. Comparative anatomy. Anatomy, Comparative. 286 COMPARATRE ANATOMY second head cavity, while the third head cavity combining with the second forms the external rectus muscles. (Figs. 237, 238.) The serial homology of the head cavities or somites with trunk somites, which for many years was a controverted problem, has now been demon- strated bv the fact that in the embryos of lower vertebrates the head somites form a series of mesodermal segments continuous with the trunk somites. Like the latter, they become differentiated into myotome and and sclerotome, are innervated by somatic motor nerve
. Comparative anatomy. Anatomy, Comparative. 286 COMPARATRE ANATOMY second head cavity, while the third head cavity combining with the second forms the external rectus muscles. (Figs. 237, 238.) The serial homology of the head cavities or somites with trunk somites, which for many years was a controverted problem, has now been demon- strated bv the fact that in the embryos of lower vertebrates the head somites form a series of mesodermal segments continuous with the trunk somites. Like the latter, they become differentiated into myotome and and sclerotome, are innervated by somatic motor nerves, and are dorsal MESCNCHYMA. TRIGEUtNUS Fig. 238.—A camera drawing of a section of the anlage of the posterior (external) rectus muscle—right side—in a 29 mm. Squalus embryo, showing its two constituents derived from the second and third myotomes. Although the two constituents persist in the adult muscle, the limiting membrane which separates them in the embryo dis- appears in ontogenesis. The section from which the figure was drawn is a parasagittal one. The embryo was preserved and stained by the vom Rath method. to notochord and dorsal aorta. Furthermore, their segmentation is independent of that of the visceral arches. Another point of resemblance is that the first and second head cavities divide during ontogenesis into dorsal and ventral moieties precisely as do the first and second post-otic myotomes in petromyzon. The fusion of portions of two myotomes, the second and the third, to form the external rectus muscle of the eye resem- bles the fusion of trunk myotomes such as occurs in the formation of the tongue muscles. (Fig. 238) Lateral Trunk Muscles. The lateral trunk muscles of man develop from myotomic segments which first appear in the fourth week of onto- genesis, and by the end of the second month, have increased to nearly fortv pairs. The original metamerism of the myotomes, which persists. Please note that these images are extracted from scanned page image
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