562 



and 4 that, accompanying the double flexure of the hind brain, the 

 rhombomeres behind the second of the series are laterally rotated 

 forwards. The auditory vesicle and the nerves on each side of it share 

 in the rotation but they do not become Temoved from their primitive 

 relationships to the brain segments in the process. The posterior 

 rhombomeres sufler however some degree of compression, and the 

 seventh for example in an older embryo is at the level of attachment 

 of the nerves indistinguishable from the sixth. The mesencephalic 

 flexure, moreover, in addition to bringing the floor of the fore -brain 

 into relationship with the structures under the floor of the hind brain 



Fig. 2. Rabbit. Reconstruction from sagittal sections. The prosomeres, meso- 

 meres and first eight rhombomeres are given in Arabic numerals, the nerves in Roman 

 numerals. The hinder margin of the head is indicated by a dotted line. 



tends to press the visceral portion of the head backwards. It is worth 

 noting in this connexion, too, that notwithstanding the large size of 

 the Gasserian ganglion, the fibres proceeding from it to the brain all 

 spring from a narrow proximal area to enter the second rhombomere. 



It is not necessary now to discuss the prosomeres and mesomeres. 

 Authors are agreed as to the number of these and also with regard 

 to the nerves. The Rabbit like other Craniates shows that the first 

 of these five segments is olfactory, the second optic, the third has no 

 nerve related to it, the first mesomere gives origin to the oculomotor» 

 and the second to the trochlear. 



It is impossible considering only the conditions in advanced em- 

 bryos to state definitely the number of segments in the hypoglossal 



