1884] 121 [Hyatt 



mass of cells, arose according to Kowalevsky after, and not 

 before, the canal. 



Such facts as these and the primitive nature of the medullary- 

 folds, which appear at first as mere outgrowths like a primitive 

 stomodeum, seem to indicate a homoplastic origin for the essen- 

 tial organs and stand in the way of choice of any known type of 

 invertebrata for an ancestor of the vertebrata. One readily sees 

 how the mechanical requirements of an elongated body might 

 lead to the formation of the medullary plate, and it also opens the 

 way by which it is possible to account for the conversion of 

 the upper part of the mesenteron into the hypophysial tube 

 and finally into a notochord, simply because of the need of 

 a stiff axis to strengthen the neural fold. Possibly the forma- 

 tion of the former may be traceable to reactions of the axial 

 portions caused by the action of the lateral myoblasts in mov- 

 ing an elongated vermiform animal with two regions divided 

 from each other by a longitudinal tube. According to the 

 well known laws of use this tube would be forcibly acted 

 upon by these muscles and react by growth against them, tending 

 continually to become solidified and more resistant. 



This view accords with Cope's explanation of the distribution 

 and formation of the neurocentra, pleurocentra and neurapophyses 

 in the vertebrae of the Permian Rhachitomi. This author with his 

 usual clear insight has illustrated the segmentation of these prim- 

 itive vertebrae by means of the folds which occur on the 

 inner side of a coat sleeve when compressed by the bend- 

 ing of the arm. We find ourselves also in accord with Spencer 

 who first suggested the segmentation of vertebrae to be due to 

 lateral flexion (Prin. Biol., u, p. 195) ; and with Ryder in his last 

 remarkable treatise on the mechanical evolution of the different 

 forms of the tail in fishes. Our hypothesis simply completes 

 their explanations by attributing the origin of the notochord 

 itself to the same causes which have probably occasioned the 

 subsequent segmentation of the calcareous plates, and the ine- 

 qualities of the tail. 1 



We think, also, that the evidence is very deficient for the 

 accepted belief that the Ascidia have degenerated widely from 



1 We have found, too late for insertion in this page,. that Spencer has completely 

 anticipated this theory, (op. cit. p. 199.) 



