148 CHARLES EUGENE JOHNSON 



of the now very compact and lobulated dorsal end of the maxillo- 

 mandibular musculature. 



The continued connection of the mass x with the M. obliquus 

 superior and the great length of the latter, are peculiar features 

 of this stage. 



The oculomotor group. The premandibular somites are now 

 well drawn away from the median line, and on the left side only 

 is there a remnant of a former stalk or cross-canal. The two 

 cavities are quite similar in form and show little difference in 

 size, but the right is a trifle more advanced in the development 

 of the muscles. The dorso-lateral wall of each is disintegrating 

 (fig. 21 &, Mes.). Its cells are passing out into the surrounding 

 mesenchyma and pushing into the somite cavity. The inferior 

 oblique muscle {ohl. inf.) is large and forms a solid club-shaped 

 mass reaching ventrad well beyond any other portion of the 

 somite. The anlage of the rectus superior {red. sup.) is well 

 established as an extensive solid thickening involving a large 

 portion of the upper half of the posterior wall of the somite. 

 It is roughly quadrangular, being narrow medially and widening 

 laterally, where it is closely covered by the ciliary ganglion. 

 This thickening of the wall takes place by an outward prolifer- 

 ation of cells and no perceptible protrusion into the interior of 

 the somite occurs. The position and relations of the anlages 

 of the right and left sides of the embryo are identical. 



In the lateral half of the somite its posterior ventral wall sends 

 out a deep sac-like evagination, the outer side of which is closely 

 apposed to the median side of the upper half of the M. obliquus 

 inferior. The posterior and ventral sides of this. sac are thick- 

 ened. The ophthalmic artery crosses the middle of its posterior 

 wall, and turns cephalad, passing in between the evagination 

 and the M. obliquus inferior, and emerging on the anterior side. 

 The large evagin.ated portion in question is the common anlage 

 of the Mm. inferior rectus and rectus medialis {inf. -\- med. . 

 recti) . 



The ciliary ganglion {cil. g.) at this stage appears as an accumu- 

 lation of cells which form a thickening of the N. oculomotorius 

 toward its distal end. On the right side the nerve extends a short 



