564 



assumption has been made, therefore, based however on the sections, 

 that the number of rhombomeres is the same as in the case of the gull. 

 Before reviewing further the elemental structure of the brain it 

 will be convenient at this point to refer briefly to the histogenesis of 

 the nerves of the rabbit. 



Histogenesis of Nerves. 



Nowhere amongst the specimens hitherto examined has the origin 



of the motor nerves by delicate roots, unaccompanied by nerve cells, 



been so clearly seen as in the embryos now under consideration. In 



the case of the oculomotor (Fig. 5), and also the abducent (Fig. 1) the 



Fig. 5. Babbit. Origin of oculomotor. 



fibres may be traced with ease, after they meet to form the nerve, to 

 their destination, and at no place in their course is a nerve cell to be 

 found. These nerves for this reason, from their position and their 

 manner of origin by fan-like roots, are truly ventral roots. Notwith- 

 standing the fact that the trochlear is a motor nerve and, as the late 

 Dr. DoHRN in his last paper pointed out, correcting thereby his former 

 impression, originates as stated above for the oculomotor and abducent, 

 it requires special consideration, and it will be treated of in the next 

 section. 



The same features of origin are quite plainly seen in the case of 

 the ventral roots which survive in the hypoglossal region, and also in 



