21^8 JOUKNAI- OF Co.MrAUA ri\ I-: NKUUOI.OCiY. 



In the fore gill region^ which extends from the eye to the 

 third primitive gill pouch, the process takes place otherwise. 

 Here the mesoderm occurs in addition, which, until the time 

 of the appearance of the dorsal root-borders at the brain, still 

 shows an entirely epithelial arrangement. There are nowhere 

 present detached cells which enter into the formation of con- 

 nective tissue. 



The root-border grows on rapidly to the dorsal border of 

 mesoderm, and spreads out so that it lies cap-like upon the 

 border cells of the mesoderm (Fig. 4). Then two tracts 

 (Ziige) of cells separate, of which the inner proceeds be- 

 tween mesoderm and brain, while the other grows on laterad 

 from the mesoderm between it and the epidermis. This latter 

 course is characteristic of the fore gill region. It is wanting 

 in the hind gill region and in the trunk. 



As these tracts (Ziige) display separate rudiments, I 

 choose for them exact designations. I call the inner tract, 

 which remains in connection with the dorsal border of meso- 

 derm and proceeds ventrad between brain and mesoderm, the 

 dorsal spinal nerve ^ since tJiis rndiinent extends in the same 

 way in all parts of the body; the outer tract I designate the 

 branchial nerve (Figs. 5 and 6, nb). 



The border (Leiste), and what proceeds from it, is not 

 continuous in the fore gill region, but at the beginning is 

 divided into three successive segments, which correspond to 

 the regions of the trigeminus, acustico-facialis and vagus, and 

 consists exclusively of elongated, serially arranged cells. 



While the separation of the border into the tracts 

 mentioned, the spinal and the branchial rudiments, takes 

 place over the dorsal border of mesoderm — it takes place 

 about the time of the appearance of the e3'e-rudiment — 

 growths of the epidermis begin in three places; they are the 

 trigeminus, the acustico-facialis and the vagus swellings. 

 They lie laterad of the dorsal mesoderm, and arise, not sim- 

 ultaneously but successively, from before backwards. The 

 method of formation of these swellings I have minutely 



