THE ANCESTRY OP THE OHOEDATA. 81 



answer to this question can only come in a fuller understand- 

 ing of the laws of growth and of variation which are as yet 

 merely terms. 



Preliminary Remarks on the Repetition of Organs 

 of the Chordata. 



In the foregoing pages it has been attempted to show (1) 

 that repetition of organs and sets of organs is of common 

 occurrence among animals, and (2) that however far back a 

 segmented ancestor of a segmented descendant may possibly 

 be found, yet ultimately the form has still to be sought for in 

 which these repetitious had their origin. Heuce it follows 

 that in no case must it be held k priori impossible that an 

 unsegmented form showing no degeneration should be related 

 to a segmented stock. But when inquiry is made in the 

 special case of the Chordata as to the condition of the repe- 

 titions found among them, it will be seen that so far are they 

 from suggesting that their immediate ancestor of the group 

 must have been segmented, that they even preclude this view. 

 As will be shown, there is a history of the actual steps by 

 which several of the organs (the nervous system, the axial 

 skeleton, and the mesoblast) acquired their repetitions within 

 the group, and certain other structures (the notochord, &c.) 

 persist in an unsegmented form. So that instead of regarding 

 a fully segmented form as their possible ancestor it is neces- 

 sary to search for a form in which these particular sets of 

 structures at least are not repeated. 



For in the first place, taken generally, the development of a 

 Vertebrate consists in the gradual appearance of repetitions, 

 first of one organ and then of another, until at last a climax is 

 reached. The mesoblast divides into blocks, paired peripheral 

 nerves grow out, and segmented tubules arise in connection 

 with the excretorj ducts, but the mesoblastic plates were at first 

 unbroken, the medullary plate continues without transverse 

 divisions, though its peripheral organs may be repeated, and 

 the excretory ducts are single tubes with single openings. That 

 many of these structures roughly correspond with each other 



