HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 143 



the ceils of the notochord itself, and, as before suggested, evi- 

 dently have been detached from the prechordal plate. The 

 oculomotor nerve has increased in length and now hes close to 

 the posterior wall of the first head cavity. 



The second head somite is now reduced to a mass of 

 mesenchymal cells, passing gradually over into the surrounding 

 mesenchyma at the periphery, yet distinguishable as a whole 

 by its denser structure and its noticeably deeper-staining nu- 

 clei. A few diminutive cavities may occasionally appear. 



The third head somite shows advances over the preceding 

 stage in that it is appreciably larger, more compact in structure 

 and more definite in form. It here consists of a larger cell-heap 

 forming the posterior portion of the somite, from the antero- 

 lateral side of which there extends forward a narrow elongate 

 mass which lies opposite the space between the two divisions of 

 the trigeminal nerve and which represents the anterior portion 

 of the somite. The maxillo-mandibular division of this nerve 

 lies against the middle of the lateral side of the cell-mass so that 

 the position and relations of the two earlier vesicular divisions 

 of the third head somite have practically remained unaltered. 



Summary 



In the foregoing account it has been observed that during the 

 developmental changes which have taken place thus far in the 

 head somites, the anatomical positions of these structures have 

 been maintained throughout, the changes having been chiefly 

 those of form and size. From the small characteristic structures 

 found in the earliest stages, the second and third head somites, 

 after developing into relatively large vesicular bodies, have been 

 resolved by the breaking down of these, into more or less compact 

 masses of mesenchymal and spindle-shaped cells. These result- 

 ing cell-masses are the anlages of certain muscles of the eye. 

 The second produces the M. obliquus superior, and the third the 

 so-called abducent muscles, the Mm. rectus lateralis and retractor 

 oculi. 



The first head somite, differing in its early ontogenetic history 

 from the second and third, has arrived at the maximum expan- 



