330 miss e. a. fraske on the development 



Notes on other Marsupials. 

 Phascolarctos cinereus. Embiyo of G.L. 4 mm. 



Our earliest stage of Phascolarctos cinereus measures 4 mm. 

 at its greatest length. The outer wall of the optic A^esicle is 

 thickened and flattened and the optic stalk is in wide communi- 

 cation with the fore-brain. 



Situated postero-medially to the optic vesicle on each side is a 

 large preraandibular head-cavity, the long axis of which lies in 

 tlie sections in an obliquely antero-posterior direction. From 

 the posterior and postero-lateral walls active proliferation is 

 taking place by means of hollow buds which are growing out from 

 the cavity, a solid mass of cells being formed by the thickened 

 walls of one bud running into those of another. Next the fore- 

 brain the walls consist of a single epithelial layer, which in some 

 places, however, is not distinctly seen. 



Unfortunately, this embryo is not well preserved and the 

 abducens muscle-mass is very diflicult to distinguish from the 

 thickened posterior wall of the head-cavity. 



Embryo of G.L. 7-5 mm. (Text-fig. 20.) 



An older embryo, measuring 7'5 mm., shows the optic vesicle 

 much further invaginated and tlie primordium of the lens in the 

 form of a thickened hollow ingrowth from the ectoderm: 



The head-cavity has increased considerably in size, having a 

 maximum diameter of '30 mm. X '26 mm. (text-fig. 20, ?'.7i.c.), and 

 its antero-ventral portion has grown forwards above the optic 

 vesicle. The whole of the posterior wall is thickened ; hollow 

 buds, many of which are long and tubular, I'un out into a mass 

 of closely packed cells, the entire region appearing as an irregular 

 mass pitted with small hollow spaces (text-fig. 20, j)-)- The wall 

 next the fore-brain, as in our first stage, is composed of a single 

 layer of cells. 



The abducens muscle-mass, which is clearly recognisable, con- 

 sists of a narrow postero-dorsal portion lying along the medial 

 side of the vena capitis medialis ; this continues into the larger 

 anterior part, which extends obliquely forwards and downwards 

 postero-laterally to the head-cavity to meet the intermediate 

 mass. The latter stretches forwards just above the optic vesicle 

 into a bulbous extension, the primordium of the m. obliquus 

 superior, and runs downwards into the maxillo-mandibular 

 mesenchyme; it is also connected with the postero-lateral wall 

 of the head-cavity. 



The oculomotor nerve is well developed, its distal end termi- 

 nating some distance from the head-cavity. 



