338 Mr. W. K. Parker on the Development of the [Feb. 13, 



The sixth nerve, or ahducens, certainly arises from the ventral 

 surface of the hind-brain ; this being so, it manifestly corresponds to 

 the anterior root of a spinal nerve ; and as Milnes Marshall suggests, 

 it may belong to the trigeminal, to the facial, or to both of these 

 nerves. 



If low forms should turn up, in which the optic nerves were truly 

 segmental, and not direct vesicular outgrowths of the fore-brain, that 

 would only affect the classification here suggested, by showing that 

 our present Yertebrata have lost a segment through the extreme 

 specialization of the optic nerves. 



As matters stand at present, we have, then, the nasal, lacrymal, 

 oral, tympanic, and branchial clefts ; of these we see that there are 

 three in front of the tympanic, and there may be eight behind it. 



Thus we get four pre-auditory and eight post-auditory clefts, with 

 their nerves ; if we add the twelfth (hypoglossal) of the "Amniota," 

 we have obtained signs and proofs of thirteen cranial (segmental) 

 nerves, all of these, except the last, forking over visceral clefts, and 

 hedged in, all but the last, by visceral bars. 



The first of the bars is in front of the first or nasal cleft, the last, 

 or thirteenth, is the hinder bar of the lamprey's branchial basket 

 work. 



Of course in this classification I do not mention, for the time, the 

 distinction between the deep and superficial cartilages. 



In the lowest kind of chondro-cranium known to us, namely, that 

 of the sucking-fish and the larvae of the Batrachia, the first post-oral 

 arch is not only very largely developed, but is also carried forwards 

 directly to the front of the head ; it does not of itself form the 

 skeleton of the oral opening, but carries the large cartilages that form 

 the peculiar suctorial mouth. 



Hence, in the lamprey and in the Batrachia, whilst they are in the 

 larval condition, the pre-oral visceral bars are arrested in their growth ; 

 in the adult of the latter types, when the permanent mouth is formed, 

 then two (of the three possible) visceral arches are developed. 



Not only in low forms are the anterior visceral arches arrested, or 

 even suppressed, but the visceral clefts also, in front of the mouth as 

 well as the one immediately behind it, are often imperfectly developed, 

 or even suppressed. 



The nasal cleft does not remain open inwards until we get to the 

 Dipnoi and Amphibia, and the lacrymal cleft not until we get to the 

 Amniota. 



No one has seen a first post-oral or tympanic cleft in the lamprey, 

 and its second post- oral, discovered a few years ago by Professor 

 Huxley ("Proc. Roy. Soc," vol. xxiii, p. 129), in the larva, or Am- 

 mocoete, is very small, and apparently has no external opening. 



In all the Urodeles, and in the lower and more generalized Anura, 



