170 r. H. EDGEWORTH. 



The lateral plate of the hyoid segment extends upwards 

 and forwards lateral to the alimentai-y canal between the first 

 and second gill-clefts ; its upper end in 6^ and 7 mm. embryos 

 (=: stages H and I of Balfoui*) is continuous above Avith the 

 "fourth myotome" of van Wijlie. 



Van Wijhe says that the " fourth myotome " in stage J is 

 separated from the lateral plate Jind is very rudimentary, 

 also that it atrophies towards the end of that period ; further, 

 that " bis in dieselbe Hohe aber niehr lateral verlangen sich 

 in spateren Stadien die Wande der jetzigen Hyoidhohle. 

 Mit dieser Verlangeriing darf . das vierte Myotome nicht 

 verwechselt werden." The Anlage of the rectus externus, 

 regarded by van Wijhe as the third myotome, was stated to be 

 continuous in stage I with the solid cell mass in the hyoid 

 arch, and in stage J to be no longer so. 



In the embryos of 6^ and 7 mm. in length ( = stages H and I) 

 examined, it was found that the Anlage of the rectus externus 

 was continuous behind with the upper end of the epithelium-^ 

 lined cavity in the hyoid arch, i.e. with the fourth myotome 

 of van Wijhe. In embryos of 9 mm. (Text-fig. 3) and 10 mm. 

 it was found that the epithelial walls of the hj'oid cavity had 

 come together, so that the cavity had disappeared, that the 

 now solid cell cohimn had extended upwards, and that the 

 Anlage of the rectus externus was continuous behind with 

 this cell column some little distance from its upper end, at a 

 site corresponding with the original upper end of the hyoid 

 cavity. No trace of a separated " fourth myotome '' was 

 seen. It is therefore possible, on the analogy of what takes 

 place in the trunk, to regard the whole of the " lateral plate " 

 and " fourth myotome " — which do not become separate — as 

 together forming the hj^oid myotome. 



This theory is supported by the difficulty of finding any 

 structure in the body region which is homologous with the 

 lateral plates of the head. Ziegler was of opinion that they 

 were homologous with the " Urwirbelkommunikatiou " (of 

 Rabl), the " Ursegmentsteil " (of Felix). But it is scarcely 

 possible that epithelial structures of the head, which develop 



