HEAD SEGMENTS 



on the whole less and less typically developed, more and more 

 specialised ; that this process of cephalisation has proceeded farther 

 in the higher than in the lower forms ; and finally, that it is less 

 pronounced in the embryo than in the adult. Observers differ as 

 to the exact number of somites in front of the segment corre- 

 sponding to the vagus nerve. The head behind that region 

 includes a very variable number of segments. The hind limit of 

 the head in fish not only may be quite indefinite, there being a 

 gradual transition from one region to the other, but also it does 

 not occupy a fixed position, and the process of cephalisation, or 

 assimilation of trunk segments, has gone much further in some 

 groups than in others. 



Without attempting to give a history of the study of the 

 segmentation of the head, it may be mentioned that Balfour, 

 Marshall [292-3], Dohrn [118], van Wijhe [495], Hoffmann [216], 

 Braus [47], Platt [331], Koltzoff [272-3], Johnston [248rt], 

 and numerous others, have worked at this difficult problem. It 

 has been fairly well established that there are 3 mesoblastic seg- 

 ments in front of the auditory capsule (prootic somites), and a 

 varying number behind (metaotic somites) ; 9 in Pristiurus, 10 in 

 Acanthias, 11 in Spinax. The fourth somite may extend below 

 the auditory capsule. These somites, first clearly identified by van 

 Wijhe [495] in Elasmobranchs, have been found in the Cyclo- 

 stomes (Koltzoff [272]), the Amphibia (Platt [331ft]), and the 

 Amniota. They appear to be strictly comparable throughout the 

 Craniata (Fig. 3). A more anterior evanescent somite has some- 

 times been seen in front of the first of van Wijhe ; it is the 

 so-called 'anterior head-cavity' of Elasmobranchs (Platt [331]). 



In all the Craniate vertebrates the myotomes of the first three 

 prootic somites are entirely subordinated to the use of the optic 

 capsule, and become converted into the ' eye-muscles.' Although 

 there are slight discrepancies between the arrangement of the six 

 eye-muscles of the Cyclostomes and Gnathostomes, yet their form 

 and innervation are remarkably constant throughout (Figs. 3 and 4). 

 With regard to the more posterior segments, the Cyclostomes seem 

 to be in a much more primitive condition than the Gnathostomes. 

 For, whereas in the former all the metaotic somites (from the fourth 

 backwards) appear to be represented in the adult by myotomes 

 forming a continuous series with ordinary trunk-muscles (Koltzoff 

 [272], Hatschek [202]), in the Gnathostomes the myotomes of 

 the first two or more metaotic somites vanish during ontogeny, and 

 the 4th somite (1st metaotic) 1 never forms muscle even in the 

 embryo (Figs. 2 and 3). 



1 Many authors, Dohrn, Kiliau, Platt, Sewertzoff, etc., cojisir' \r that there are 

 here two or more fused somites ; but the evidence, especially of the nerves, seems to 

 be against this view. 



