74 THE ANCESTRY OF VERTEBRATES 



is cut into segments by the gill-slits, is, I think, nothing 

 but this anterior part of the unsegmented lateral plate. 



Diagrams of VAN Wyhe and ZlEGLER.— Before proceed- 

 ing now to the second of the above questions, viz: whether 

 the auditory vesicle belongs to this unsegmented head 

 region or to the region of the somites, we will first turn 

 a moment to the diagrams of the metameric structure of 

 the head drawn by the adherents of the views of Gejenbaur 

 and VAN Wyhe. As mentioned before, the scheme lastly 

 given by ZlEGLER (1908, 1915), and based on the researches 

 of his disciples KLINKHARDT (1905), GUTHKE (1906) and 

 BROHMER (1909), is one of the simplest. On the whole 

 it agrees with that of VAN WYHE (1882), but the hyoid- 

 segment is not considered by him as a double segment, 

 and also ZlEGLER's conception of the polymerism of the 

 vagus differs somewhat from that of VAN WYHE. 



Myomerism, neuromerism ^) and brarrchiomerism corres- 

 pond; each gill-slit has pierced through the lateral plaie 

 between two somites, thus causing the stalks connecting 

 each myotom with the unsegmented lateral plate to appear 

 much longer than in the trunk. They each run through a 

 branchial bar and communicate at their inferior end with 

 the pericard, in the same way as the somites of the trunk 

 with the general coelome. Only in the occipital region 

 are myotoms distinctly developed at their superior ends. 

 The spiraculum is the first gill-slit, it originally resembles 

 completely the gill-slits in shape and situation and forms 

 witn the latter a series of which the mouth seems to be 

 again the forward continuation. To each branchial bar 

 a segment corresponds ; that between the spiraculum 

 and the first gill-slit represents the hyoid-segment, that be- 

 tween the spiraculum and the mouth the mandibular segment, 

 from which the mandibula is formed and which at its upper 

 end dilates into a vesicle which, projecting far forward, 

 provides nearly all the mesenchyme of the anterior part of 

 the head. Anterior to the mouth we then have a last couple 

 of mesoderm segments, situated in front of the anterior end 



') This is not to be confounded with the attempts to claim a 

 segmental structure for the medullary tube and the brain, of which 

 the beginning has been found even in the still open medullary and 

 cerebral plate. Most authors now recognize that this can only be a 

 secondary phenomenon, caused by the segmentation of the mesoderm 

 and the segmental arrangement of the nerves and their ganglia. 



