THE VASCULAR SYSTEM 341 



arises, to the tip of the tail, lying in the median line, just ven- 

 tral to the intestine. At the very first it consists of a pair of 

 fine vessels running- very near one another, but these soon 

 coalesce into a single median vessel, much as in the case of the 

 aorta. At the level of the cloacal opening the two original 

 elements remain distinct, and run along the sides of the in- 

 testine, but fuse again posterior to it, forming a loop or 

 ring. 



Previous to this the cardinal system has begun its develop- 

 ment in the form of minute vessels which grow out from the 

 sides of the sinus venosus, and as they extend farther and be- 

 come of larger size the free ends of the posterior cardinals 

 form several anastomoses with the subintestinal vein anterior 

 to the cloacal ring and at the place about which the kidneys 

 (mesonephros) are to develop. This connection furnishes two 

 large lateral channels for the blood from the subintestinal 

 system, a change which has two direct results, first, the gradual 

 usurpation of function of that part of the subintestinal vein 

 which lies anterior to the anastomoses, a relationship that leads 

 to its ultimate disappearance, and second, the retention of the 

 part posterior to the connection as the caudal vein, now become 

 a part of the cardinal system. At the point where the original 

 anastomoses occur, the development of the kidneys causes the 

 formation of a rich capillary net-work, a process which ends in 

 the establishment of the renal portal system with caudal veins 

 for conveying the blood to the kidneys and posterior cardinals 

 for re-collecting it and conveying it to the heart. 



Although we know that stages like those just described 

 no longer exist in living adult animals, it is quite certain that 

 in these embryonic changes an early phylogenetic history is 

 recapitulated ; that in some past group of animals, dimly 

 foreshadowing the vertebrate type, a well-developed subin- 

 testinal vein existed, and that the usurpation of its function 

 by the cardinal system, repeated with great faithfulness to 

 detail in selachian embryos, was once actually experienced 

 and slowly worked out in adult animals through the action 

 of natural selection or whatever other forces are and have been 

 in operation for the gradual improvement of organisms. 



