neal: nervous system in squalus acanthias. 217 



beginning opposite encephalomere VI. In parasagittal sections the 

 Anlage of the glossopharyngeus appears clearly distinct from that of the 

 vagus, while in the median plane they are seen to be continuous portions 

 of the neural crest. 



When the embryo has 28 to 30 somites (Fig. 13) the conditions, so 

 far as the trigeminus is concerned, are practically unchanged. Neural- 

 crest cells still persist in the regions of constriction between the primary 

 brain vesicles. Thus, three strands of neural-crest cells are seen to lie 

 in the region of constriction between the brain vesicles, just as they do 

 in the trunk between the myelomeres. The ganglionic Anlage of the 

 acustico-facialis, which had fused with the thickened auditory epithelium 

 in the early stages of its development, now, as the nerve Anlage recedes 

 from the ectoderm, retains this connection, forming thus the Anlage of 

 the acusticus. The acusticus therefore in its development and relations 

 resembles a ramus dorsalis of a cranial nerve. 



The cells of the glossopharyngeus have been further displaced. In all 

 the specimens of this stage which I have examined, two distal portions 

 of the nerve Anlage may be distinguished. The fate of the posterior of 

 these is unknown to me. The cells of the anterior portion pass ventrally 

 into the third visceral arch, and are related to the constriction between 

 van Wijhe's somites 4 and 5. In precisely the same way the Anlage 

 of the seventh nerve occupies the cleft between the third and fourth 

 somites. The advancing ganglionic Anlagen pass close to the superficial 

 ectoderm in the plane of the constrictions between the somites. Simi- 

 larly the Urvagus Anlage meets the mesoderm at the posterior cleft of 

 the fifth somite. This fact seems to me to be of some importance in 

 considering the question whether the branchial nerves are soraitic or in- 

 tersomitic in position, and to warrant the conclusion that the cranial 

 nerves resemble the dorsal nerves of Araphioxus in being intei'somitic, 

 as well as in other respects. At a stage with 33 or 34 somites (Plate 3, 

 Fig. 14) the trigeminus Anlage retains connection with the mid-dorsal 

 line of the neural tube in only two restricted regions, anteriorly by the 

 " thalamic " portion, and posteriorly (in the region of the constriction be- 

 tween midbrain and hindbrain) by a strand of cells to which Miss Piatt 

 has given the name " primary trochlearis." Posteriorly the cells of the 

 trigeminus Anlage are grouped into a somewhat thickened mass opposite 

 the posterior part of encephalomere III, the first indication of the differ- 

 entiation of the Gasserian ganglion. The Anlagen of the acustico-facialis 

 and the glossopharyngeus have become farther separated by the invagina- 

 tion of the auditory epithelium, the displacement affecting the cells of 



