150 CHARLES EUGENE JOHNSON 



and downward, so that they now form a triangular mass at the 

 anterior extremity of a slender stalk of cells, as seen in figure 

 22. This stalk of inflowing cells is the belated caudal end of 

 the muscle, and in it lies imbedded the abducent nerve. 



The M. rectus lateraUs in the former stage occupied a more 

 anterior position than the M. retractor oculi, but this is no longer 

 the case since its forward movement has been proportionately 

 less. It is now an elongate, more or less conical mass, taking 

 the same general ventral direction as the M. retractor oculi, 

 but it is also directed laterad at an angle of about fifteen degrees 

 from that muscle. Its base abuts against the median side of 

 the broad upper portion of the retractor, and its apex points 

 towards the posterior equatorial region of the eye-ball. It is 

 a much smaller mass than the retractor portion. 



The M. ohliquus superior is here an elongate mass, narrow 

 and cylindrical at its posterior end, but broad anteriorly, where 

 it is flattened in the dorso-ventral direction. It lies close to the 

 surface of the eye-ball, parallel to and slightly mediad of the 

 equator, projecting out beyond the surface of the eye anteriorly. 

 Its posterior end is in close association with the distal end of 

 the M. rectus superior. These ends are destined to be extended 

 further forward on the eye-ball before their insertion becomes 

 established. The N. trochlearis can be traced to within a short 

 distance of the posterior end of the M. obliquus superior, but 

 actual connection cannot be demonstrated. 



With respect to the ventral portion of the second head somite, 

 embryo 'b' of the 10-mm.. stage and the 11-mm. specimen show 

 practically identical conditions, both in structure and in relation 

 to the developing maxillo-mandibular muscles. In the 11-mm. 

 embryo, however, conditions are somewhat clearer and here 

 small branches from the adjacent veins have penetrated the 

 ventral cell-mass of the second somite. The lateral and medial 

 portions have a more open and decidedly mesenchymal appear- 

 ance, but towards the middle of the mass the structure becomes 

 more compact, and the spindle-shaped cells with their elongate 

 nuclei resemble muscle-forming tissue. As a whole, the mass 

 appears to be undergoing retrogressive changes. 



