THE PRIMITIVE KIDNEY OF SKULLED ANIMALS. 407 



knot of blood-vessels (glomerulus, an arterial net, Fig. 

 317, -B, c). Afferent arterial branches (vasa afferentia) con- 

 vey arterial blood into the coiled branches of the "glome- 

 rulus" (d), and efferent arterial branches (vasa efferentia) 

 again carry it out of the glomerulus (e). 



In Primitive Fishes (Selachii) also there is a longitudi- 

 nal series of segmental canals, which open outwardly in 

 the primitive kidney ducts. The segmental canals (a pair 

 in each metameron of the central part of the body) open, in 

 this case, freely into the body-cavity, through a ciliated 

 funnel (as in Ringed Worms, or Annelids). A part of this 

 organ forms a compact primitive kidney, while the rest is 

 employed in the formation of the sexual organs. 



The primitive kidney in the embryo of Man and in that 

 of all other Skulled Animals (Craniota) is first formed in 

 the same simple shape which persists throughout life in 

 Myxinoides, and partly in Selachii. We found this primi- 

 tive organ in the human embryo at that early period just 

 succeeding the separation in the skin-sensory layer, of the 

 medullary tube from the horn-plate, and the differentiation, 

 in the skin-fibrous layer, of the notochord, the primitive 

 vertebral plate, and the skin-muscle plate. As the first 

 rudiment of the primordial kidneys, a long thin, thread-like 

 string of cells, which is soon hollowed out into a canal, 

 appears in this case, on each side, immediately below the 

 horn-plate ; this extends in a straight line from front to 

 back, and is plainly seen in the cross section of the embryo 

 (Fig. 318) in its original position in the space between the 

 horn-plate (K), the primitive vertebrae (uw), and the skin- 

 muscle plate (hpl). The first origin of this primitive 

 kidney duct is still a matter of dispute, some ontogenists 



