460 Mr. S. Hatta. Mesodermic Origin and the Fate of the 



branchiomeres, and the proximal part of each branchiomeric piece is 

 swollen up (Schalk's text-fig. 18, p. 6), giving rise to the cartilaginous 

 visceral bar.* The ectoderm lying in close contact with the thickened part 

 of the mesectodermal epithelium, which is shown in the figure by Schalk as 

 continuous with the ectoderm, is a thickening produced by active multiplica- 

 tion of the component cells of the ectoderm (text-fig. 1, a). 



This ectodermal thickening assumes an oval outline with its long axis 

 vertical and grows inwards (text-fig. 1, b), pressing against the rudiment of 

 the cartilaginous visceral bar. But the conical bottom of the entodermal 

 visceral pouch in front pushes its way laterally and backwards, and presses 

 upon the invaginating ectodermic pouch, finally fusing with it. On the 

 14th day this spot becomes perforated and forms an oval slit with its long 

 axis vertical, and the gill slit is thus established (text-fig. 1, c). The 

 rudiment of the cartilaginous visceral bar is found close behind this orifice. 

 "Now, the thickening of the ectoderm which Schalk saw was the developing 

 gill-cleft and had no genetical relation to the cartilaginous visceral bar. 



A thick section might induce one to assume continuity of the thickened 

 ectoderm and the likewise thickened mesectoderm ; but, in reality, both the 

 parts are separated by a sharply defined boundary line, as careful observations 

 of thin sections prove without difficulty. 



In spite of his efforts, Schalk could not detect, as he says, a corresponding 

 placode in the hyoid segment. Here, in fact, no ectodermal thickening for 

 the gill-cleft takes place at all, because the hyomandibular pouch in front of 

 this visceral area does not break out to the exterior, but is transformed into 

 the velar cavity ; it has no direct respiratory function, but performs an 

 auxiliary service to it. 



I may be permitted to add a few words on the placodes of nervous nature 

 in the branchial region, in order to avoid possible confusion of them with the 

 ectodermal thickening for the gill-cleft above stated. V. Kupffer (94) was the 

 first who described the ganglionic placodes, termed by him the epibranchial 

 placodes. The placodes of the epibranchial ganglia are situated at the 

 level of the dorsal edge of the lateral plates, consequently at a much higher 

 level than that at which the ectoderm thickens for the gill-clefts, and the 

 placode appears in each branchiomere from the vagus segment, which 

 represents the fourth branchiomere, counting from the premandibular arch 

 to the hindmost branchiomere behind the last or eighth visceral pouch. 

 These placodes are, of course, purely nervous and have nothing to do with 

 the mesectoderm ; they represent the posterior section of what I have called 



* The cartilaginous visceral bar in Petromyzon is not to be confounded with the true 

 visceral arch formed in higher craniota. 



