OF THE EYE MUSCLES IN MARSUPIALS. 



333 



Embryo of G.L. 11 mm. 



In oiii" next embryo, which measures 11 mm., all traces of the 

 head-cavity have disappeared, the m. obliqiius inferior has begun 

 to move forwa,rds below the eye and the m. rectus inferior is 

 further developed. Phascolarctos has now reached a stage in 

 development intermediate between that of the Trichosuriis 

 embryos of 10 and 11 mm. 



The later history of the eye muscles is similar to that of 

 Trichosurus and need not again be described. 



Phascglomys mitchelli. (Text-fig. 22.) 



The two youngest embryos of Phascolomys oniichelli measure at 

 their greatest length 9 mm. and 8-5 mm. respectively ; the former 

 is slightly the younger of the two. 



Tiie optic vesicle is closed and the optic stalk is in wide com- 

 munication with the fore-brain ; the lens is a hollow vesicle shut 

 off from the ectoderm. 



Text-fiijure 22. 



V c a 



Op-C 



o.inf 



v.c.m: 





* "''. • i % \ , ' ♦ , ♦' » <• * ^ •' .' 1 



max. md. 



JPhascolomys mitchelli. G.L. 8'o mm. 



Longitudinal section (S 2-1-1) passing througli the lateral side of the right head- 

 cavity {r.h.c.) and showing the proliferation from the dorso-lateral wall {d.l.) 

 and the primordium of the m. obliquus interior {o.inf.). The intermediate mass 

 {int.m.) situated between the cavity and the Gasserian ganglion (//. F.) is seen to 

 he connected with the maxillo-mandibular mesenchyme (max.md.) by a baud 

 of cells lying just on the medial side of the vena orbitalis inferior (v.o.i.). 

 X 110 and reduced by i. 



a.c.fr. = arteria cerebri anterior. oj9.c.=optic cup. ■y.c.a.=vena cerebralis anterior. 

 y.c.jH. =vena capitis medialis. v.o.i.— vona, orbitalis inferior. IIL = oculomotor 

 nerve. 



24* 



