411 



two of the bridges had changed their position since the embryo was 

 in the same stage as the last one, and he also assumed that two others 

 had fused ; but no attempt was made to account for the irregular dis- 

 position of the remainder. A more justifiable assumption is that sepa- 

 ration is initiated at somewhat different points in different embryos. In 

 both hypochord and bridges were isolated spaces which Stöhr ranked 

 as lumina, though they were merely intercellular spaces, and maybe 

 vacuoles also. Neither bridges nor lumina seem to have been found 

 in the posterior trunk region and tail. The maximum development of 

 the hypochord coincided with the appearance of the lumina; and de- 

 generation began immediately after it had separated from the hypoblast. 

 In the tail the gut disappeared before the hypochord, and was in small 

 fragments when the latter was still continuous. A small piece of the 

 hypochord, which was believed to have a separate origin, lay in the 

 auditory region in front of the main part, and to it the term "head 

 part" was applied. Calling to his aid the supposed metameric arrange- 

 ment of the bridges, the supposed fact that lumina existed, and the 

 fact that in his youngest embryo there were extensions of the lumen 

 of the gut into its dorsal wall in the anterior trunk region, Stöhk ad- 

 vanced the hypothesis that primitively a series of metamerically- arranged, 

 tubular outpushings of the gut-wall extended along the dorsal line, 

 and that their distal ends had secondarily fused to form the hypochord. 

 The bridges were the proximal parts of these outpushings. 



BifiRGFELDT (4) was the next to give special attention to the hypo- 

 chord. It appeared in the trunk and tail of Alytes as a strip of cells 

 lying within the dorsal wall of the gut, and was scooped out, as it 

 were; but in the head it became a ridge before separation took place. 

 The posterior borders of the auditory vesicles was reached in front; 

 and behind it was lost in the undifferentiated mass of cells into which 

 merged also the tail-gut, the notochord and the neural tube. The 

 bridges were not metamerically arranged, and lumina were absent. 

 An isolated piece of the hypochord lay in some embryos in the audi- 

 tory region. At an early stage there developed a highly refractive 

 cuticle which did not select the same stains as the cuticula chorda, 

 and which, like the cuticles of Hasse and Field, did not closely invest 

 the hypochord. The latter attained its maximum development when 

 the perichordal tissue was collecting and was inserting itself between 

 it and the notochord. Degeneration was complete. And contrary to j 

 what Stöhr noted in Rana, the tail-gut was healthy when the hypo- 1 

 chord above was degenerating. 



Klaatsch (22), with scanty Selachian material, could not describe 

 completely the development of the hypochord in his paper devoted to 

 that structure. When ridge-like, the cavity of the gut extended into 

 it; the bridges were not metameric; and lumina were not found. 

 Stöhr's hypothesis was pronounced against and one of his own advan- 

 ced : that the hypochord is homologous with the epibranchial groove of 

 Amphioxus. A secondary extension of the groove throughout the whole 

 length of the Ichthyopsidan gut had to be assumed; and it was men- 

 tioned that in the young Amphioxus the groove extends a little beyond 



