. Comparative anatomy. Anatomy, Comparative. THE HEAD PROBLEM 615 with those of the trunk, as would be expected if they were members of a continuous series; that the prootic segments do not become differentiated into myotome and sclerotome as do trunk somites; that the muscles in the trunk region are differentiated from the median wall of the somite, while the musculature of the pre-otic segments arises in large part from the lateral wall; that the mesenchyma of trunk somites comes from a constricted region of the somite, while in the head segments it comes from the entire median wall; that re
. Comparative anatomy. Anatomy, Comparative. THE HEAD PROBLEM 615 with those of the trunk, as would be expected if they were members of a continuous series; that the prootic segments do not become differentiated into myotome and sclerotome as do trunk somites; that the muscles in the trunk region are differentiated from the median wall of the somite, while the musculature of the pre-otic segments arises in large part from the lateral wall; that the mesenchyma of trunk somites comes from a constricted region of the somite, while in the head segments it comes from the entire median wall; that relations of the nerves to the head segments differ from those of spinal nerves to the trunk somites. In addition to these general objections, special objections have been raised against the inclusion of the firSt and second pro-otic segments as true somites. Most of these assertions have proved to be erroneous. EAR PLACODE'. POSTERIOR HEAD FOLD/ Fig. 508.—A parasagittal section of an eight-day Petromyzon embryo viewed from the left side showing the mesodermal somites. The first somite is shown in an adjacent section, not in the figure. According to Koltzoff all of the head somites of Petromyzon form permanent muscles. The first three form the eye muscles, while the fourth is the first myotome of the lateral trunk muscles. (Redrawn after Koltzoflf.) That the segments discovered by van Wijhe in elasmobranch embryos are true somites homologous with those of the trunk, now seems sufficiently estabhshed by the fact that their presence in diverse groups of vertebrates has been repeatedly confirmed; that their segmentation is independent of the visceral segmentation represented in the visceral arches; that they form a series continuous with that of the trunk; that they are dorsal in relation to the dorsal aorta and notochord; that they develop consecu- tively, beginning in the neck region; that they become differentiated like trunk somites into myotome and sclerotome; that their musc
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