328 



MISS E. A, FRASER ON TUE DEVELOPMENT 



unite distally to have a single insertion on the posterior side of 

 the bulbus. 



Text-fifi-ure 19. 



r.e. 



re. 



-r. Inf. 



r.i. 



Trichoswus vulpecula. G.L. 5'2 cm. 



Longitudina] section (S 23-1-5) passing through the optic nerve (II.) and the 

 surrounding eye muscles. The m. retractor bulbi (r.b.) forms a circular sheet 

 of muscle within the other eye muscles. The two portions of the m. rectus 

 externus {r.e.) are also seen. X 35. 



?.jp.s.=levator palpebrse superioris. o.s.=m. obliquus superior. VI.=abducens 

 nerve. r.i.=va. rectus internus. r.inf. = m. rectus inferior. r.s. = m. rectus 

 superior. 



Swmmary of Events in Trichosurus vulpecula. 



In early stages a large premandibular cavity surrounded by 

 a single epithelial layer of cells is present on each side of the 

 fore-brain posterior to the optic vesicles. 



In the 7 mm. embryo an extensive thickening occurs on the 

 lower two-thirds of the posterior wall, later (in the 6 mm. 

 embryo) extending on to the ventral wall of the cavity. From 

 this thickening the m. obliquus inferior and the common 

 primordium of the m. rectus inferior and m. rectus internus 

 develop, the first to appear being the ni. obliquus inferior which 

 grows out laterally as a solid protuberance from the ventro- 

 lateral side ; the m. rectus inferior arises a little later (in the 

 embryo of 8-5 mm.) as a foi'ward extension from the thickened 

 wall immediately medial and slightly ventral to the m. obliquus 

 inferior. 



