DEVELOPMENT OF CRANIAL NERVES IN THE CHICK, 19 



be traced for a short distance in successive sections running 

 downwards and slightly outwards^ lying close to, but perfectly 

 independent of, the external epiblast. At this period there is a 

 hardly perceptible thickening of the external epiblast at the 

 spot where the olfactory pit will shortly afterwards appear ; it 

 is towards this point that the outgrowth is directed. These out- 

 growths have exactly the same histological appearance as the 

 other cranial nerves at a corresponding stage of development. 

 Their position, relations and subsequent history leave no doubt 

 in my mind that they are the olfactory nerves in an early stage 

 of development. We shall find that they resemble the auditory 

 nerves in attaining a certain size before the corresponding in- 

 volution of the external epiblast has commenced. 



The next point, "What is the relation of these nerves — for so 

 we may now call them — to the anterior part of the neural ridge 

 as shown in fig. 6, is not so easy to determine. We shall find 

 that all the other nerves soon lose their attachment to the ex- 

 treme summit of the neural canal, and travel down the sides a 

 certain distance; so that the fact that the attachment of these 

 nerves is not quite to the summit, speaks in favour of, not against, 

 their being derived from the neural ridge. I have observed a 

 mass of spherical cells occupying nearly the same position as 

 that just described, but not extending so far down, in embryos of 

 thirty-six, forty, and forty-six hours; and I believe that these 

 represent intermediate stages. 



I wish, however, to state distinctly that though I myself have 

 but little doubt that the '' olfactory nerves " of a fifty-hours' 

 chick are really derived from the anterior extremity of the neural 

 ridge, yet I am fully aware that I have not proved my case. 

 To do so it would be necessary to trace their development hour by 

 hour, inasmuch as the relations of the parts change so quickly 

 and to so great an extent about this time, owing to the rapid 

 growth of the fore brain, and the rapidly occurring cranial 

 flexure, that it is exceedingly difficult, and in some cases impos- 

 sible to make perfectly satisfactory comparisons of sections taken 

 through the same parts at so short an interval as four or five hours. 



I have not figured the stage just described ; but a very good 

 and fairly exact idea of the appearance can be gained from 

 figs. 19 and 20 by supposing the olfactory nerves (I, in fig. 19) 

 to be rather nearer one another, and the dilatations of the fore- 

 brain {cfb) in fig, 20 to be not the cerebral hemispheres, but the 

 optic vesicles ; the former not having yet appeared. 



The later stages are easier to work. The fore brain grows 

 forwards in front of the optic vesicles, carrying with it the olfac- 

 tory nerves ; it then bulges out laterally to form the cerebral 

 hemispheres. I have already figured the condition of the parts at 



