacs and the 

 side of the bladder, persist as the superior vesical arteries. 



The DEVELOPMENT of the LYMPHATIC SYSTEM rosembles more or 

 less that of the blood-vascular system. The small ///inphaiics 

 originate by the junction of nucleated cells, and they spread and 

 increase through the development of peculiar cells, which branch 

 out and join fine processes given off from the lymphatics formed 

 earlier. The lymphatic glands appear to be developed from clusters 

 and aggregations of lymphatic vessels from which diverticula grow 

 out. The liimphatic corpuscles probably originate from subdivision of 

 cells contained within the early formed lymphatic vessels and glands, 

 also in the spleen and perhaps in the thyroid and thymus glands. 



The fluid lym]3h may be regarded, like the fluid blood, as the 

 blastema of a tissue of which the white corpuscles represent the 

 nucleated cells. 



§ 16. The LUNGS are at their origin small outgrowths from the 

 aUmentary canal (Fig. 155, I). This canal, at a point immediately 

 behind the heart, becomes laterally constricted by two lateral in- 

 foldings of its walls, which divide it into a narrower upper, and a 

 broader lower (or ventral) channel, while the lower channel tends 

 to be subdivided by a median longitudinal upgrowth from its floor. 

 This process continuing, three tubes come to be formed, whereof the 

 dorsal one is the future ossophagus, while the two ventral ones 

 become the two lungs, which thus arise as two diverticula, lined 

 with hypoblast, from the ventral aspect of the oesophagus. These 

 little sacs elongate by degrees, and ultimately come to hang by a 

 single and common supporting tube, the future trachea. Ramifica- 

 tions of the primitive cavity into the mesoblastic tissue surrounding 

 it, form the bronchial tubes, mth their branches. 



In contrast with the liver, the lung.s of the fffitus are exceedingly 

 small, but as soon as the first inspiration has taken place, they 

 rapidly increase in size and weight, while their consistency changes 

 from that of a granular, compact, heavy substance, to a light and 

 spongy texture. 



