NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 229 



vraud immer noch unmittelbar an, dies ist aiich jetzt noch grosstentheils 

 der Fall, aber mit ihrem medialen Eand fangt sie jetzt an sich von der 

 Gehirnwand zuriickzuziehen, bleibt aber mit ihr durch einen kurzeu, 

 dicken Zellstrang kontinuirlicli vei'buuden. Dieser Zellstrang bildet die 

 Anlage des Nervus olfactorius, aber es ist nicht moglich zu sageu, wel- 

 chen Antheil die Epidermis und welchen das Gehirn an der Anlage der 

 Eiechnerven nimmt, denu er entsteht aus dem letzten Rest des kontinu- 

 irlichen Zusammeiihanges von Epidermis und Medullarwand, welcher 

 von Anfang an bestanden hat." 



His ('89') had previously found in the human embryo that the first 

 step in the formation of the olfactory nerve was the migration of mesen- 

 chymatous cells between the olfactory plate and wall of the brain. 

 Later the olfactory ganglion is formed by the migration of cells from the 

 lateral walls of the olfactory epithelium. Finally, the olfactory nerve 

 results from the assumption by these cells of a bipolar form and the 

 elongation of the poles both centripetally and centrifugally to form 

 fibrillar connection with brain and olfactory pits. 



My own observations concerning the development of the olfactorius are 

 as yet incomplete, and I am not able to add much to the evidence which 

 has been given. In agreement with Hoffmann ('96) I find that, as the 

 olfactory plate and the brain wall separate, they retain connection with 

 each other by faintly staining fibrils in the region of the future olfactory 

 pits. Whether these fibrils enter into the formation of the definitive 

 olfactorius I am not able to state, and the observations of Hoffmann 

 appear to me insufficient to establish this fact. My results and those 

 of HoflFmaun do not agree; for he finds in embryos of 16 mm., and still 

 more clearly in embryos of 18-20 mm., that mesenchymatous tissue 

 " schiebt sich von alien Seiten zwischen Medullarwand und basale Nasen- 

 grubewand ein, und in demselben Grade als beide sich entfenien, nimrat 

 natiirlich der Eiechnerv an Lange zu." I infer this mesenchymatous tissue 

 to be the same as that which HoiTmann previously states to be derived 

 from the " anterior head cavities." My observations, however, lead me 

 to agree with Marshall ('78), that the cells which appear between the nasal 

 pit and the brain wall, as these separate, are neural-crest cells. Van 

 Wijhe may be technically correct in stating that the neural crest has 

 disappeared in the region of the forebi'ain at the time when the olfactory 

 nerve is established ; but it is certainly not true that the neural-crest 

 cells in the region of the forebrain have done so at this stage. They 

 persist in the region of the forebrain which lies opposite and anterior to 

 the optic vesicle, and in my opinion are the cells which migrate between 



