144 BIOLOGICAL LECTURES. 



of great interest had we opportunity for their discussion. Most 

 of these deal with the components of the several nerves, with 

 the true relationships of the ciliary ganglion, with the position to 

 be accorded the eleventh nerve, and with the question whether 

 there be primitively a dorsal root to the hypoglossal. The head- 

 cavities, too, are discussed. On the one hand, Rabl, in a most 

 valuable summary of our knowledge of the whole subject, can- 

 not find all of Van Wijhe's cavities, while Dohrn does not 

 regard the eye muscles as comparable to the muscles of the 

 trunk, a conclusion to which he is led, among other reasons, 

 by his view that the lens of the eye is a modified branchial 

 cleft. On the other hand, there are others who find more head- 

 cavities than Van Wijhe, and among these is to be enumerated 

 Miss Platt, who has found one undoubted cavity in front of the 

 most anterior one of the Dutch anatomist. Dohrn and Killian 

 go much further, and find eighteen or nineteen of these struc- 

 tures, but their work can be dismissed with fevfr words. It is, in 

 fact, difficult to say what Dohrn's opinions are. He is most 

 fertile in hypotheses, and he never takes the trouble to bring 

 his later views into any harmony with the old ones. At one 

 time every thickening of ectoderm or entoderm is a gill cleft, 

 again every hole in the mesoderm is a head-cavity, in the third 

 view the abducens is a complex of at least six nerves. But we 

 are getting ahead of our story. 



To Froriep and Beard we owe the introduction of a new ele- 

 ment into the discussion, that termed by Beard " branchial sense- 

 organs." These authors independently discovered that certain 

 of the cranial nerves fuse with the ectoderm a short distance 

 from the brain. From this fusion two structures are developed : 

 one the ganglion of the nerve, from which the fibers of the 

 permanent nerve grow back into the brain ; the other, the more 

 superficial portion, forms the Anlage of a sense-organ situated 

 in the branchial region just above a gill cleft. These sense- 

 organs are regarded by Beard as segmental in nature, and 

 hence, if we count these sense-organs, we at the same time 

 count the metameres of the head. So, proceeding on this basis, 

 Beard finds eleven segments in the head, and arranges the 

 nerves to fit in the way which can readily be seen from the 



