SEGMENTATION OF HEAD 249 



mental data regarding the remaining head-muscles ; but it is probable that the 

 series supplied by the motor roots of the glossopharyngeal, vagus, and spinal 

 accessory nerves, which are primarily branchial in their distribution, are to be 

 looked on as muscles of the branchial arches, corresponding to those originating in 

 connexion with the branchial sacs of the lower vertebrates. The sternomastoid and 

 trapezius, though they wander far back in development, may with some reason be 

 regarded as, in part at least, branchial derivatives. 



Segmentation of the head.- The foregoing paragraphs necessitate here a brief statement 

 on this obscure and intricate subject. 



In the higher Amniota there is certain evidence of segmentation in the occipital region. 

 Here at an early stage there are three myotomes, and, related to these, three or four occipito- 

 spinal nerves, which are primarily segmental, but united into a single trunk, the hypoglossal. 

 In front of this clearly segmented portion of the head there is no trace of segmentation 

 except in the lower vertebrates. In Selachia the number of ' head-segments ' has been very 

 variously estimated ; but according to the classical account of van Wijhe there are in all nine, 

 four metotic (the occipital) and five pre-otic. The pre-otic are much modified, and their claim 

 to rank as segments has been disputed. They are cavities the walls of which give rise to muscles. 

 The first three provide the eye-muscles, while the fourth and fifth disappear. In the branchial 

 arches there is a series of cavities regarded as representing the lateral plates of certain segments. 

 Their walls give origin to the branchial muscles. The evidence of typical segmentation is more 

 striking in the Cyclostomata. In Petromyzon (Koltzoff) typical segments occur, the anterior 

 only being modified in having no skle-plates. They are derived from pouches of the archenteron, 

 and the lateral plates show cavities, one in each branchial arch. There are two main views 

 regarding the head-segmentation. The first, maintained by van Wijhe, Miss Platt, Koltzoff, 

 and others, represents the whole series of segments as belonging to the head proper, and corre- 

 sponding to trunk-segments. The anterior or pre-otic are rudimentary and greatly modified, 

 the posterior or metotic are more complete. Their dorsal portions persist and give rise to 

 myotomes supplied by the occipital nerves (hypoglossal). Their ventral portions give rise to 

 the muscles of the branchial arches, and their splanchnic and sensory nerves are collected into 

 the vagus-complex. The second view, maintained by Gegenbaur, Froriep, Fiirbringer, and others, 

 represents the series of ' head-segments,' as divided into two distinct categories. The metotic 

 are segments belonging to a part of the trunk which has become included in the head 

 comparatively recently in phylogeny. The pre-otic are primarily segments of the head proper, and 

 to them belongs the mesoderm of the branchial arches which have been displaced backwards, 

 while the occipital segments have been displaced forwards, so that the two regions overlap. 

 There are thus two genetically distinct parts of the head ; the palingenetic and ccenogenetic of 

 Gegenbauer, palseocranium and neocranium of Fiirbringer, the prespinal and spinal of Froriep. 

 The occipital nerves (hypoglossal) belong to the occipital myotomes, and therefore to the 

 neocranium ; the vagus-complex (including the glossopharyngeal in lower forms as well as the 

 accessory), belongs to the palseocranium, being formed by^the coalescence at their proximal ends 

 of the splanchnic fibres of the segmental nerves of the posterior palingenetic segments, of which 

 only the splanchnic or branchial portions now remain. According to Agar in a recent paper 

 on the anterior mesoderm in Lepidosiren, 1 the preponderance of evidence is in favour of 

 the Gegenbauer-Fiirbringer view. It seems quite certain that the occipital myotomes really 

 belong to the trunk, but with regard to the palseocranium the matter is more doubtful. 

 It is not yet proved that the prechordal ' head-segments ' and branchial segmentation 

 correspond to the trunk-segmentation, and the ontogenetic facts which have induced Hubrecht 

 and others to attribute a radial symmetry to the fore-part of the head must not be left out 

 of sight. 



1 Trans. Eoy. Soc. Ed. xlv. Part III. 1907 



