158 CHARLES EUGENE JOHNSON 



continuous. In this condition (5-mm. stage) the position of the 

 somite is such that the maxillo-mandibular division of the 

 N. trigeminus passes across the mid-lateral wall of the somite and 

 the vesicular anterior part lies in the angle formed by this nerve 

 trunk and the ophthalmic division. The wall of the anterior 

 division of the somite next collapses, and becomes transformed, 

 in a manner similar to that of the second head somite, into a 

 rather narrow mass of spindle-shaped cells which are continuous 

 with the cells of the larger sohd mass resulting from proliferation 

 in the posterior division; so that two unequal masses are now 

 formed, representing corresponding divisions of the somite. The 

 anterior mass is the anlage of the M. rectus lateralis, and the 

 posterior of the M. retractor oculi. 



In regard to the idea expressed by some authors, that the 

 head cavities form reservoirs for excretory products resulting 

 from the activities of the developing somites, these observations 

 on Chelydra can contribute nothing. The technique employed 

 brought out no evidence that there is normally any substance 

 present in these large cavities. In two instances, however, one 

 of the first head somites was found gorged with blood cells. The 

 first case occurred in an embryo Chrysemys marginata from the 

 collection of Dr. B. M. Allen; the other in a 10-mm. Chelydra 

 serpentina of the writer's series, in which the oculomotor muscles 

 had begun to develop. Compared with the other side where 

 conditions appeared normal, the cavity was more nearly spheri- 

 cal, but it was not appreciably larger, and no disturbance of the 

 developing muscles had resulted. It could not be ascertained 

 positively just where the contents had entered the cavity, but 

 there were some indications that a rupture had occurred in the 

 ophthalmic artery, or perhaps in an adjacent part of the carotid 

 artery itself. 



The eye muscles. The first of the eye muscles to be laid down 

 are those arising from the third head somite, or the abducent 

 muscles. But these muscles, however, on account of the greater 

 distance which they have to traverse in reaching their destina- 

 tion, are not the first to attain their adult position; the M. retrac- 

 tor oculi, due to its great complexity, is the last eye muscle to 

 reach its definite position. 



