409 



The mesencephalon similarly shows on its floor and sides a sub- 

 division into two regions which I shall call Alesomeres 1 and 2 (ü^^, M2). 



The rhombencephalon is marked at this stage by 6 neuromeres 

 which I shall call Rhombomeres 1 — 6 (-Ri_(;). The auditory in- 

 vagination takes place opposite the two last (E5, Eg), and immediately 

 behind these the rest of the hind brain, which in its complete condition 

 for the sake of uniformity I venture to propose should be named the 

 metencephalon, is indistinguishable from the spinal cord. Moreover 

 the anterior border of the first myomere is situated somewhat in ad- 

 vance of the posterior border of the sixth rhombomere. 



Two further neuromeres make their appearance later, behind the 

 sixth rhombomere, and the anterior two myomeres are reduced and 

 carried back to make room for them (Figs. 2 and .3). 



Aud. organ 



central roots 



dorsal roots 



"*V'S: Pronephros 



Fig. 3. Eeconstruction from the '94' sagittal sections of the hind brain showing 

 relationships of neuromeres, nerves and somites. 



It will be convenient to at once state the nerve relationships of 



