PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 617 



The dorsal, or somatic, head mesoderm consists anteriorly of three segments, 

 equivalent to trunk myotomes, from which arise the muscles of the eyeball. The two 

 posterior ones of these are continuous ventrally through the first two visceral arches 

 with the anterior portion of the lateral plate mesoderm. 



Behind these three segments comes an unsegmented portion of somatic mesoderm 

 ( = fourth somite of van Wijhe). This represents an unknown, probably large, number 

 of myotomes which have lost their individuality and become condensed, owing to the 

 fact that they now give rise to no muscles in the adult. All the visceral arches behind 

 the first two belong primarily to this region, though in development a certain number 

 of them appear further back — that is, the lateral plate portion of these myotomes has not 

 become condensed like their myotomic portions, owing to the presence of the functionally 

 active gill pouches. This concludes the palingenetic part of the head. Primarily 

 behind this, but now overlapping it owing to this condensation of the dorsal, myotomic 

 portion, but not of the ventral, lateral plate part, we have the occipital myotomes form- 

 ing the coenogenetic head region, and continuous with the series of trunk myotomes. 

 Like the hinder palingenetic head myotomes, some of the anterior ones belonging to 

 the coenogenetic part of the head have been lost, but at a much later period phylo- 

 genetically, as shown by the fact that some of these still appear and atrophy in the 

 ontogeny of so many different forms of vertebrates. 



It has just been maintained that it is the fourth somite of van Wijhe which repre- 

 sents the condensed somatic portion of the hinder palingenetic head somites. The 

 variation in its extent and position in different forms is interesting. In Scyllium and 

 Pristiurus it is under the auditory vesicle (van Wjihe) and also in Acanthias (Hoffmann). 

 In Ceratodus it is pro-otic (Gregory), in Petromyzon it is metotic (though the first part 

 of it is hypotic). In Lepidosirevi and Protopterus its homologue is a long stretch of 

 mesoderm, extending in front of, underneath, and behind the auditory vesicle. Some 

 have concluded, from the result of direct observation, that this " somite " is an aggregate 

 of two or more. Brads finds that in Spinax it probably represents two "somites" 

 fused, the simpler condition in Scyllium and Pristiurus being probably due to a more 

 complete fusion. Miss Platt thinks that in Acanthias " three mesodermic segments 

 lie above the hyoid arch" (quoted from Braus). Sewertzoff finds, behind the usual 

 first three somites in the head of Torpedo, a fourth pro-otic and a fifth hypotic. In 

 other cases, such as Petromyzon (Koltzoff), there is nothing to suggest that it may 

 have a composite nature. 



The above considerations show that direct observation of the embryological con- 

 ditions of the myotomic portion of this "somite" are not against the view maintained 

 above, namely — that its real nature is a mass of mesoderm, lying in the region of the 

 auditory vesicle, and of various extent corresponding with the amount of reduction 

 which the numerous segments more or less completely fused together to compose it 

 have undergone. Besides the palingenetic segments which have gone to form it, it 



