No. 3-] THE VERTEBRATE HEAD. 503 



of secondary origin, depending on the segmentation of the 

 mesoblast. Nevertheless, I hope to show that these segments 

 are the first to appear in the head region, and that they are 

 entitled to more serious attention on the part of anatomists, as 

 representing the most primitive segmentation of the head of 

 which any traces are preserved. 



PART I. — METAMERISM OF THE HEAD. 



I. Basis for the Discussion. 



The more recent discussions on the metamerism of the head 

 are based upon segmental divisions as shown in (i) cranial 

 nerves and branchial clefts, (2) mesoblastic head cavities and 

 (3) segments of the neural tube. 



The first-mentioned basis may now be set aside as involving 

 too much conjecture. McClure has stated the objections to it 

 as follows : " We have positive proof that the degeneration of 

 certain branches has taken place. This being the case, we 

 have every reason to assume that whole segmental nerves may 

 have once existed, which have completely degenerated, leaving 

 no trace whatever of their previous existence. If such be the 

 case, the segments originally connected with these degenerated 

 nerves must necessarily be overlooked, if the existing nerves 

 are made use of as a means of determining the original num- 

 ber of segments. 



" Furthermore, the vagrant changes in the position of some 

 of the cranial nerves must necessarily cause confusion. For 

 example, take the sixth nerve, which in the frog and tadpole 

 stages is situated between the first and second roots of the 

 ninth nerve, a position somewhat posterior to its place of origin. 

 This remarkable shifting clearly shows not only what great 

 changes in position the cranial nerves are capable of under- 

 going, but it also goes to prove that we can find no reliable 

 means of determining the primitive segments by means of their 

 connection with the exit of the existing cranial nerves. Beard 

 in taking up this problem made use of an important series of 

 sense-organs for which he has proposed the name of 'Bran- 

 chial Sense Organs,' from their development from thickenings 



