64 NERVOUS SYSTEM OF VERTEBRATES. 



At an early stage in the embryo the dorsal border of the sheets of 

 mesoderm becomes divided into a series of segments, which soon 

 separate from the undivided lateral mesoderm and lie as indepen- 

 dent blocks or sacs at the sides of the neural tube. These are known 

 as mesodermic somites. The first somites to be formed lie in 

 the region which will form the neck and the segmentation continues 

 from that point forward and backward. In front of this point 

 there are formed in selachians and cyclostomes a definite number 

 of somites which were first described by vanWijhe in 1882. These 

 head segments bear constant relations to other organs and may 

 be designated by numbers indicating their order from before 

 backward. Somite i lies behind and below the optic vesicle 

 and gives rise to those eye-muscles which are innervated by the 

 III nerve (mm. rectus superior, rectus inferior, rectus internus, 

 obliquus inferior). In front of somite i there is found in selachians 

 a pair of lateral masses of mesoderm known as anterior he^d 

 cavities. These are believed to represent a still more anterior 

 pair of somites which disappear in vertebrates without forming 

 any recognizable structure. Somite 2 is larger and is directly 

 connected with the mandibular arch. It gives rise to the muscle 

 innervated by the IV nerve (m. obliquus superior). Somite 

 3 is connected with the hyoid arch and gives rise to the muscle 

 innervated by the VI nerve (m. rectus externus). Somite 4 

 lies mesial to or slightly behind the auditory pit and is known 

 as the first postauditory somite. It gives rise to muscle on the 

 dorsal surface of the head in cyclostomes; in all other vertebrates 

 breaks down and disappears. Somite 5 in cyclostomes gives 

 rise to a muscle lying next behind that of somite 4; in all other 

 vertebrates disappears. In cyclostomes all the following somites 

 form permanent myotomes, and myotome 10 together with several 

 following it send muscle buds down behind and forward beneath 

 the branchial apparatus to form the sub-branchial muscles corre- 

 sponding to the tongue musculature of higher forms. 



In selachians a variable number of postauditory somites may 

 first form some muscle fibers and afterwards break down and 

 only contribute to the formation of mesenchyme. The first somite 

 to form permanent muscles is number 7 or 8 in most selachians. 



