NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHI AS. 



201 



i'lGUKE 1). 



connection with the ectoderm immediately posterior to the iufimdibulum. 

 Still later (40 somites) cavities appear botli in the median connecting- 

 stalk and in tlie lateral meso- 

 derm, and these by their fusion <^<=''"' 

 form the continuous cavity in 

 the manner already described 

 by Miss Piatt ('91"). It follows 

 therefore that the premandib- 

 ular cavities comprise dorsal 

 and only dorsal mesoderm/ 



1 HofEmann ('94, p. 648), how- 

 ever, finds evidence of a splanchnic 

 portion of the premandibular so- 

 mite in a " Zellstrang, welcher 

 dem Mandibularliogen parallel ver- 

 lauft und der Vorderfliiche deises Bogens unmittelbar aufliegt" (his Fig. 4 x, 

 p. 648). He adds, "Ein Lumen dieses Bogens habe ich im dieseni Strange nie 

 gesehen," and he uses this evidence to support his conclusion that the mandibular 

 arch is double. I can confirm Hoffmann's statement as to the presence of this 

 "Zellstrang " in the anterior portion of the mandibular arch; but there is another 

 cord, not mentioned by Hoffmann, which is in every respect similar to this one and 

 extends parallel and close to the posterior wall of the arch. I hold Hoffmann's in- 

 terpretation, however, to be incorrect, since, according to my determination, the cells 

 of these strands are in large part if not entirely ectodermal in origin, i. e. derivatives 

 of the neural crest. The cells of the Anlage of the Trigeminus may be followed in 

 closely connected stages as they migrate ventrad until they enter the mandibular 

 arch, where they come to surround the mesothelium as a ring of loose cells between 

 the mesothelium and the superficial ectoderm. This evidence confirms the pre- 

 vious results of Kastschenko, Piatt, and Goronowitsch ('93). While the fate of 

 these cells is not clear to me, Miss Piatt ('94 and '97) finds that in Necturus they 

 contribute in large part to the formation of the cartilage of the mandibular arch. 

 Considering the similarity in the origin of the anterior and posterior cell strands, 

 as seen in parasagittal sections through the mandibular arch, it becomes note- 

 worthy that Hoffmann ('94) in his preliminary paper failed entirely to reproduce in 

 his figures the posterior, while in his later paper ('96, Taf. III. Fig. 22), he figures 

 two cell strands as histologically quite different from each other. This appears to 

 me a notable illustration of the prejudicial influence of a theory. Although I 



Fig. D. Cross section of a Squalus embryo with 11 or 12 somites in a plane cor- 

 responding with that of the line a /8 of Fig. C. X 50. The section shows clearly 

 the dorsal position of the connecting stalk of van Wijhe's first somite (I) in rela- 

 tion to the pre-oral pouch (ar'ent.). 



I, mesoderm of the connecting stalk of van Wijiie's first somite; ar'ent., arch- 

 enteron = pre-oral pouch; c1. crs. n., neural-crest cells; ec'dnn., ectoderm; ent., 

 entoderm ; i'/b., infundibulum ; tb. n., neural tube. 



