HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 121 



To morphologists looking to the problem of the segmentation 

 of the vertebrate head, the important question is whether these 

 so-called head somites or cavities represent true somites, compar- 

 able to the somites of the trunk and occipital regions, and are 

 therefore marks of a primitive segmentation of the head. The 

 answer to this naturally depends upon each investigator's indi- 

 vidual conception of what constitutes a true somite; and, as 

 Filatoff ('07) has pointed out in this connection, the variability 

 of a somite in the head region of higher vertebrates, due to the 

 disturbing influence of a greater development in brain and sense 

 organs, must be taken into consideration. Some forms, for 

 instance, may be so highly specialized that all the characteristics 

 of a typical somite have dropped out. As will be noted in the 

 review of literature, the question has been answered in the af- 

 firmative by investigators of the head cavities in the Reptilia. 

 For the Aves, Rex ('05), from extensive studies on this group, 

 believes that these structures are products of the visceral meso- 

 derm, and hence cannot be considered true somites, which are 

 differentiations of the dorsal, or paraxial, mesoderm only. 



REVIEW OF LITERATURE 

 THE HEAD SOMITES 



The first observations on the head somites of the Reptilia 

 were made by Van Wijhe ('86). This author, in embryos of 

 Lacerta, found what he considered to be the homologue of the 

 first head somite of selachians, in the form of a large sac the wall 

 of which consists of a single layer of cells, lying on each side 

 close to the posterior surface of the optic vesicle. There was no 

 connection between the cavities of these two sacs. Because of 

 their position, their association with the oculomotor nerve, and 

 the later transformation of parts of their walls into the same 

 eye muscles that arise from the first head somite in selachians, 

 Van Wijhe declared them homologous structures. 



In regard to a s6cond head somite in reptiles. Van Wijhe makes 

 no mention. 



