334 



MISS E. A. PRASER ON DEVELOPMENT 



111 the 9 mm. embryo the head-cavity is large and ti'iangular 

 in shape, resembling that of the 9 mm. stage of Phascolarctos, 

 while in the 8'5 mm. specimen it is much reduced in size (text- 

 fig. 22, r.h.c). In both embryos a dorso-lateral proliferating 

 muscle-bud is present (text-fig. 22, d.l.)^ and the m. obliquus 

 inferior extends down from the ventro-lateral wall posterioi' and 

 ventral to the optic cup (text-fig. 22, o.inf.). 



Tlie anterior portion of the abducens muscle complex is joined 

 with the intermediate mass, and the connection of the latter with 

 the maxillo-mandibular mesenchyme by means of a wide band of 

 cells is well seen in the 8"5 mm. embryo (longitudinal series) 

 (text-fig. 22). 



The m. obliquus superior has the same appearance as in the 

 9 nun. stage of Phascolarctos; on its inner border it practically 

 suri'ounds the supra-orbital branch of the trigeminal neive. 



The oculomotor nerve is prolonged ventrally into the m. obliquus 

 infei'ior and the abducens nerve runs into the hinder end of the 

 postero-dorsal portion of the abducens muscle complex. 



The examination of the two older stages, measuring 15-5 mm. 

 and 17*5 mm. respectively, shows that the further development of 

 the eye muscles proceeds as in Trichosurus. 



Macropus ruficollis. (Text-figs. 23 & 24.) 

 Only one embryo of Macropus ruJicoUis, with a maximum length 

 of 6'7 mm., came under observation. The optic vesicle is 



Text-fioure 23. 



a.c.a 



a.c.a. 



Macropus ncficoUis. G.L. 6"7 mm. 



Horizontal section tlivougli the head (S 4-2-1) showing the head-cavity on 

 each side {I.h.c. & r.h.c). Size of cavities: lelt ='29 mm.X"14 mm.; right 

 = •30 mm.X'16 mm. X50 and reduced by i. 



a.c.a. = arteria cerebri anterior. J^i?. =fore-Lraiii. 7/p. = hypophysis. 



