HEAD SOMITES AND EYE MUSCLES IN CHELYDRA ' 125 



In an embryo of the latter species of 9 segments, two wing-like 

 cell-masses, similar to those of Anguis fragilis, partly enclose the 

 ventral wall of the brain tube; but no structure having somite 

 characteristics is present. The 'Stiel' or connecting-stalk of 

 the somite, in the midline, is connected with the entoderm of 

 the foregut, but Corning makes no mention of a prechordal 

 plate. Later a cavity appears in the lateral part of the wing- 

 like cell-mass, which gradually expands into a large sac with 

 walls of cubical or even flat cells. The cavity has continued 

 through the cellular 'Stiel' so that a very wide canal now connects 

 the right and left somites. 



Corning evidently found no structure corresponding to the 

 second head somite of Oppel and Hoffmann, and treats of the 

 development of the M. obliquus superior in connection with the 

 gill-arch musculature, as it arises, according to him, from the 

 dorsal portion of the trigeminal muscle anlage which grows out 

 anteriorly above the eyeball. 



For the third head somite this author recognized a structure 

 which he states agrees in every respect with the third head somite 

 of Anguis, and later gives rise to the muscles innervated by the 

 abducent nerve. This somite, he says, is difficult to locate in 

 younger stages, but later is easily found as a cell-mass lying 

 close to the lateral side of the internal carotid artery and some- 

 what medial to the trigeminal ganglion. 



For the Chelonia, the only work on the head somites known 

 to the writer is Filatoff's article ('07) on Emys lutaria. Accord- 

 ing to this author, the first head somite of Emys is developed 

 from a mass of cells which grows out laterally from the thickened 

 dorsal wall of the anterior end of the foregut. This thickened 

 part of the gut-wall forms at that stage the common origin of 

 both notochord and first head somite. The middle portion of 

 this thickening then differentiates into the chorda, the lateral 

 portions grow outward and give rise to the first head somites. 

 In an embryo of 18 segments the laterally lying first head somites 

 are still connected in the midline by the cell-mass which pushes 

 out from the intestinal wall, which Filatoff now calls the 'Zwisch- 

 enplatte,' and which corresponds to the Traechordalplatte' of 



