286 



COMPARATRE ANATOMY 



second head cavity, while the third head cavity combining with the 

 second forms the external rectus muscles. (Figs. 237, 238.) 



The serial homology of the head cavities or somites with trunk somites, 

 which for many years was a controverted problem, has now been demon- 

 strated bv the fact that in the embryos of lower vertebrates the head 

 somites form a series of mesodermal segments continuous with the trunk 

 somites. Like the latter, they become differentiated into myotome and 

 and sclerotome, are innervated by somatic motor nerves, and are dorsal 





MESCNCHYMA 



TRIGEUtNUS 



Fig. 238. — A camera drawing of a section of the anlage of the posterior (external) 

 rectus muscle — right side — in a 29 mm. Squalus embryo, showing its two constituents 

 derived from the second and third myotomes. Although the two constituents persist 

 in the adult muscle, the limiting membrane which separates them in the embryo dis- 

 appears in ontogenesis. The section from which the figure was drawn is a parasagittal 

 one. The embryo was preserved and stained by the vom Rath method. 



to notochord and dorsal aorta. Furthermore, their segmentation is 

 independent of that of the visceral arches. Another point of resemblance 

 is that the first and second head cavities divide during ontogenesis into 

 dorsal and ventral moieties precisely as do the first and second post-otic 

 myotomes in petromyzon. The fusion of portions of two myotomes, the 

 second and the third, to form the external rectus muscle of the eye resem- 

 bles the fusion of trunk myotomes such as occurs in the formation of the 

 tongue muscles. (Fig. 238) 



Lateral Trunk Muscles. The lateral trunk muscles of man develop 

 from myotomic segments which first appear in the fourth week of onto- 

 genesis, and by the end of the second month, have increased to nearly 

 fortv pairs. The original metamerism of the myotomes, which persists 



