i So 



THE ORIGIN OF VERTEBRATES 



C.N.S 



LVS. 



line of two longitudinal venous channels, which were originally situ- 

 ated one on each side of the notochord, in what was then the ventral 



surface of this part of the embryo. 

 This history is especially in- 

 structive in showing how the 

 pharyngeal region is formed by 

 the growing round of the lateral 

 mesoblast, i.e. the muscular and 

 other mesoblastic tissues of the 

 branchial segments, and how the 

 two longitudinal veins take part 

 in this process. The phyloge- 

 netic interpretation of this em- 

 bryological fact seems to be, 

 that the new ventral surface of 

 the vertebrate in this region is 

 formed, not only by the branchial 

 appendages, but also by the 

 growth ventrally of that part 

 of the original ventral surface 

 which covered each longitudinal 

 venous sinus. 



The following out of the 

 consecutive clues, w 7 hich one 

 after the other arise in har- 

 monious succession as the neces- 

 sary sequence of the original 

 working hypothesis, brings even now into view the manner in which 

 the respiratory portion of the alimentary canal arose, and gives 

 strong hints as to the position of that part of the arthropod which 

 gave origin to the notochord. Here I will say no more at present, 

 for the origin of the new alimentary canal of the vertebrate and of 

 the notochord will be more fittingly discussed as a whole, after all 

 the other organs of the vertebrate have been compared with the 

 corresponding organs of the arthropod. 



The strong evidence that the vertebrate heart was formed from a 

 pair of longitudinal venous sinuses on the ventral side of the central 

 canal, carries with it the conclusion that the original single median 

 dorsal heart of the arthropod is not represented in the vertebrate, 



Fig. 72. — Diagram (Upper Half op 

 Figure) of the Original Position 

 of Veins (H) which come together 



TO FORM THE HEART OF A VERTEBRATE. 



C.N.S., central nervous system; 71c, 

 notochord; m., myotome. 



The lower half of figure shows compara- 

 tive position of the longitudinal venous 

 sinus (L.V.S.) in Limulus. C.N.S., 

 central nervous system ; A I., alimentary 

 canal; H., heart ; m., body-muscles. 



