142 BIOLOGICAL LECTURES. 



First, the olfactory nerve was brought into the category of 

 the segmental nerves, for Marshall points out that in its devel- 

 opment it resembles the other nerves, and is not a prolongation 

 of the brain substance, that it possesses a ganglion, and finally 

 it splits distally to embrace the olfactory organ, just as the facial 

 or glossopharyngeal splits to pass on either side of the corre- 

 sponding visceral cleft. From this relationship he is led to 

 regard the olfactory organ as a gill cleft, comparing the folds of 

 the Schneiderian membrane with the gills themselves, regard- 

 less of the fact that the one is ectodermal, the other entodermal 

 in origin. Second, the fact that the trochlearis arises from the 

 dorsal crest of the brain, and that the oculomotor was thought 

 to be primarily connected with the ciliary ganglion, leads Mar- 

 shall to assign these motor nerves segmental value. The 

 increase in segments behind is due to the fact that Marshall 

 regarded sharks like Heptanchus, with seven gill slits, as the 

 more primitive. Regarding the rest of the table nothing need 

 now be said, but we must point out, thirdly, that the history 

 of some of the head-cavities was traced. They were found to be 

 connected at first, and their separation into distinct bodies was 

 shown to be independent of the formation of gill slits. The 

 history of two of these cavities was followed, and these two 

 were found to give rise to some of the muscles which move 

 the eyeball. 



At almost the same time that Marshall published his last 

 paper, Van Wijhe gave to the world the results of his studies, 

 and his paper is one of the most frequently quoted in connec- 

 tion with the subject. Head-cavities form the basis of his 

 work, and of these he finds nine in the shark. Since his num- 

 bering of these has been almost universally followed, we 

 may enumerate them with some detail, giving their fates as 

 determined by Van Wijhe. The first pair of cavities are pre- 

 mandibular in position, and in the later development give rise 

 to the muscles rectus superior, inferior, internus, and inferior 

 oblique, all of which are controlled by the oculomotor nerve. 

 The second cavity is above the mandibular arch, and sends a 

 branch into it. This cavity gives rise to the superior oblique 

 eye muscle, and is innervated by the trochlearis nerve. The 



