126 CHARLES EUGENE JOHNSON 



Oppel. The end of the chorda approaches this closely, but is 

 separated from it by an insignificant cell-mass which later be- 

 comes more sharply differentiated from both the 'Zwischenplatte' 

 and the chorda, but eventually degenerates into mesenchyma. 

 When fully developed the first head somites in Emys are large, 

 thin-walled 'sacs' connected with each other by a narrow canal 

 resulting from the 'Zwischenplatte.' 



The second and third head somites were found differentiated 

 in an embryo in which spiracular and first gill clefts had appeared. 

 The second somite Ues just below the developing N. trigeminus; 

 it has a lumen and its upper or dorsal wall, especially, is formed 

 by a distinct layer of close-set cells. The third head somite is 

 represented b}^ a heap of cells lying between the second somite 

 and the auditory vesicle, and above the spiracular cleft. In 

 this stage it possesses neither lumen nor the characteristic radi- 

 ation of cells. Later, however, a rather indistinct radiation 

 appears, and this is the only character, according to Filatoff , which 

 gives this structure claim to being a true somite. A cavity is at 

 no time de\^eloped. A compound nature of the somite such as 

 described for the Lacertilia, was not observed in Emys. 



THE EYE MUSCLES 



Investigations of the development of the eye muscles in Rep- 

 tilia have been fragmentary. The works of Corning ('00) and 

 Filatoff ('07) contain the most complete accounts. 



According to Corning the oculomotor muscles arise at definite 

 places on the wall of the first head somite. These are chiefly 

 the dorsal and ventral regions, while the lateral region, and the 

 antero-medial wall which is directed towards the optic cup, take 

 no part in the formation of the eye muscles. From the first 

 mentioned parts muscle-forming cells grow out forming muscle 

 'buds,' and at the same time out-pocketings or folds of the wall 

 occur, but to no great extent. The muscle buds or an! ages 

 thus formed grow out dorsally and ventrally, and then take an 

 anterior and lateral direction towards the eye-ball. The ventral 

 outgrowth takes the lead; it is bifurcated at its anterior end, 



