neal: nekyous system in squalus acaxthias. 205 



a G mm. embryo. In consideration of the facts that he does not even 

 know that these muscle cells become differentiated into the eye muscles, 

 and that he has not determined their innervation, the doubt which he 

 seeks to throw upon the results which differ from his own appears quite 

 unwarranted. Furthermore, I find that the anterior and posterior velar 

 muscle strands described by Kupffer are in essentially the same relations 

 to the eye capsule in stages of 6-9 ram. as in those of 5 cm., and that 

 these strands show no relation — except that relation of the posterior 

 (mandibular) muscle strand described by Hatschek ('92) — to the eye 

 muscles, which are already clearly differentiated in the latter stage. I 

 must therefore conclude that Kupffer has not seen the early stages of 

 the development of the eye muscles of Ammocoetes. I regard the 

 determination of their origin in this animal as an enibryological task 

 yet to be accomplished, — a task in which the well known difficulty of 

 obtaining material in stages between 9 mm. and 30 mm. will be encoun- 

 tered. For it is in these stages, in my opinion, that the eye muscles 

 ai'e differentiated. 



I turn now to the development of the "anterior cavity," which has 

 been so thoroughly studied by Miss Piatt ('91, '91*) and by Hoffmann 

 ('96) that I need say but little, and that of a general nature. It 

 seems very clear, since the " anterior " mesoderm segment develops 

 from a perfectly solid mass of cells anterior and lateral to the infundi- 

 bulum of the brain, that the statement of their formation as lateral 

 diverticula of the alimentary canal is purely hypothetical. It seems also 

 warrantable to infer that the connecting stalk which unites the lateral 

 halves of the segments in early stages of development, the cells of which 

 according to Hoffmann ('96) entirely disappear, represents in part the 

 anterior continuation of the alimentary canal. But it is impossible 

 to state, because of want of such criteria as chorda and dorsal aorta, 

 whether we have here to do with dorsal mesoderm. Without proof to 

 the contrary, and with the evidence that these cavities assume a histo- 

 logical appearance similar to that of the following ones, I conclude with 

 Piatt and Hoffmann that the " anterior " mesoderm segment, which 

 appears, so far as is known, in only two Selachii (Squalus and Galeus), 

 is serially homologous with those behind it. I am able to confirm the 

 evidence given by these two observers, that mesenchyma cells migrate 

 into the lumen of the cavity in the later stages of its development, and 

 to confirm the former, that such cells first migrate from the median wall 

 (Figure E), in which also some cells assume an elongated spindle form, 

 possibly indicating rudimentary muscle cells. Such histological evi- 



