34 FROM FISH TO PHILOSOPHER 



Though the soft organs of the body are never preserved 

 in the fossil record, the evolution of the kidney can be 

 reconstructed with reasonable accuracy from data ob- 

 tained from hving forms. In this reconstruction we can 

 begin with the premise that the body of the protoverte- 

 brate was divided into many similar, regularly repeated 

 muscular segments. This segmentation persists in the 

 skeletal muscles, nerves, and backbone of all the higher 

 vertebrates. The viscera or hollow organs were, however, 

 not segmentally divided but extended continuously 

 from mouth to anus, and were contained in an unseg- 

 mented body cavity known as the coelom {koiloma = 

 a hollow). 



In the embryos of all vertebrates the viscera, on the 

 one hand, and the segmented muscles and backbone, 

 on the other, develop from different germinal layers— the 

 viscera from the inner germinal layer or endoderm; the 

 muscles and backbone from the middle germinal layer 

 or mesoderm. The kidney develops not as one might ex- 

 pect from the endoderm with the viscera, but from the 

 mesoderm (as also do the gonads and adrenal cortex), 

 and, Hke the muscles and backbone, it starts out as a 

 segmented structure. From this fact, and from the struc- 

 ture of the adult kidney in the primitive fishes, it is in- 

 ferred that the kidney of the protovertebrate was also 

 segmented, and that each segment of the body through- 

 out the length of the coelom carried a bilateral pair of 

 nephric (nephros = kidney) tubules. There is some evi- 

 dence that the membrane lining the coelomic cavity 

 primitively played a role in excretion, and it appears 

 that the nephric tubules were first formed by multipHca- 

 tion of cells belonging to the coelomic membrane. On 

 the interior of the body each of these nephric tubules 

 communicated freely with the coelom by means of an 

 open mouth or 'coelomostome' {coelom + stoma = 

 mouth, also frequently called nephrostomes') ; on the ex- 

 terior they drained either through separate vents in the 



