ORIGIN AND STRUCTURE OF THE HEAD 111 



stage I, it was found in the fourth and in an embryo in 

 stage O in the seventeenth trunksegmenl. This shows that 

 the growth of the interior region of the gut surpasses that 

 of the rest of the body, a process which causes the hind- 

 most gill-slits to be found finally under myotomes which 

 were originally situated behind ihem. But this does 

 not mean to say that a// the myotomes found 

 above the gill-slits have only secondarily 

 acquired this position. That myotomes behind 

 the auditory capsule have been, so to speak, crushed out 

 of existence by the strong development of this capsule can 

 not be doubted. FuRBRlNGER (1897, p. 440), however, 

 assumes a "stetiges Vorriicken" and dissolution of somites 

 in the occipital region, for which embryology has not yiilded 

 serious evidence. On the contrary, no embryologist, with 

 the exception of BRAUS (1899), has assumed such a process, 

 and Goodrich (1918, p. 13), the last to have worked on 

 the subject, asks: what definite evidence is there that such 

 a procession of myotomes which plunge one after the other 

 below the capsule and vanish in a cloud of mesenchyme 

 really occurs? I think with GOODRICH that there is good 

 reason to believe that for the most part myotomes once 

 laid down persist. 



We have seen that in Petromyzon, as a consequence of 

 the backward extension of the branchial basket, the post- 

 otic myotomes 7 — 12 become secondarily epibranchial; in 

 the same way this occurs with a considerable number of 

 myotomes in Selachians but of these only the anterior one 

 or two, as we have seen, belong to the skull. The other 

 occipital myotomes, in front of these, and their nerves belong 

 from the beginning to th e branchial region, just like the anterior 

 epibranchial myotomes in Petromyzon. In Selachians, however, 

 this epibranchial musculature is much less developed than in 

 the latter, though in the more primitive froms with the greatest 

 number of gill slits it is more developed. In other groups 

 of Vertebrates, and even in rays, it is no longer found. 



FuRBRlNGERs rule applied to the hypoglossus. — Petromyzon 

 shows us that the opinion that the hypoglossus ab origine 

 has nothing to do with the vagus (vago-accessorius) is not 

 incorrect. If we compare the situation of the hypoglossus- 

 roots in the main groups of Vertebrates (cf. Plate I), we 

 see that there can be no question about an individual 

 homology of the segments to which they belong in the 



