OF THE EYE MUSCLES IX MARSUPIALS. 329 



As development proceeds, the m. obliquus inferior moves 

 forwards below the eye, its distal end turning inwards in the 

 11*5 mm. embryo to become attached to the ali-nasal cartilage. 

 Jt becomes independent in the 13"5 mm. embryo and its origin- 

 ally proximal end moves outwards to reach its final insertion on 

 the antero- ventral side of the bulbus. This muscle thus under- 

 goes a noteworthy change in position. The m. rectus inferior 

 grows forwards and outwards, becomes independent and finally 

 extends downwards fi'om below the inner end of the optic cup to 

 become inserted in the ventral side of the bulbus close to the 

 m. obliquus inferior. 



From the dorso-lateral region of the cavity in the 6 mm. 

 embryo appears a bud-like outgrowth whose surrounding walls, 

 especially that of the posteiior side, proliferate and give rise to 

 the m. rectus superior. The primordium of this muscle grows 

 antero-posteriorly over the eyeball, becomes independent and 

 very early attains its final position (11 to 12 mm.). 



The remaining walls of the head-cavity undergo degeneration 

 and become obliterated in the surrounding mesenchyme. 



The m. rectus internus is the last of the eye muscles to make 

 its appearance. It first develops in the 11 mm. embryo as an 

 outgrowth from near the proximal end of the m. rectus inferior 

 and grows dorso-laterally to become inserted on the anteiior side 

 of the bulbus. 



The abducens complex is first seen as a solid mass of cells 

 lying posterior to the premandibular cavity and consisting of 

 postero-dorsal and antero-ventral portions. The latter grows 

 out laterally as the m. rectus extern us and the former grows 

 forwards and outwards, crossing the inner end of the m. rectus 

 externus, its distal end finally surrounding the optic nerve as the 

 m. retractor bulbi. The m. retractor bulbi is the last of the eye- 

 muscles to assume its final position. 



The m. obliquus supei-ior takes its origin from a, mass of cells, 

 termed by me the intermediate mass, which can be traced down- 

 wards into continuity with the maxillo-mandibular mass of 

 mesenchyme. The anterior band-like part of the intermediate 

 mass separates ofi' to form the primordium of the m. obliquus 

 superior. It grows inwards to form a muscular band, which 

 extends out from above the n^. rectus internus to the anterior 

 side of the bulbus. 



The more posterior portion of the intermediate mass, attached 

 to the maxillo-mandibular mesenchyme, is wider and composed 

 of more loosely arranged cells, and is also connected in our earlier 

 stages with the lateral side of the primordium of the m. rectus 

 externus and for a still shorter period with the postero-lateial 

 wall of the head-cavity. After the m. rectus externus and the 

 m. obliquus superior become independent the intermediate mass 

 disappears. 



