. Comparative anatomy. Anatomy, Comparative. THE HEAD PROBLEM 6ii of this assumption has been persistently challenged, and experience has demonstrated that it should not be pressed too far. In a very general way, however, the law has proved helpful, especially in relation to the head problem. In 1878 Francis Balfour made the important discovery that in elasmo- branch embryos the coelom extends continuously throughout head and trunk. He thus demonstrated that in this fundamental respect head and trunk are similar to a degree not previously realized. Balfour also stressed the fact that, by the f
. Comparative anatomy. Anatomy, Comparative. THE HEAD PROBLEM 6ii of this assumption has been persistently challenged, and experience has demonstrated that it should not be pressed too far. In a very general way, however, the law has proved helpful, especially in relation to the head problem. In 1878 Francis Balfour made the important discovery that in elasmo- branch embryos the coelom extends continuously throughout head and trunk. He thus demonstrated that in this fundamental respect head and trunk are similar to a degree not previously realized. Balfour also stressed the fact that, by the formation of gill-pouches, the cephalic coelom is divided into a series of mesodermal "cavities," which Balfour CEREBELLUM HEAD CAVITIES Y. MOUTH HYPOPHYSIS OPTIC RECESS ^THYROID Fig. 505.—A parasagittal section of a squalus embryo showing the three "head cavities" from which the six eye muscles develop. The somatic motor nerve relations are also shown. (After Neumayer modified.) erroneously compared with trunk somites. The falsity of this comparison was subsequently recognized when it was demonstrated that the trunk somites develop from the epimere, while the mesoderm of the visceral arches comes from the hypomere. In 1881, Balfour's pupil, Marshall, emphasized the distinction between the segmentation of the somites and that of the mesoderm of the visceral arches, and asserted the independence of the two. Only segmentation of the dorsal somites of head and trunk are comparable. The mesodermal cavities within the visceral arches are peculiar to the head. This contrast between "mesomerism" and "branchiomerism" has been recognized by later morphologists. Whether or not these two kinds of segmentation originally corresponded has remained a controverted issue to this day. Marshall also showed that the first or premandibular "cavity" gives rise to the four eye muscles innervated by the oculomotor nerve, while the external rectus mus
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
