102 ME. E. S, GOODRICH ON THE OCCIPITAL REGION 



like those of the trunk. A process of cephalisation leads to the 

 fusion and partial suppression of a number of skeletal segments, 

 or scleromeres, which combine into a compact occipital region 

 continuous with the remainder of the skuil in front. Through 

 its wall issue segmental nerves. At the same time, there is a 

 tendency for the corresponding muscular segments to become 

 reduced. The history of this subject has been so often told that 

 it need not be repeated here (SeAvertzoif 9, Gaupp 3, and 

 myself 6). 



The occipital region in the Amniota has been found to include 

 behind the vagus nerve four scleromeres enclosing three roots of 

 the hypoglossus nerve. There are therefore probably at least 

 live segments altogether between the auditory capsule and the 

 atlas — the first corresponding to the glossopharyngeal, the next 

 to the vagus, and the last three to the hypoglossal. Possibly 

 there are a few more. 



In the Pisces the posterior limit of the head is both less 

 definite and more variable in position ; but the postanditory 

 region probably always includes at least seven segments. The 

 Selachians have been most thoroughly studied, and in them there 

 are about eight segments behind the auditory capsule (metaotic 

 segments). The first corresponds to the glossopharyngeal nerve 

 and the fourth mesoblastic somite (three of these being prootic) ; 

 the next four segments belong to the vagus, and the last three to 

 the hypoglossus, much as in Amniotes. But in the Selachian the 

 anterior sclerotomes and myotomes are more distinct. The first 

 metaotic somite produces no myotome, and therefore preserves no 

 ventral nerve-root. The remaining occipital somites contribute to 

 the hypoglossal musculature, and their corresponding ventral roots 

 are the spino-occipitals of Fiirbinger (s — z). But the muscles of 

 these segments tend to disappear in ontogeny from before back 

 wards. In adult Selachians some two or three hypoglossal roots 

 are usually found piercing the occipital region of the skull. An 

 examination of the early stages, however, reveals the complete 

 series of somites, nerves, and skeletal segments, which make 

 up the postauditory region of the head. 



"Very different is the state of things in the Batrachia (Am- 

 phibia). Here the skull appears to end immediately behind the 

 vagus foramen ; and nerves contributing to the hypoglossal issue 

 from the vertebral column behind the occipital condjdes. 



The first, second, and third spinal nerves may form the 

 complex hypoglossal ; the second being the main, and often the 

 only, hypoglossal nerve in the adult. 



These facts immediately suggest several questions : — Does the 

 occipital region of the Amphibian really include fewer segments 

 than that of the other Gnathostomes, or have certain segments 

 been telescoped and practically crushed out ? Are the hypo- 

 glossal segments of the Gnathostomes really represented by the 

 first three trunk-segments of the Amphibian, or have these simply 

 assumed the function originally fulfilled by others farther forward ? 



