DEVELOPMENT OF BLOOD-VESSELS. 101 



veins of Rathke, which are persistent in fishes. The union of these two 

 conjugate sub vertebral veins in the human subject, and the subsequent 

 termination of the true vena azygos, thus formed, in the vena cava su- 

 perior, is little more than a repetition of what occurs in the Myxionoid 

 fishes, where the symmetry of the venous system observed in fishes generally 

 is destroyed ; the union of the anterior and posterior subvertebral trunks 

 (the jugular and cardinal veins of Rathke) taking place on one side only, and 

 the two posterior veins uniting before joining the single anterior trunks. 



Development of Blood-vessels generally. From his researches on the 

 mode of development of the tissues in young Batrachians, Kolliker * has 

 obtained some important results in relation to the formation of blood-vessels, 

 which are especially valuable since they tend, in great measure, to re- 

 concile the opposite opinions of those physiologists who adopt Schwann's 

 view f in its fullest extent, by attributing the formation of blood-vessels 

 exclusively to the direct transformation of nucleated cells, and of those 

 who, with Plattner and others, consider that new blood-vessels are never 

 formed except as off-shoots from previously existing vessels. Kolli- 

 ker finds, that in the tail of tadpoles it is by the combined metamorphosis 

 of cells and the production of off-shoots from tubes already in connexion 

 with the general circulation, that new vessels are developed. In the tails 

 of these larva? all the vessels have originally the microscopic characters of 

 the finest capillaries, being composed of a delicate, perfectly homogeneous 

 membrane, with nuclei scattered along its internal surface. The mode 

 of formation of the main arterial trunk and its corresponding vein, which 

 run along the axis of the tail, cannot be observed, owing to the opacity of 

 the surrounding tissues at the period of their development; but these two 

 trunks, which at their distal extremity communicate with each other by a 

 simple arch, elongate, as the tail increases in length, by pushing forth off- 

 shoots which join and coalesce with embryonic cells situated around the 

 posterior extremity of the chorda dorsalis. The first lateral vessels of the 

 tail have the form of simple arches, passing from the artery to the vein, 

 and are produced by the junction of prolongations sent from both the 

 artery and vein, with certain elongated or star-shaped cells, in the sub- 

 stance of the tail. When these arches are formed, and are permeable to 

 blood, new prolongations pass from them, join other radiated cells, and thus 

 form secondary arches. In this manner the capillary net-work extends in 

 proportion as the tail increases in length and breadth, and it, at the same 

 time, becomes more dense by the formation, according to the same plan, 

 of fresh vessels within its meshes. The prolongations by which the vessels 

 communicate with the star-shaped cells, consist at first of narrow pointed 

 projections from the side of a vessel, which gradually elongate until they 

 come in contact with the radiated processes of the cells. The thickness 

 of such a prolongation often does not exceed that of a fibril of fibrous 



* An. des Sc. Nat. Aout, 1846. t See Miiller's Physiology, pp. 404, and 1537. 



