HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 141 



trigeminal nerve, which it barely crosses. This represents the 

 anterior vesicular portion of the somite. Its cells are spindle- 

 shaped, and all lie in a general longitudinal direction. In the 

 posterior portion of the somite the cells are for the most part 

 promiscuously heaped, but they assume a distinct spindle form 

 anteriorly on the outer side, where they pass over into the anter- 

 ior division. 



7 -mm. embryo; sagittal series 



In this embryo the beginning of the epiphysis is present as a 

 distinct epithelial bud. In general form the first head somite 

 is the same as in the preceding stage, but it is somewhat larger. 

 On one side the narrow distal end of its ventro-lateral diverticu- 

 lum has fused with the neighboring end of the tubular process 

 from the muscle anlage of the mandibular arch, and only a few 

 loose cells on the inner wall of the latter, separate their cavities. 

 This is the most intimate association between the first head cavity 

 and the cavities of the mandibular arch found in the present study. 

 The other side of the embryo, however, shows relations essen- 

 tially as in the preceding stage. 



The prechordal plate is almost entirely absorbed into the 

 median stalk, in this case the left. It is simply a slender solid 

 cell-cord which rapidly broadens into the stalk. The end of the 

 notochord has severed its connection with the prechordal plate, 

 and is more sharply bent ventrally and backward, so that it 

 touches the plate with its dorsal side. 



A second embryo of the same measurements, and of equally 

 advanced development, shows a condition of the prechordal 

 plate which is a further instance of the irregularity of the develop- 

 mental changes in this structure. In this embryo the plate 

 presents an appearance almost identical with that described for 

 the distinctly younger second series of the 6-mm. stage, i.e., it 

 is a relatively large tubular structure with epithelial walls, lying 

 transversely at the end of the notochord which is closely attached 

 to its posterior wall. In the older embryo the cells of the wall 

 at this point are partially drawn out, as if unable to separate 

 from the end of the notochord. That a part of the prechordal 

 plate may thus remain attached to the chorda and become ab- 



