SEGMENTATION OF THE HEAD. 435 



that, of a series of homodynamous segments 1 . While the 

 researches of Huxley, Parker, Gegenbaur, Gotte, and other 

 anatomists, have demonstrated in an approximately conclusive 

 manner that the head is composed of a series of segments, great 

 divergence of opinion still exists both as to the number of these 

 segments, and as to the modifications which they have under- 

 gone, especially in the anterior part of the head. The questions 

 involved are amongst the most difficult in the whole range 

 of morphology, 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 con- 

 sideration of these organs points. There are three sets of organs, 

 whose development has been worked out, each of which presents 

 more or less markedly a segmental arrangement: (i) 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 adiilt has frequently 

 been used in morphological investigations about the skull, but 

 there are to my mind strong grounds against regarding it as 

 affording 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 

 supply. On this account it is a matter of great difficulty, amount- 

 ing 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 



1 Semper, in his most recent work, maintains, if I understand him rightly, that 

 the head is in no sense a modified part of the trunk, but admits that it is segmented 

 in a similar fashion to the trunk. 



