OX THE EMBRYOLOGY OF THE TELEOSTS. 207 



homologous with the canal connecting the neural and alimentary canals which has recently 

 been demonstrated to exist in all vertebrates. An unfortunate accident occurred to the 

 egg forming the subject of this figure and hence no consecutive observations were 

 made upon it. No other eggs in such favorable conditions were found. In fig. 30 I have 

 shown the medullary folds extending slightly farther over the deutoplasm than does the 

 rest of the blastoderm, and between them the medullary groove, the epiblast of which of 

 course forms the lining of the neural canal. Continuous with the epiblast which passed 

 down into the notch between the medullary folds is the hypoblast which extends up the 

 under surface of the blastoderm. Whether this notch eventually closes up leaving a tube 

 connecting the neural and alimentary canals I do not know, but it seems impossible to 

 escape the conclusion that it well represents the neurenteric canal of other forms although 

 the enteric canal of the fish does not at this time have the closed condition which obtains 

 in other forms. 



According to various observers, before the formation of the neural canal the epiblast 

 separates itself into two layers, an outer or epidermal and an inner or nervous layer, the 

 latter being confined more closely to the embryonic area. This differentiation we were 

 not looking for and hence no allusion to it appears in the foregoing account of the for- 

 mation of the neural cords ; whether, it exists or not we cannot positively say though it 

 probably does and was overlooked in our studies. According to those observers who 

 make this distinction, the nervous layer forms the bulk of the neural canal and Cal- 

 berla('77) claims that a thin layer of epidermal cells penetrates into the nervous layer and 

 eventually forms the epithelial lining of the neural canal, but Gotte ('78) denies this 

 and Balfour studying Lepidosteus and the teleosts ('81) has not been able to confirm Cal- 

 berla's observations. It would however seem probable that Calberla is right though farther 

 observations are necessary to settle the point. 



Schapringer ('71 p. 555) does not afford any information on this point, for he merely states 

 that the medullary canal does not form in bony fishes, as in Birds, Batrachia and Mammals 

 by a folding in of the medullary folds but through a process of separation on the inside. 



According to our observations the method of origin of the neural ridges has an almost 

 exact parallel in that of the Elasmobranch as given by Balfour ('78) and also closely 

 resembles Klein's observations on the trout '62 pi. 17, fig. 2. The figures of His, ('75) in 

 text, and which are copied by Balfour, '81 fig. 33, are greatly different from anything 

 which we have seen as will be noticed on an inspection of our plates. 



Optic Bulbs and Protovertebrae. 



Shortly before the stages shown in figs. 29 and 32 the fore-, mid- and hind-brains are 

 differentiated and almost immediately the optic lobes begin to be segmented off. At this 

 time both the brain and the rudimentary lobes appear to be solid bodies without any 

 internal cavity nor does there appear to be any trace of the neural canal in the spinal 

 cord. A fissure appears on either side of the brain cutting off a portion which forms the 

 optic lobe. This fissure begins above and behind and gradually extends forwards and 

 downwards until at last but a slight connection is left, the rudimentary optic nerve (fig. 

 36). The fissures progress until at last the nerve going to the right eye is connected only 

 with the left side of the brain while the optic nerve of the left eye arises from the right 



