254 THE DOCTRINE OF DESCENT. 



let also, the anterior end of the prhnitive intestine 

 becomes the branchial cavity. But with reference to the 

 vertebrate animals, the most important parts of the Asci- 

 dian larva are the following. It possesses a true spinal 

 cord with a vesicularly expanded brain {ra). The distri- 

 bution and position of this organ agrees accurately with 

 the corresponding parts of the vertebrate animal, and 

 Kupfer has even discerned the rudiments of nerves 

 {s s s), which, if the observation is confirmed, will still 

 more incontrovertibly establish the homology of the 

 organ in question with the spinal cord of the Vertebrata 

 and the nerves proceeding from it in pairs. But we 

 know that it is not the spinal cord alone, but its combi- 

 nation with the vertebral column which constitutes the 

 characteristic feature of the vertebrate animal. This ver- 

 tebral column the Ascidian larva likewise possesses (f) 

 in the form of the noto-chord, and, as in the vertebrate 

 animal, this embryonic vertebral column lies between 

 the intestine and the spinal cord. So far goes the ac- 

 cordance ; henceforth, the development of this part, so 

 important to the vertebrate animal, becomes retrogres- 

 sive in the Ascidian. The rudder-like tail, with the 

 spinal cord contained in it, and the noto-chord, are CdSt 

 off wlien the animal becomes fixed ; the larval brain 

 which promised so well, shrinks into an insignificant 

 nervous ganglion, and the complete animal gives no 

 cause for suspecting its analogy with the Vertebrata. 



These laborious observations prove that the Vertebrata 

 are not the sole proprietors of the spinal cord and verte- 

 bral column, but received these organs as a heritage from 

 lower grades of organization as their progenitors. It 

 does not occur to the Darwinists to regard man as the 



