565 



the spinal accessory. In the V and VII also the proximal and distal 

 fibres from the ganglia are unaccompanied by nerve cells. There can 

 be no question that in the rabbit His's teaching of the origin of the 

 nerve fibre receives abundant proof. 



Cephalogenesis. 

 A. The Nervous System. 



The results of this and preceding enquiries, referring to a Se- 

 lachian, a bird, and a Mammal, tend, with others of a like nature i), 

 to show that the elemental structure of the brain and the cranial 

 nerves is the same throughout the Craniata. The attempt may now 

 be made therefore to state a little more fully what that fundamental 

 structure is. 



Like the spinal cord the brain consists of a transversely segmented 

 canal, the segments in the cranial region being definite in number, viz, 

 3 prosomeres, 2 mesomeres, and 13 [rhombomeres : 18 altogether. 

 There were originally 13 ganglia associated with the 13 rhombomeres. 

 Posteriorly the last four or five of these as those of the trunk have 

 been absorbed evidently in the lateral line. The anterior eight are 

 accounted for in the X, IX, VIII, VII, VII, V, V, V ganglia, — v. 

 paper on the brain of the gull. There are now the five segments 

 which form the mid and fore brains. Leaving the eye for the moment 

 since the ganglion arises from an expansion of the brain, the only 

 ganglion at all comparable with those of the hind brain is the ol- 

 factory with the so-called nervus terminalis, and these are purely 

 sensory. 



Yet presumably there ought to be two for the segments of the 

 mesencephalon, and a third for the third prosomere. The latter is 

 more than probably represented by the fleeting thalamic nerve of 

 Miss Platt, and the next by the still more fleeting and delicate mes- 

 encephalic nerve of Tketjakoff (in the Ammocoet). There is room 

 for still another. Is it the trochlear? Both the thalamic and the 

 mesencephalic are connected with the ophthalmic. This is usually de- 

 scribed as ophthalmic V, but it is probable from the manner of origin 

 of the ophthalmic VII that the latter actually takes up the sensory 

 elements of these nerves. Many writers have shown that the trochlear 

 takes origin from the neural crest in a manner similar to the thalamic 

 and also in connexion with the ophthalmic. It occupies the position 



1) Froriep and others — v. literature quoted in the paper on 

 Larus, loc. cit. 



