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 muscles develop 

 from the median wall; and that they correspond numerically with the 

 neuromeric segments. Consequently, the conclusion that they form 

 the most convincing evidence of the fundamental similarity of the seg- 

 mentation in head and trunk seems warranted. 



Van Wijhe noticed in the embryo of Galeus, an elasmobranch, a pair 

 of mesodermal segments or "cavities" which he interpreted as an anterior 



