THE EYE MUSCLES 1^ MARSUPIALS. 



Text-fio-ure 20. 



331 



Fhascolarctos cinereus. G.L, 7'o mui. 



Horizontal section through the head (S 5-3-7), passing immediately above the 

 optic vesicle and just below the middle ot the right head-cavity [r.h.c.) The 

 posterior wall of the cavity is actively proliferating {p.). Size of cavity 

 •30 mm.X'26 mm. XllU and reduced by \. 



a.c.a. — arteria cerebri anterior. ^^2?. = Fore-brain. /?j5. = hypophjsis. 



Embiyo of G.L. 9 mm. (Text-fig. 21.) 



At 9 mm. the aptic vesicle is dosed except on tlie medial side 

 where the liollow optic stalk runs inwards to the fore-brain. The 

 lens has now the foi'm of a vesicle and is no longer connected 

 with the ectoderm. 



The head-cavity is still larger (text-fig. 21, l.h.c.) and resembles 

 in shape that of Trickosurus, embryo («) 7 -25 mm. A large bud 

 is present on the dorso-lateral side, the posterior and ventral walls 

 of which are thickened, whilst the whole of the postero- lateral 

 wall of the cavity is actively proliferating. 



The primordium of the m. obliquus inferior runs out from the 

 postero-lateral side of the cavity behind the optic vesicle, and 

 medial to this muscle there is an indication of the future m. rectus 

 inferior. 



The abducens muscle complex is well marked in this embryo, 

 and its connection with the intermediate mass and the m. obliquus 

 superior is clearer than in any of the embryos of Trichosurus 

 examined (text-fig. 21). The abducens nerve runs into the 

 postero-dorsal poi'tion which is situated in its usual position 

 medial to the vena capitis medialis ; the anterior portion (text- 

 fig. 21, r.e.) lies along the postero-lateral side of the cavity in 



Proc. Zool. Soc— 1915, No. XXIY. 24 



