HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 131 



layer of tall cells, apparently two or three deep, arranged about 

 a well formed cavity. The one on the left side is the larger and 

 better developed of the two, but in actual size is smaller than 

 the dorsal member of the more anterior group. The cavity can 

 be traced through seven sections. The entire body with its 

 lumen is slightly compressed laterally, so that its dorso-ventral 

 diameter is greater than the latero-medial. Its long axis is paral- 

 lel to the long axis of the embryo, agreeing in this respect with 

 the two components of the group first described. A short 

 distance caudad the auditory plate appears, and no further 

 mesodermic structures are seen till the first opisthotic somite 

 is encountered just posterior to the auditory plate. 



Turning our attention now to the most anterior region of the 

 head mesoderm, in the same section represented by figure 2 

 there may be observed an elongated area on each side of the 

 forebrain vesicle, which is marked off from the adjacent meso- 

 derm only by the closely packed condition of its cells (S 1). Each 

 mass extends from near the median line antero-laterally, follow- 

 ing the border of the fore-brain wall. Followed out, these cell- 

 heaps expand somewhat anteriorly and laterally, and a section 

 passing through the antero-dorsal wall of the foregut shows a 

 rather short bridge of cells connecting the right mass with the 

 left, producing a dumb-bell shape. At its middle this connecting 

 bridge is seen to form a part of a mass of cells pushing out from 

 the anterior dorsal wall of the foregut (fig. 3) . The anterior end 

 of the notochord enters this outgrowing cell-mass from the pos- 

 terior side and becomes indistinguishably fused with it. The 

 two laterally growing cell-masses, therefore, arise from a median 

 mass which pushes out from the entoderm of the anterior dorsal 

 wall of the foregut. 



From the position and relations of the three sets of structures 

 here described, it is clear that they represent the three prootic 

 head somites described by Filatoff for Emys lutaria, and by other 

 authors for a number of representatives of the Lacertilia. From 

 anterior to posterior they are designated the first, second, and 

 third head cavities or somites, or the premandibular, mandibu- 

 lar, and hyoid somites (S 1, S 2, S3). 



