212 SEGMENTATION OF THE HEAD. 



as to the modifications which they have undergone, especially 

 in the anterior part of the head. The questions involved 

 are amongst the most difficult in the whole range of morpho- 

 logy, and the investigations recorded in the preceding pages do 

 not, I am very well aware, go far towards definitely solving 

 them. At the same time my observations on the nerves and on 

 the head-cavities appear to me to throw a somewhat new light 

 upon these questions, and it has therefore appeared to me worth 

 while shortly to state the results to which a consideration of 

 these organs points. There are three sets of organs, whose de- 

 velopment has been worked out, each of which pi-esents more or 

 less markedly a segmental arrangement: — (1) The cranial nerves ; 

 (2) the visceral clefts ; (3) the divisions of the head-cavity. 



The first and second of these have often been employed in 

 the solution of the present problem, while the third, so far as is 

 known, exists only in the embryos of Elasmobranchs. 



The development of the cranial nerves has recently been 

 studied with great care by Dr Gotte, and his investigations have 

 led him to adopt very definite views on the segments of head. 

 The arrangement of the cranial nerves in the adult has fre- 

 quently been used in morphological investigations about the skull, 

 but there are to my mind strong grounds against regarding it as 

 a-ffording a safe basis for speculation. The most important of 

 these depends on the fact that nerves are liable to the greatest 

 modification on any changes taking place in the organs they 

 suppl}^ On this account it is a matter of great difficulty, 

 amounting in many cases to actual impossibility, to determine 

 the morphological significance of the different nerve-branches, 

 or the nature of the fusions and separations which have taken 

 place at the roots of the nerves. It is, in fact, only in those 

 parts of the head which have, relatively speaking, undergone 

 but slight modifications, and which require no s]3ecial elucidation 

 from the nerves, that these sufficiently retain in the adult their 

 primitive form to serve as trustworthy morphological guides. 



I propose to examine separately the light thrown on the 

 segmentation of the head by the development of (1) the nerves, 

 (2) the visceral clefts, (3) the head-cavities ; and then to com- 

 pare the three sets of results so obtained. 



The post-auditory nerves present no difficulties ; they are all 

 organized in the same fashion, and, as was first pointed out by 



