OF THE EYE MUSCLES IN MARSUPIALS. 335 



flattened, its outer layer being much thickened and bulging into 

 the cavity, and the wide opening in the optic stalk is connected 

 with the fore-brain. The ectoderm opposite the vesicle is as yet 

 only slightly thickened on the ventro-lateral side of the latter. 



This embryo possesses a large premandibular head-cavity on 

 each side of the fore-brain in the usual position medial and 

 posterior to the optic vesicle (text-figs. 23 & 24, l.h.c. & r.h.c). 

 The dorsal portion of the cavity is somewhat irregular in shape 

 with a small area marked ofi" towards the median plane by a 



Text-figure 24. 







& 





Macropus ruHcolUs. G.L. 6'7 mm. 



Left liead-cavit^' {l.h.c.) showing its lining of a single layer of epitlielial cells. 

 Size of cavity=-29 mm.X'M mm. X200. 



consti-iction as in Trichosurus ., embryo («), 5 mm, ; further 

 ventrally, it assumes a more oval form (text-figs. 23 & 24, l.h.c. & 

 7'.h.c.), measuring on the left side '29 mm. x "29 mm. x*14 mm. 

 and on the right '31 mm. x "30 mm, X'16 mm., the shortest 

 measurement being the ti'ansverse one. It is lined throughout 

 by a single layer of epithelial cells. 



The abducens muscle complex, the intermediate mass and 

 the primordium of the m. obliquus superior are ignited in the 

 usual way, but their respective limits are difficult to determine 



