160 CHARLES EUGENE JOHNSON 



of cells from the dorsal portion of the mesenchymal cell-mass 

 which results from the second head somite. It remains connected 

 for a relatively long period with the ventral portion of the cell- 

 mass, and thereby attains great length. It passes forward over 

 the dorsal side of the eye-ball, separates from the ventral part 

 of the somite, and soon after the 11-mm. stage it becomes attached 

 by its posterior end (insertion) at about the mid-dorsal surface 

 of the eye, lying close to the surface of the latter along the equa- 

 tor. From this position the muscle gradually swings medio- 

 ventrally, rotating about its inserted end, till it lies in a meridional 

 direction (8.5-mm. carapace stage) when it continues its growth 

 directly towards its origin, passing just dorsad of the nasocihary 

 nerve. 



The N. trochlearis does not reach the M. obliquus superior 

 until a comparatively late period — shortly after the 11-mm. 

 stage. It enters the muscle near its insertion. 



FilatofT states than in Emys the ventral portion of the second 

 head somite, i.e., the cell-mass resulting from it, becomes a part 

 of the musculature of the mandibular arch. Except for the 

 small part of this mass which is directly adjacent to the muscula- 

 ture of the arch, I am inclined to believe that such is not the 

 case in Chelydra. At least the evidence here gives little support 

 to Filatoff's interpretation. The embryo of 8.5-mm. carapace 

 showed no trace of this cell-mass and gave no clue to its proba- 

 ble fate. In younger stages its structure indicates that it is of 

 muscle-forming nature but its progressive development seems 

 interrupted or suppressed. As late as the 11-mm. stage it still 

 remains unchanged in position and form, and its more lightly 

 stained elements and open structure are in marked contrast 

 with the heavily stained and dense masses of the maxillo-man- 

 dibular musculature, and with the progressive development char- 

 acteristic of other muscle-masses. In view of these facts it seems 

 that this part of the second head somite must be interpreted in 

 some other way. 



In elasmobranch embryos it was first shown by Miss Piatt 

 that from the ventro-median wall of the mandibular somite 

 proper there is formed a muscle 'E,' which early reaches an ad- 



