310 Mr MacBride, The Relationship of [May 24, 



The simplest animal which is universally recognised as a 

 Vertebrate is Amphioxus, and the study of its anatomy and 

 development has thrown a flood of light on the meaning and 

 origin of many of the organs in the higher Vertebrates. If 

 however we desire to trace the Vertebrates further back than 

 Amphioxus, we find no consensus of opinion as to the direction 

 in which we should prosecute our search. Dohrn supposes that 

 we ought to go in the direction of the marine Annelid worms. 

 This Society has recently had presented to it by a distinguished 

 fellow, an elaborate paper, pleading for the affinity of Vertebrates 

 with the Arthropoda ; whilst, as we all know, Bateson's view, 

 that Balanoglossus is the lowest animal with traces of Vertebrate 

 structure, has been widely accepted here, though it has not met with 

 the same favour in Germany. 



It must be admitted that Balanoglossus is about as unlike 

 a Vertebrate externally as one can well conceive, and it is difficult 

 to imagine that the dominant Vertebrates should have taken their 

 origin from a mud-inhabiting worm. One of the most prominent 

 features in the organization of the lower Vertebrates is the 

 division of their muscles into segments, a feature which they 

 share with the Annelids, and to which it is hardly possible to 

 compare the division of the body of Balanoglossus in three regions 

 — head, collar and trunk. The main object of this essay is to 

 show that there exists in the Amphioxus larva an analogous 

 division of the body which precedes in order of development the 

 segmentation of the muscles. 



In a former paper on this subject, I showed that the ccelom 

 in Amphioxus originated at three, distinct points from the 

 alimentary tube. 



(1) At the dorsal lateral corners a pair of hollow ridges 

 become constricted off from the rest of the gut, and then by 

 the obliteration of their lumen at various points, divided into a 

 series of distinct sacs. These processes go on progressively as 

 the animal grows in length — the ridges continuing to com- 

 municate with the gut behind, long after they have been 

 completely separated from it and divided into segments in 

 front. 



(2) A pair of pouches grows out of the gut in front of the 

 foregoing and nearer the mid-dorsal line, these give rise to the 

 first pair of muscular segments and to other structures as well. 



(3) The most anterior portion of the alimentary canal becomes 

 separated off from the rest, and becomes immediately divided into 

 a pair of cavities of which the right becomes large and thin- walled, 

 whereas the left becomes a small thick-walled sac which acquires 

 an opening to the exterior. 



