204 JoiTRNAi. OK Comparative Nei!koi.o(;v. 



brates the three cephalad encephalomeres are much larger. 

 These svibsequently divide, the primitive prosencephalic 

 vesicle into two, the mesencephalic and the third each into 

 three. The avithor thinks the three first are originally com- 

 pound, and that for some reason the subdivision is retarded in 

 this region. The thirteen encephalomeres are therefore 

 homologous. These are especially prominent in Mustelus 

 and Acanthus (Selachii). 



The first encephalomere forms the secondary prosenceph- 

 alon, with only a dorsal nerve root represented by the olfac- 

 tory. The second forms the thalamus, and is devoid of nerve 

 roots. The third, or first mesencephalic encephalomere, is 

 also devoid of nerves. The fourth bears the ciliaris as a 

 dorsal aud the oculo-motor as a ventral root. The seventh 

 is doubtfully credited with the trochlearis. The eighth has 

 the sensoi'y root of the trigeminus as its dorsal and the motor 

 trigeminus as its lateral root. The ninth has no nerves. The 

 tenth with the profacialis (sensory part -|- acoustic) as a 

 dorsal root and a lateral root. The eleventh bears the 

 acoustic ganglion as dorsal root, a lateral branch entering 

 the genu of the facialis, and the abducens. The twelfth 

 bears the sensory glosso-pharyngeal, a lateral branch, and a 

 ventral branch which passes dorsad behind the glosso-phai-yn- 

 geal. The thirteenth also has three roots, the sensory fibres 

 of the tenth, a lateral branch chiefly in the accessory, and a 

 ventral branch passing dorsad behind the vagus. 



The four following neuromeres enter the skull in mammals 

 and have dorsal roots in the vagus, lateral ones in the acces- 

 sory, and ventral ones in the hypoglossus. 



The vagus of mammals includes five dorsal roots. Prof. 

 Froriep remarks that he has detected two neuromeres in the 

 diencephalon. This latter statement the present writer is 

 able to substantiate from his own observations on Cavia and 

 Canis embryos, though very doubtful as to the homology of 

 these segments with the neuromeres of the spinal regions. 

 -[Ed.] 



