146 CHARLES EUGENE JOHNSON 



notochord has a group of cells attached to its tip, which in appear- 

 ance are quite distinct from its own; and a loose patch of similar 

 cells extends from it to the postero-ventral side of the transverse 

 connecting-canal, in the midline, that is, to that part of the 

 canal which corresponds to the prechordal plate. These cells 

 here also are clearly a part of the latter structure, which has 

 become detached along with the chorda. 



From a relatively small area on the extreme ventro-lateral 

 wall of the first head cavity, a rather narrow outgrowth of cells 

 extends forward. The basal portion of the outgrowth leceives 

 a slight and irregular extension of the somite cavity; its distal 

 part is simply a mass of spindle-formed cells directed towards 

 the ventral side of the eye-ball. The outgrowth is the anlage of 

 the M. obliquus inferior {obi. inf.) (See also figure 13.) 



The oculomotor nerve has grown further ventrad, and lies in 

 close contact with the mid-posterior wall of the somite.- In 

 this region there appears a thickening in the somite wall, but 

 it is so slight that without the aid of the stages following it would 

 be considered of no significance. It is the anlage of the M. 

 rectus superior, and is destined shortly to become the largest 

 member of the oculomotor group. 



The opposite side of the embryo presents practically identical 

 conditions. 



lO-mm. embryo (a); sagittal series; reconstruction: figures 

 21a, 21b; sections: figures I4 to 17 



The abducent muscle-mass in the 9-mm. embryo already de- 

 scribed has the form of a stout rod which is somewhat convex 

 externally. In the 10-mm. specimen the anterior portion of the 

 rod, which gives rise to the M. rectus lateralis, has grown out 

 laterally, as seen in figure 216. This lateral portion is closely 

 wedged into the angle formed by the anterior cerebral vein in 

 front, as it bends downward to enter the vena capitis medialis, 

 and the V. capitis medialis^ behind, as it ascends and passes 



' The vena capitis medialis is the primary vein of the head, situated medial 

 to the cerebral nerves, close to the wall of the brain. Grosser, O. Die Elemente 

 des Kopfvencnsystems der Wirbeltiere. Verh. der Anat. Gesellschaft, Wiirzburg, 

 1907, p. 180. 



