THE HEAD PROBLEM 

 Head Segments According to Marshall 



613 



The year following the publication of Marshall's paper, van Wijhe 

 discovered in elasmobranch embryos an unbroken series of somites 

 beginning with the premandibular and extending throughout head and 

 trunk. He was thus able to demonstrate an "acraniote stage" of the 

 vertebrate embryo, which, so far as the mesodermal segmentation is 

 concerned, resembles the acraniote amphioxus. While all the myotomes 

 of amphioxus persist throughout life, in elasmobranchs some of those in 

 the region of the ear degenerate. As a result, the three anterior somites, 

 which form eye muscles, become separated from the metaotic. In 

 cyclostomes, however, according to Koltzoff ('02), all the embryonic 

 myotomes form muscles, which persist in the adult lamprey as in amphi- 

 oxus. There is, therefore, in the lamprey no hiatus between prootic and 

 metaotic muscles. (Figs. 507, 508) 



Head Segments According to van Wijhe 



Somite 



Muscles from the somite 



Rect. sup., inf., int., and inf. 

 oblique 

 Superior oblique 



Rectus externus 



None 



None 



Very rudimentary 



Muscles from skull to shoul- 

 der girdle, with anterior 

 part of sternohyoid 



Ventral nerve root 



Dorsal nerve root 



Oculomotor 



Trochlearis 



Abducens/ 



None ) 



None 



Not recognizable/ 



Hypoglossus ) 



Ophthalmicus profundus 



Trigeminus less ophth. 

 prof. 



Acusticofacialis 



Glossopharyngeal 



Vagus 



