DEVELOr-MBNT OP CRANIAL NERVES IN THE CHICK. 25 



much more compactly arranged than they were in the earliest 

 stages. At forty-three hours there is a slender outgrowth 

 from the extreme top of the widest part of the mid brain on 

 either side ; this passes downwards, lying in close contact with 

 the walls of the mid brain, for a short distance; its outlines are 

 very difficult to distinguish from the mesoblast cells. There is 

 no doubt, however, that this is identical with the outgrowth 

 observed at the twenty-ninth hour (fig. 8, m). At fifty-four hours 

 there is a rather larger mass of cells, which appears to be con- 

 nected with the mid brain about half way down its sides. 



On considering all these facts — the a priori probability that 

 the outgrowth from the mid brain of a twenty-nine-hours' 

 chick should develope into the third nerve ; the condition of 

 what is unquestionably the third nerve at the sixtieth hour ; and 

 the evidence furnished by such intermediary stages as I have 

 been able to observe, of vv^hich that furnished by my forty-three- 

 hours^ specimens is perfectly definite, I am led to the belief that 

 the third nerve is developed directly out of the outgrowth (m) 

 from ihe top of the mid brain shown in fig. 8, and that at some 

 period between the forty-third and sixtieth hours its attachment 

 shifts down from the top of the mid brain to the lower part of 

 its sides. 



The necessary assumption of this shifting liaving occurred is 

 the most st rious difficulty in the way of the above view. It is 

 very considerably diminished by the consideration that at a cor- 

 responding stage of development all the other cranial nerves 

 and the posterior spinal roots undergo a precisely similar shifting 

 of their points of attachment. I propose to notice in detail 

 the different stages of this process in the case of the seventh 

 nerve. It is true that the change is rather greater in the case 

 of the third nerve than of the others — the third nerve being 

 brought ultimately nearer to the median ventral line than the 

 others ; but this, I think, is explicable by its very early appearance 

 and the enormous growth of the mid brain, which exaggerates the 

 causes which we shall see lead to the shifting down of the other 

 nerves. 



Moreover the seventh nerve exceeds the posterior root of a 

 spinal nerve in the amount of displacement it undergoes far more 

 than the third exceeds the seventh ; so that a view that regards 

 the third nerve as originally arising from the mid dorsal line is 

 not so paradoxical as it appears at first sight. 



Erom the ganglionic termination of the third nerve seen in 

 fig. 22 two branches arise. Of these the smaller and later deve- 

 loped one is very short, and runs forward above the ophthalmic 

 branch of the fifth to the superior rectus (vide fig. 2Q,rs). The 

 posterior and larger branchy which is also the earlier developed, 



