OF THE EYE MUSCLES IN MAESUPIALS. 337 



layer of epithelium having been observed only in the section 

 figured where the cavity is larger than it is elsewhei-e. There 

 appear to be proliferations of cells round parts of the cavity 

 but no definite muscle-masses can be distinguished. 



The m. obliquus superior and the abducens muscle-mass are 

 connected by means of the intermediate mass and show the 

 same relations as described for other genera. A portion of 

 the intermediate mass is apparently joined with a side of the 

 head-cavity but the limits between the parts are again difilcult 

 to make out. 



In the next embryo, which measures 7 mm., the cavity of the 

 optic vesicle is almost closed and the lens forms a hollow vesicle 

 but the optic stalk is still widely open. Small head-cavities are 

 again found, the one on the left side being larger and better 

 developed than in the 5' 7 mm. embryo, while that on the right 

 is more or less broken up. On both sides a thickening is present 

 on the postero-lateral side of the head-cavity and the primordium 

 of the m. obliquus inferior extends down below the cavity ; on 

 the left a proliferating dorso-lateral bud is present. 



In the 8"75 mm. embryo of P. obesula the cavities have almost 

 disappear-ed and may be compared with the traces of the head- 

 cavity still left in the 8"5 mm. embiyo of Trichosurus. The 

 muscles are developing as in the latter. 



DiDELPHYS MARSUPIALIS. (Text-fig. 26.) 



We had at our disposal only one embryo of Bidelphj/s 

 measuring 8*5 mm. The optic vesicle is completely invaginated 

 and its cavity obliterated ; the optic stalk on the right side 

 contains a prominent cavity connecting it with the fore-brain 

 but on the left it is almost solid, only a few very small hollow 

 spaces remaining, and these are chiefly on the side next the brain. 

 On the left side the lens has just separated ofi" from the ectoderm 

 but lies close to the latter and is of a remarkably small size, but 

 on the right no lens could be seen and had apparently not yet 

 begun to develop. 



No head-cavities are present but in the usual position of the 

 latter, posterior and dorsal to the eye, lies a condensed mass of 

 mesenchyme cells from which the eye muscles are developing. 



On careful examination, the postero-lateral side of this mass, 

 which extends obliquely forwards in front of tlie vena capitis 

 medialis, is recognised without difiiculty as the abducens muscle- 

 mass (text-fig. 26, r.e.) ; lying parallel to this, on the anterior 

 side of the whole group of cells, we see the future m. rectus 

 superior (text-fig. 26, r.s.), between the medial side of which 

 and the abducens muscle-mass runs down the oculomotor nerve. 

 The lateral side of the abducens muscle-mass is joined with the 

 intermediate mass which, stretching forwards above the eye, is 

 continued into an outgrowth corresponding to the primordium 

 of the m. obliquus superior (text-fig. 26, o.s. and int.m.). The 



