NEAL: nervous system IX SQUALTJS ACANTHIAS. 193 



Muskelteile ist eine vollkommene. Dieses Yerhaltnis iihnelt in hohem 

 Grade jenem, welches van Wijhe als ein embryonales von d'eu Selachiern 

 abgebildet hat. Seine Deutung ist aber darin zu korrigieren, dass der m. 

 obliqu. sup. nicht dem parachordalen Muskelblatte, sondern den Seiten- 

 platten zugehort. Die iibrigen Augenmuskeln, die in Form eines Kegel- 

 mantels an* der raedialen ventraleu Seite des Augapfels sich fiuden, 

 bilden in Bezug auf ihre Lage und histologische BeschafFeuheit eine 

 dritte Gruppe, deren Ableitung nicht gauz sicher erscheiut. Sie sind 

 wahrscheinlich von den Konstriktoren des Visceral-apparates abzuleiten. 

 Keinesfalls konnen 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 tlie oculomotorius from the epithelium 

 of this cavity. According to Kupffer all the eye muscles (with the possible 

 exception of the muse, 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. I 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 embrj-os of Torpedo ocellata, kindly- 

 given me by my friend, Professor A. N. Sewertzoff, leads me to agree with Sedg- 



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