48 A NEW CRANIAL NERVE IN SELACHIANS. 



greater uniformity. The fibres have been traced into the epistriatum in Squalus, 

 Mustelus, and Carcharias, but they have not yet been studied in the other forms. 

 There is also much variation as regards the ganglionic enlargements on the nerve. 



III. EMBRYONIC HISTORY OF THE NEW NERVE IN SQUALUS. 



The embryonic history of this nerve was described by me (Locy, '99) some four 

 years ago, and the main facts are repeated here. It can be found in embryos of 

 Squalus acanthias about 10 millimetres in length running from the apex of the fore- 

 brain near the neuropore to the olfactory epithelium. Its previous history is difficult 

 to clear up. I have given much attention to sections of embryos from 6 to 8 milli- 

 metres long, and I have repeatedly observed that there exists a cellular connection 

 between the olfactory plate and the brain-wall as described by Hoffmann ('96). 

 The new nerve has at first a connection (placode) with the thickened surface exist- 

 ing just above the shallow depression that marks the beginning of the olfactory pit. 

 This connection is a group of closely packed cells in which I have failed, at this 

 stage, to recognize fibres. In embryos about 10 millimetres long, however, fibres 

 are to be seen that run from the sides of the neuropore to the olfactory epithelium. 

 These I take to be fibres of the new nerve, but have not been able to satisfy myself 

 as to the position of the neuroblasts that give rise to them. The neural crest is dis- 

 appearing in this region, and my observations incline me to the view that the neuro- 

 blasts of the fibres of the new nerve are derived from the cells of that structure. 



For a short time there is a single connection, on each side, between the brain- 

 wall and the nasal epithelium. Very soon a second fibrous connection, more lateral 

 in position, is established between the brain- wall and the olfactory pit. The two 

 connections are entirely independent as to their union with the brain-wall, but are 

 close together in the olfactory epithelium. The earliest of these fibrous tracts repre- 

 sents the new nerve, the later one the main olfactory. The latter is present in em- 

 bryos 13 millimetres long * (and perhaps in still smaller ones). 



By the time the embryo has reached a length of 16 millimetres the two inde- 

 pendent brain connections are clearly differentiated. The connection belonging to 



* The length is, of course, no sure criterion as to its age. Those who have compared a large assortment of 

 embryos of any animal must have been impressed with their variations. Some embryos are longer than others 

 which are clearly more advanced in development, and there is variation as to the number of gill-clefts broken 

 through on the two sides of the body as well as in other anatomical landmarks. The difference between embryos 

 10 millimetres and 13 millimetres in length is slight, and in individual cases the longer one might be the younger. 

 The chief point is that my sections show stages in which the median fibres are present and the more lateral ones 

 absent. 



