324 MISS E. A. FRASER OX THE DEVELOPMENT 



increased in length and runs from the dorsal anterior side of the 

 bulbus inwards and slightly downwards (text-fig. 18, o.s.). 



The anterior portion of the abducens muscle-mass (text-fig. 18, 

 r.e.), which we may now call the m. rectus externus, stretches 

 out laterally in the usual position posterior to the other muscles 

 and just behind the ciliary ganglion, its pointed lateral end 

 extending outwards towards the posterior border of the eye. 

 The inner dorsal portion of the mass, however, is now shorter 

 and its anterior end, which is united with the m. rectus externus, 

 has thickened, the thickening being the preliminary stage of the 

 later forward movement of this poi'tion which is the primordium 

 of the m. reti^actor bulbi. 



The oculomotor nerve passes ventrally from the mid-brain 

 anterior to the Gasserian ganglion, gives off a twig into the 

 proximal end of the m. rectus superior, then goes downwards 

 through the medial side of the ciliary ganglion immediately 

 behind the optic stalk. Leaving the ganglion, it gives oflf a 

 branch to the m. rectus inferior and runs outwards on the 

 postero-lateral side of the latter to send an offshoot to the 

 m, obliquus inferior (text-fig. 18, III.). Thus all the oculomotor 

 muscles receive their nerve supply at practically the same time. 

 The ciliary ganglion is slightly larger than at the last stage and 

 forms a transversely elongated mass of cells lying on a level with 

 the optic stalk between the ventral end of the m. rectus externus 

 and the proximal end of the m. rectus inferior. 



The trochlear nerve can now be followed from the dorsal 

 sui'face of the bi-ain downwards immediately in front of the 

 Gasserian ganglion, its distal end turning forwards to run into 

 the m. obliquus superior. 



As in the last stage, a strand of mesoderm cells extends 

 laterally from the proximal end of the m. rectus inferior to the 

 ventral border of the eyelid, but here, owing to the forward 

 movement of the m. obliquus inferior below the bulbus, this 

 strand now lies doi-sal and posterior to the latter muscle. 



The embryo of 12 mm. only diflTers in the following respects 

 from the 11 "5 mm. The eyelids have folded completely over the 

 eye; all the muscles have increased in size and every trace 

 of the walls of the former head-cavity has disappeared. The 

 attachment of the distal end of the m. obliquus inferior to 

 the ali-nasal cartilage has begun and the anterior end of the 

 primordium of the m. retractor bulbi, only apparent as a 

 thickening in the last embryo, now projects slightly forwards 

 towards the optic stalk. 



Embryos of G.L. 13 & 13-5 mm. ( = Stage XIY.). 



The stage consists of three embryos, two measuring 13 mm. 

 (a) (IX. '01) & {b) (XXV.), and the third (c) (XXIY.) with a 

 length of 13"5 mm. 



The optic stalk is still hollow, though its cavity is very small 



