NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 193 
Muskelteile ist eine vollkommene. Dieses Verhältnis ähnelt in hohem 
Grade jenem, welches van Wijhe als ein embryonales von den Selachiern 
abgebildet hat. Seine Deutung ist aber darin zu korrigieren, dass der m. 
obliqu. sup. nicht dem parachordalen Muskelblatte, sondern den Seiten- 
platten zugehört. Die übrigen Augenmuskeln, die in Form eines Kegel- 
mantels an der medialen ventralen Seite des Augapfels sich finden, 
bilden in Bezug auf ihre Lage und histologische Beschaffenheit eine 
dritte Gruppe, deren Ableitung nicht ganz sicher erscheint. Sie sind 
wahrscheinlich von den Konstriktoren des Visceral-apparates abzuleiten. 
Keinesfalls können sie nach ihre Lage, Verlaufsrichtung und Struktur 
zu den Seitenrumpfmuskeln in Beziehung gebracht werden.” 
Kupffer’s (94) results are essentially a confirmation of those of Hat- 
schek, Finding that the premandibular cavity entirely disappears, and 
that its cells contribute in no part to the formation of the eye mus- 
cles, Kupffer is led to doubt the conclusions of those investigators who 
derive the muscles innervated by the oculomotorius from the epithelium 
of this cavity. According to Kupffer all the eye muscles (with the possible 
exception of the musc. rectus posterior) are derived from two visceral 
arches, the “ trabecular ” and the mandibular. This evidence, as well 
as that given by Hatschek, obviously stands in direct contradiction to 
the somite theory. I am, however, after my study of the literature, 
inclined to be optimistic concerning the ultimate settlement of the ques- 
tion as to the somatic value of the pre-otic mesodermal segments, for 
the differences of opinion are not due to equivocal evidence, but to 
directly contradictory and equally positive statements. We have chiefly 
to determine who has stated the facts correctly in order to determine 
whether we shall accept the opinion of van Wijhe, or that of Kast- 
schenko, Rabl, and Froriep. The evidence obtained by me, which leads 
me unhesitatingly to accept the view of the first, that the head somites 
are serially homologous with trunk somites, is as follows. 1 find the pre- 
otic mesodermal segments as described by van Wijhe (82) most clearly 
defined by mesodermal constrictions or clefts in embryos of Squalus 
with 28 or 30 somites (Plate 3, Fig. 13, Plate 6, Fig. 40, Plate 7, 
Fig. 46)" They are so distinctly marked that they may be seen in 
whole specimens properly cleared, as well as in sections. Moreover, 
they are found to be the same on both sides of the embryo.’ 
1 Van Wijhe's post-otic mesoderm segments have indisputable somatic value, 
and need not be brought into discussion. 
2 An examination of some finely preserved embryos of Torpedo ocellata, kindly 
given me by my friend, Professor A. N. Sewertzoff, leads me to agree with Sedg- 
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