OF THE EYE MUSCLES IN MARSUPIALS. 321 



The ciliary ganglion first appears at this stage as a trans- 

 versely elongated mass of ceils, situated between the ventral end 

 of the anterior portion of the abducens complex and the point of 

 origin of the m. obliquus inferior and the m. rectus infeiior 

 (text-fig. 14, cil.g.). 



The trochlear nerve was observed for the first time in the 

 longitudinal series but it could not be made out in the horizontal. 

 It arises in the usual way from the dorsal side of the brain and 

 runs ventrally almost as far as the level of the dorsal border of 

 the Gasserian ganglion. 



The first indication of the formation of muscle cells is seen in 

 series (6) and is most marked in the mm. recti superior, inferior 

 and externus, less definite in the m. obliquus inferior, and not 

 recognisable at all in the m. obliquus superior. 



Embryos of G.L. 9-5 mm. (V. '01) & 10 mm. (VI. '01) 

 ( = Stages X. & XI.). (PI. I. figs. 1, 2 & text-fig. 17.) 



These two embryos, measuring 9*5 and 10 mm. respectively, 

 resemble in many respects those of the last stage but show in 

 some details a slight advance in development. 



The outline of the eye is more oval and the evelid has closed 

 over at the extreme ventral side of it. At 10 mm. the cavity of 

 the lens is fm-ther reduced and the opening in the optic stalk is 

 narrower. 



A slit-like remnant of the head-cavity is still present in both 

 embryos (text-fig. 17, l.h.c). Figs. 1 & 2 (PI. I.) show posterior 

 and anterior views of a w^ax-plate model of the embryo of 

 9*5 mm. The m. rectus superior is now quite conspicuous and 

 has grown forwards and dorsally above the optic cup (figs. 1 & 2, 

 r.s.). Below the latter the m. obliquus inferior extends down 

 from the postero-lateral corner of the solid mass of mesodenn 

 which has taken the place of the former head-cavity (fig. 1, 

 o.inf.). The m. rectus inferior is now more marked and stretches 

 out anteriorly below the optic stalk (fig. 2, r.in/.). The m. 

 obliqiius superior (fig. 2, o.s.) lies in the same position as in the 

 last stage ; but is larger and more prominent and still retains 

 the narrow prolongation running backwards towards the m. rectus 

 superior (fig. 1, r.s.). The two portions of the abducens complex 

 are well seen in the model (PI. I. fig. 1 and text-fig. 17, r.b. & 

 r.e.) and are very similar to the embryo of 8"5 mm. 



At 10 mm. the differentiation of muscle fibres is well estab- 

 lished and can be seen in all the developing eye muscles. 



In both embryos, at the point of origin of the m. rectus 

 inferior and m. obliquus inferior, a narrow band of mesodermal 

 cells runs directly outwards immediately anterior to the m. 

 obliquus inferior and becomes united with the ventral side of 

 the eyelid posterior to the optic cup (PI. I. figs. 1 & 2, -{-). 



