HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 145 



entiation becomes mope pronounced in the following stages, and 

 is the beginning of a process by which the M. rectus lateralis, 

 represented by the greater part of the anterior division, becomes 

 more and more distinct from the retractor oculi portion, which 

 is posterior. 



The abducent nerve (n. ahd.) could first be found in this stage. 

 It may be seen as a short slender strand parallel with the ventral 

 wall of the brain, and the first impression is that it lies isolated 

 in the mesenchyma, having connection neither with the brain 

 nor with the abducent muscle-mass. Even under careful exami- 

 nation with high power, its distal end could not be traced to the 

 muscle, but its proximal end was seen to be connected with the 

 brain by a number of very delicate filaments or rootlets, three 

 of which can be clearly followed out. In following stages a 

 greater number of such rootlets occur. By the union of these 

 delicate strands the stouter portion of the abducent nerve is 

 formed, which is readily observed; but before their union the 

 rootlets may easily escape notice. 



The muscle-mass of the second head somite shows important 

 changes. Its dorsal portion is moving forward as a strong stream 

 of spindle-shaped cells, which is dorsad of the eye-ball and just 

 laterad of the nasociliary nerve. The remaining ventral portion 

 of the somite is in immediate contact with the maxillo-mandibu- 

 lar muscle-mass, but is easily distinguishable from it. Its cells 

 are spindle-formed and directed ventrally. In sagittal sections 

 the entire mass thus presents two divergent cell-streams: the 

 dorsal forward growing part is the M. obliquus superior {ohl. 

 sup.) ; the ventral portion {x) is of doubtful destiny. 



The upper or dorsal portion {max.) of the maxillo-mandibular 

 muscle-mass has become more sharply differentiated from the 

 longer ventrally extended portion {man.) and is a dense structure 

 lying at right angles to the latter. It is narrowly separated 

 anteriorly from the ventro-Iateral wall of the first head somite. 



The oculomotor muscles. The right and left premandibular 

 somites in- this stage are united by a narrow transverse canal, 

 formed by the union of the somite stalk of each side with the 

 prechordal plate. As noted also in the 8-mm. specimen, the 



