. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. MUSCULAR SYSTEM. Ill head and those myotomes which form the muscles of the limb. The process for the formation of the posterior fin is essentially the same. In the head region, although no little study has been de- voted to the subject of the mesoderm segments, naturalists are not in unison as to their results. Not only is there a difference of opinion as to the number of myotomes developed in this region, from the nine recognized by van Wijhe to the seventeen or more claimed by Dohrn and Killian, but it is even disputed whe
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. MUSCULAR SYSTEM. Ill head and those myotomes which form the muscles of the limb. The process for the formation of the posterior fin is essentially the same. In the head region, although no little study has been de- voted to the subject of the mesoderm segments, naturalists are not in unison as to their results. Not only is there a difference of opinion as to the number of myotomes developed in this region, from the nine recognized by van Wijhe to the seventeen or more claimed by Dohrn and Killian, but it is even disputed whether these be true somites. The questions involved will be taken up in another place. Setting these points aside, it may briefly be said that in those forms which have been most carefully studied (the elasmobranchs) there are ten ^ myotomes developed in the head region. Each of these which occur in front of the ear is completely separated from its fellows, a fact which leads some to believe that we have to do here, not with the whole mesothelial structures as in the trunk region, but with merely the myotome zone. This matter, however, would seem to have less importance than is sometimes given to it; for we must re- member that the vertebrate head is an extremely complicated structure, all the parts of which have been greatly modi- fied, while the appearance of the gill slits is of itself sufficient to explain the ab- sence of a continuous metacoele. To the muscle fibres, the development of which was out- lined above, other parts of mesenchymatous origin are added in the development of the definitive muscle. This connective ^ There is clearly one pair of ccelomic cavities in front of the first recognized by van Wijhe (Fig. 121, ac).. Fig. 121. Section through the head of ext^- htyo Acanthias at about the stage of Fig. 122. a, aoita; ac, anterior (premandibu- lar) head cavity; e, pig- mented epithelial layer of eye; f, fore brain; gg, Gasserian ganglion; k, hind brain ; /, lens o
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