6i4 



COMPARATIVE ANATOMY 



Van Wijhe confirmed the conclusions of Marshall in regard to the fate 

 of the first and third somites, and also asserted that the superior oblique 

 muscle is derived from the second somite. In the metaotic region the 

 first permanent myotome develops from the seventh somite, and van 

 Wijhe concludes that altogether nine segments enter into the formation 

 of the head, a conclusion very similar to that reached by Huxley and 

 Gegenbaur on anatomical grounds. In a later paper, van Wijhe ('22) 

 adds another metaotic segment, making a total of ten in the head of 

 elasmobranchs. 



Since van Wijhe's first paper, many embryologists have investigated 

 the mesodermal segmentation of embryos in all classes of vertebrates. 



PROFUNDUS Yn 

 SOMITE 



FOREBRAIN 



MANDIBULAR IS V. 

 SOMITE 3. 



ACUSTICO-FACIALIS. 



OTIC PIT 



SOMITE 4. 



GLOSSO- 

 PHARYNGEUS. 



VAGUS. 



Fig. 507. — A diagram of a Squalus embryo, showing the mesodermal somites of van 

 Wijhe. The nerve anlagen are stippled. (Redrawn after Braus.) 



While their observations and conclusions have not been wholly consistent, 

 it may be said that van Wijhe's general results have been confirmed. 

 Somites like those of elasmobranchs have been found in cyclostomes and 

 amphibia. Their presence indicates that the ancestors of vertebrates, 

 like the Hving amphioxus, were segmented throughout the length of the 

 body, that the metamerism was primarily muscular, and that head seg- 

 ments were like those of the trunk. 



The conclusion that the mesodermal segments in the head region 

 are comparable with those of the trunk has by no means been accepted by 

 morphologists without opposition. Among the objections raised are 

 that the head segments are due to the mechanical influence of surrounding 

 parts such as the gill-pouches; that the head somites are irregular in size; 

 that the continuity of trunk and head segments does not prove their 

 serial homology; that the head segments do not develop in succession 



