NERVES OF THE DOGFISH 295 



head, but in seeking to establish a theory we have too often over- 

 looked the obvious facts. A review of the extensive investiga- 

 tions into the anatomy of the selachian nervous system would be 

 of little value in the present research, for, notwithstanding the 

 voluminous literature, few attempts have been made in an exact 

 analysis of the cranial nerves of these forms. Strong ('03, '04) 

 published very brief abstracts of very extensive and thorough- 

 going analyses of the cranial-nerve components in Squalus acan- 

 thias. Landacre ('16) described the embryonic condition of the 

 cranial ganglia and nerve roots in the same species. Houser 

 ('01) made a careful study of the neurones of the selachian brain, 

 using chiefly Mustelus canis. Allis' ('01) studies upon Mustelus 

 laevis have made valuable contributions to our knowledge of the 

 selachian nervous sytem, but the writers are unable to follow 

 him in many of his conclusions. 



It is in the hope of clarifying our present knowledge of the basic 

 structures that constitute the peripheral portions of the cranial 

 nerves of the elasmobranch fishes that the writers have under- 

 taken an analysis of the composition of these nerves. At the 

 very outset of the presentation of our results, it may be said 

 that there has been throughout the investigation a remarkable 

 clearness as to the certainty of the facts. Long and wearisome 

 tracing out of details has characterized the progress of the re- 

 search, but there has been a satisfying certainty about all the 

 minutiae that has made the task a pleasant one. Probably in 

 no other vertebrates are the nerve components more distinct 

 histologically than in the selachians. Given the proper tech- 

 nical treatment, there need be little uncertainty about the 

 findings. 



For general bibliographies on the cranial nerves of the selach- 

 ians the reader is referred to the papers of Allis ('89, '97, '01), 

 Cole ('98), and Neal ('98, '14). 



The writers are greatly indebted to Prof. H. V. Neal, of Tufts 

 College, for embryos of Squalus acanthias, to Prof. J. F. Daniel 

 and Mr. F. H. Ballou, of the University of California, for em- 

 bryos of Mustelus californicus, and to Mr. Fred T. Lane, of Rock- 

 port, Massachusetts, whose kindly cooperation has been that of a 



