410 



after my own observations were completed. Hatta tells that before the 

 chorda-hj'^poblast began to fold, when it was still the median part of 

 the archenteric roof — and after the mesoblastic diverticula had parted 

 from the hypoblast — it was divided from the hypoblast on either side 

 by "peculiar spindle-shaped hypochordal cells". These cells were "prob- 

 ably derived from cells" lying originally between the chorda-hypoblast 

 and the mesoblastic diverticula. After the withdrawal of the notochord,^ 

 the median part of the gut roof was formed by these hypochordal cells ; 

 and as they passed upwards behind the notochord to become the hypo- 

 chord, their place was taken by hypoblast cells. 



Hoffmann (18) noticed that the hypochord in Acanthias arose from 

 the hypoblast ; but some of his statements were of a remarkable nature, 

 having presumably been founded upon a hasty examination of defective 

 material. We may safely believe that they were later tacitly aban- 

 doned. It was claimed that the most dorsal part of the gut — his 

 "Aortadarm" — became constricted off to form the aorta, while the 

 most ventral part — his "Herzdarm" — also became part of the vas- 

 cular system. To him the hypochord was a thickening of the dorsal 

 wall of the "Aortadarm" and was a blood-forming organ still functional 

 in Acanthias. 



Klaatsch (20) asserted that the ligamentum longitudinale ventrale 

 was partly built out of the hypochord. In a succeeding paper (21) 

 there was also mooted the question of the mixing of hypochordal cells 

 with the others forming the perichordal tissue. His paper devoted to 

 the hypochord is referred to below. 



Field (7) saw the hypochord first appear as a ridge in the pro- 

 nephric region of Amblystoma embryos. It developed as far as the 

 auditory vesicles and the end of the tail. Eventually it left its position 

 against the notochord, became surrounded by skeletogenous tissue, and 

 disappeared. Field homologised it with the "Nebendarm" of Polychaets. 

 He claimed that it was enclosed b}^ a cuticle strictly comparable with 

 the cuticula chorda. Curiously enough, this cuticle is not shown in his 

 figures of Amblystoma, but appears in those of Rana, a type in which 

 I cannot find it. And surely it is a remarkable cuticle in that there 

 is a space nearW all the way round between it and the hypochord. 

 As this feature is characteristic also of a supposed cuticula chorda 

 shown in some figures of Amblystoma, it would seem that these puta- 

 tive cuticles were artefacts of a familiar kind ; and this may be true 

 also of the cuticle depicted by HAsse. 



Stöhr (45), with material of Rana, was the first to make the hypo- 

 chord an object of special research. There was the usual beginning in 

 the anterior trunk region as a ridge. In an older embryo separation of 

 hypochord from hypoblast was completed in this region except where 

 isthmuses of cells maintained a union between the two. Stöhr called 

 these isthmuses "bridges" and claimed for them a metameric arrange- 

 ment. In the next older embryo, where the area of separation was 

 more extensive and the bridges were more numerous, they had varying 

 distances between them, while they were varied in anterior-posterior 

 bx'eadth. To account for this absence of metamerism, he assumed that 



