NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 205 
a 6 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 
secks 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 mm. 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 Ammocetes. I regard the 
determination of their origin in this animal as an embryological 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 
are differentiated. 
I turn now to the development of the “anterior cavity,” which has 
been so thoroughly studied by Miss Platt (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 atimentary 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 
Platt 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 «u 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 „udimentary muscle cells. Such histological evi- 
