190 The Development of the Ej-e Muscles in Acanthias 



A considerable lumen can now be distinguished in the lateral somites. 

 The median stalk is continuous at its middle with the notochord above 

 and the alimentary canal behind. Hoffmann describes three processes 

 arising at this stage (32 segments) from the 1st somite : 



I. A process extending backwards and downwards, running close to, 

 and j)arailel with, the visceral prolongation of the 2nd somite. He 

 considered it probable that this process, although he had never seen 

 a lumen in it, was comparable to the hollow process found by Zimmer- 

 mann, 91, in Pristiurus, connecting the premandibular somite with the 

 ventral coelom. Neal, 98, p. 201 note, while confirming the. presence 

 of this " Zellstrang," was very certain that it was derived not from the 

 mesoderm but from the neural crest, having followed the migration of 

 neural crest cells ventrally into the mandibular arch. Neal also called 

 attention to a similar strand of cells situated posterior to the visceral 

 portion of the mandibular cavity. While I found that this strand of 

 cells is apparently continuous with a slight outgrowth from the 1st 

 somite, as figured by Hoffmann, this continuity seems to be more appar- 

 ent than real. In the first place, I am able to confirm the presence of 

 the posterior strand described by Neal. Further, I found this posterior 

 strand not only continuous laterally with the anterior strand, but dor- 

 sally with neural crest cells. This is especially evident at a 32-33 

 somite stage. I therefore conclude with Neal that this " Zellstrang " 

 is not a process of the somite, but is rather derived from the neural 

 crest. 



II. A process extending ventrally forward below the anterior cavity. 

 A similar process was seen by A'^an Wijhe in Pristiurus and was homo- 

 logised by him with the anterior head cavity of Galeus. Since the 

 anterior cavity in Galeus is in all probability homologous with that in 

 Acanthias, if Hoffmann's process in Acanthias is homologous with that 

 found by Van Wijhe in Pristiurus, then, since both occur at once in 

 Acanthias, the homology drawn by Van Wijhe could not 1)e maintained. 

 Hoffmann believed this to be the case. 



1 have been able to find but very slight evidences of this process. I 

 am led therefore to doubt the homology drawn by Hoffmann between 

 this process and the process described by Van Wijhe in Pristiurus. 

 Consequently, I cannot on this ground take exception to the homology 

 drawn by Van Wijhe between the process of the 1st somite in Pristiurus 

 and the anterior somite in Galeus. 



III. A process extending dorsally along the anterior surface of the 

 2nd somite. It is small in extent and transitory in appearance. I am 

 inclined, here as before, to doubt the real continuity between this 



