OF THE EYE MUSCLES IN MARSUPIALS. 325 



and difficult to see in the mid-region of its extent. In (a) the 

 eyelids have folded completely over the eye, but in (b) they have 

 closed over the lower half only and here the lens is solid. 

 Otherwise the eye resembles that of the last stage. 



A vestige of the walls of the former head-cavity is still present 

 in embryo (a) only, and is seen as a strand of cells running back- 

 wards and inwards from the medial end of the m. rectus supei-ior. 

 In (a) the latter is an independent muscle, whereas in (b) and (c) 

 it is apparently still joined with the m. rectus inferior. 



As the positions of the muscles at this stage are very much 

 the same as at 15 mm., of which a wax model has been con- 

 structed, they will be described in detail in the sequel. A few 

 remarks may, however, be made here. The m. rectus internus 

 is as yet very short and at 13 mm. the m. rectus inferior is still 

 attached to the m. obliquus inferior, but the sepai-ation of the 

 two muscles is complete at 13"5 mm. The m. obliquus inferior 

 is now definitely attached to the ali-nasal cartilage ; it has thus 

 changed its position, its outer end turning inwards to the point 

 of attachment, while the proximal end is inserted into the 

 antero- ventral side of the bulbus. 



The part of the abducens muscle complex destined to form 

 the m. retractor bulbi has grown still further forwards and 

 oiitwaxds towards the medial wall of the eyeball. From its 

 lateral side the m. rectus externus extends almost directly out- 

 wards to the posterior lower wall of the bulbus, the two muscles 

 running out at an acute angle to each other, the m. reti'actor 

 bulbi crossing the inner end of the m. rectus externus. 



Embryos of G.L. 14, 14-5, & 15 mm. ( = Stage XY.). 

 (PI. II. figs. 3-5.) 



In this stage we have an embryo of 14 mm. (XXIII.), two 

 pouch young of 14-5 mm., a (XV. '02) and b, and one of 15 mm., 

 the last three being recently born. 



The eyelids are now quite folded over but the optic stalk 

 still contains a minute cavity. 



The m. rectus superior (PI. II. fig. 4, r.s.) runs from behind 

 the inner end of the optic stalk just above the proximal end of 

 the m. rectus inferior slightly dorsally and outwards, its pointed 

 outer extremity being inserted in the eyeball dorsal to the 

 posterior side of the latter and at a slightly lower level than the 

 m. obliquus superior. 



The m. obliquus superior (PL II. fig. 3, o.s.) is now a stronger 

 better developed muscle than at 13 mm. It extends directly 

 outwards over the anterior side of the bulbus, its broad inserted 

 end, which is flattened antero-posteriorly, stretching further 

 laterally over the eyeball than any of the other eye muscles. 



The m. rectus inferior (PI. II. figs. 3 & 5, r.inf^ arises from 

 below the inner end of the optic stalk and passes downAvards and 

 slightly anteriorly below the &talk a^ a broad muscle which tapers 



