HEAD SEGMENTS 



his famous Cronian Lecture in 1858 [225]. It received its final 

 blow from Gegenbaur (1872), and has been gradually replaced by 

 a ' Segmental ' theory of the Craniate head founded on sound 

 anatomical and embryological evidence (Gegenbaur [153, 161], 

 Fiirbringer [143], Froriep, Balfour [27], Marshall [292-3], van 

 Wijhe [495], von Kupffer [275], and others). 



It is now recognised that the remote ancestor of the Craniata 

 must, like Ampkivxus, have been fully segmented to its anterior 

 extremity ; that the great differences in structure between the 

 head and the trunk must be considered as due to the divergent 

 specialisation of two regions of the body, which primitively 

 resembled each other closely ; and that there is no hard and fast 

 line between the two, the distinction having been gradually estab- 

 lished, and being more pronounced in the higher than in the lower 

 forms. The limit of the head-region varies according as we adopt 

 the skeleton, the nerves, or the gill-slits as our criterion. 



To unravel the complex structure of the head, to enumerate 

 and identify the segments of Avhich it is composed, is one of the 

 most interesting and difficult problems of the morphology of the 

 Craniata. 



For this purpose three chief sets of structures must be studied : 

 the nervous system and sense-organs ; the mesoblastic somites ; 

 and the gill arches and slits. Of these it is the second, perhaps, 

 which affords the most trustworthy evidence. It is well known 

 that in the trunk-region the mesoblast becomes differentiated in 

 ontogeny into two main divisions : the segmental dorsal somites 

 and the ventral unsegmented ' lateral plate.' The former may 

 contain segmental transient coelomic cavities (myocoel), the latter 

 the unsegmented coelom or permanent body-cavities. Indeed, it 

 is one of the main characteristics of all Craniata, as distinguished 

 from the Cephalochorda, that the ventral mesoblast is continuous 

 and has lost its segmentation, though traces of it may be seen in 

 the development of the trunk segments of Cyclostomes (Hatta 

 [202]), and in the head region of all Craniates. The dorsal 

 somites become further differentiated into an outer ' cutis layer ' 

 yielding connective tissue, an inner muscular layer forming the 

 true myotomes from which are derived all the segmental muscles 

 of the body, and a ventral inner outgrowth, the sclerotome, the 

 chief source of the connective tissues. From the lateral plate are 

 derived the coelomic epithelium and the splanchnic or visceral 

 muscles. Now it is important to notice that, while the myotomes 

 and the muscles derived from them (such as the limb-muscles) receive 

 their motor nerves exclusively from the ventral roots of the spinal 

 nerves, the splanchnic muscles, the skin, the mucous membranes, 

 and their sense-organs are supplied from the mixed dorsal roots 

 (Fig. 1). 



