412 



the branchial region. Klaatsch had forgotten that the thyroid is much 

 later than the hypochord in beginning its development. The latter is 

 first apparent in a 2.5 mm skate embryo ; while the former, once part 

 of the same system as the epibranchial groove, is absent in a 12 mm 

 embryo, and is at a very early stage in one of 17 mm. Further, I 

 have found in a 21 mm embryo a tubular outpushing of the branchial 

 wall opposite to the thyroid which can with better reason be homo- 

 logised with the epibranchial groove. 



Franz (8), another worker applying himself particularly to the 

 hypochord, observed its earlier stages in the trout, the later in the 

 salmon. In the trunk and branchial region, at the top of the hypo- 

 blastic fold — the "Darmstraug" — and therefore against the noto- 

 chord, was a mass of cells distinguishable from those below by the 

 clearness of their cytoplasm, by their more spherical shape, and by 

 the distinct border of the mass. Franz did not magnify the latter 

 feature into a cuticle, for it would be no more than the so-called 

 ectoplasm. The part of the fold between the hypochordal rudiment and 

 the definitive hypoblast began to degenerate, but at first only over 

 certain areas so that bridges were left opposite to the myotomes. They 

 were absent from the tail and part of the trunk. In the tail (salmon) 

 the mode of formation was the same as that noted by Wilson in the 

 case of Serranus, for in that region, as the gut degenerated, it left 

 behind a rod of cells. While degenerating, the hypochord was merely 

 enclosed in the ligamentum longitudinale ventrale and did not assist 

 in its formation. 



Wheeler (47) mentioned that in Petromyzon the hypochord does 

 not become a permanent organ. 



SoBOTTA (44), having at hand the some material as Franz, contra- 

 dicted his assertion that the bridges were metamerically arranged. 



Reinhardt (39) has written a paper entirely about the hypochord ; 

 in this case, of the Salamander. A ridge was raised above the hypo- 

 blast, and coextensive with it was a shallow groove in the ventral sur- 

 face of the hypoblast below. Upon the free surface of the ridge and 

 below it there early developed a cuticle which was comparable with the 

 cuticula chorda. In his oldest embryos, while the hypochord was con- 

 tinued into the indifferent mass of cells at the posterior end, the tail- 

 gut stopped short of the mass. The anterior end reached the auditory 

 vesicles; and in some embryos a "head part" lay in front. Degeneration 

 began when the hypochord was surrounded by mesenchyme; and after 

 its disappearance the cuticle remained in evidence for a while as a 

 tubular structure, which, when compressed by mesenchyme, was distin- 

 guishable with difficulty. 



UssoFF (47), in a preliminary communication, has recorded some 

 results of his labours upon the hypochord. The types examined were 

 Pristiurus, Scyllium, Bufo, Tropidonotus, and Buteo. Owing to some 

 malobservation and misinterpretation, arising partly, I take it, from 

 defects in the material, this is a difficult paper to deal with. There 

 are three courses open to me: to summarise it briefly here, to write a 

 separate critique upon it, to wait until the completed results have been 

 published. I have adopted the last course. 



