HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 133 



precisely the same number of segments was available, and in 

 exactly the same position a cell-group surrounding a small but 

 distinct lumen was found in one section, but as the sections adja- 

 cent had been partly lost or mutilated, nothing further could be 

 learned. In the embryo described, the opposite side (the left) 

 presented less advanced development. No such radial cell struc- 

 ture was present, but a rather inconspicuous mass of cells indi- 

 cated what undoubtedly would have resulted in a corresponding 

 structure. In regard to the second smaller component of the 

 second head somite, it is doubtful whether it is constant, or has 

 any other significance than that it indicates a tendency of the 

 second head somite to split up, — a step towards the diffused 

 condition of this area in birds, as found by Rex. The present 

 phase of the somite is evidently maintained for only a brief period, 

 for in the embryo with ten segments, it was still undeveloped 

 on the left side, and, as will subsequently appear, relatively 

 large cavities soon take its place, which little suggest an earlier 

 somite form. 



The third head somite of Chelydra differs markedly from the 

 corresponding somite in Emys in that it possesses a very clear 

 radial cell structure and a well defined conspicuous cavity. In 

 Chelydra, therefore, this somite approaches closely the typical 

 somite form. 



4--mm. embryo {13 segments); sagittal series: figures 4 to 7 



Distinct advance is apparent in this stage. Cavities have 

 appeared in the first head somite. In the preceding stage, it 

 will be recalled, the anterior end of the notochord disappeared 

 in the thickened dorsal wall of the foregut, from which the cell- 

 masses constituting the first head somites grew out laterally. 

 At the point of cell outgrowth there now lies a median oval thick- 

 walled epithelial body, with narrow central cavity, closely wedged 

 in between the infundibular region and the dorsal anterior wall 

 of the foregut. Its anterior end abuts ventrally against the 

 ectoderm where the invagination of the hypophysis later appears 

 (fig. 4). This structure is the Traechordalplatte' of Oppel or 



