278 ZOOLOGY. 



condition of Amphioxus, whose tubular spinal cord has its lining 

 epithelium pigmented and sensitive to light for practically its whole 

 length. Such a condition is only possible in a translucent animal. 

 If we next imagine a series of forms intermediate between Amphioxus 

 and the dog-fish, they will be successively more and more opaque, 

 and the only way in which the usefulness of the sensitive lining 

 epithelium can be retained is by its extending outwards to the 

 surface, as the primary optic vesicle actually does in development. 

 In a retina developed in such a way, the Vertebrate peculiarities 

 are inevitable. We may therefore reasonably regard those pecu. 

 liarities, not as meaningless caprices, but as indications of the 

 stages by which the eye of Vertebrates has come to be what it is. 



13. TJrinogenital System. We have seen how, in the 

 tadpole, a series of pronephric tubules, in many ways com- 

 parable to the nephildia of Amphioxus, and actively 

 functional in excretion, are replaced in later larval life by 

 another set of excretory tubules, forming the mesonephros. 

 In the chick the pronephros appears, but in a most rudi- 

 mentary condition, and disappears almost at once ; while in 

 the rabbit it is doubtful if it appears at all. In these types, 

 then, the history of the urogenital system begins with the 

 archinephric duct and mesonephros. The first appearance 

 of the duct is shown in fig. 130. It appears close under the 

 epiblast, and it has even been suggested that it originates 

 from the epiblast (an origin that would meet the difficulty 

 mentioned in chap, xviii., 12), but the evidence is against 

 this origin. It lies just on the outer side of the mesoblastic 

 somites, and gradually sinks through the somatic mesoblast 

 till it lies close to the coelom. Meanwhile, the mesonephric 

 tubules have been formed, as in the frog, from the mesoblast 

 of a number of somites. Their structure and relations are 

 practically the same as in the frog, except that the nephros- 

 tomes last longer and do not become connected with the 

 veins. A germinal ridge forms, as in the frog, nearer the 

 median line than the mesonephros. 



The mesonephros does not, however, become the perma- 

 nent kidney. Just as notochord is replaced by cartilage and 

 cartilage by bone, so pronephros is replaced by mesonephros 

 and mesonephros by metanephros. The metanephros is, how- 

 ever, not so distinct f rom the merpnephros as that is from 

 the pronephros. 



