OF THE EYE MUSCLES IN MARSUPIALS. 307 



Another embryo (/3 '98) measuring 4-5 mm. ( = Stage II a) at 

 its greatest length, may be mentioned here. It is cut trans- 

 versely and almost at right angles to the last and is slightly 

 older, the optic vesicle being still further invaginated. 



The head-cavities are narrow from side to side and have the 

 form in the sections of an elongated triangle, the long pointed 

 end of which runs ventrally and medially (text-fig. 3, 7\h.c.). 

 They are lined by a single layer of epithelial cells which can be 

 distinguished all round the cavity but which, on the side next 

 the optic vesicle and extending round this side laterally for a 

 short distance, is considerably attenuated. The exact limits of the 

 abducens muscle complex are again very indefinite (text-fig. 3, r.e.). 



Embryo of G.L. 7 mm. (a '97) (=Stage III.). 

 (Text-figs. 4-6.) 



The cavity of the optic vesicle is very narrow, the outer or 

 retinal layer being much thickened and well invaginated and the 

 a(lja.cent ectoderm now forms a rounded solid inbulging, the 

 primordium of the lens. 



Text-fiffure 4. 



Trichosurus vtilpecula. G.L. 7 mm. {a '97). 



Horizontal section tlirough the head (S 3-2-11), passing above the optic vesicle and 

 tlirough approximately the middle of the head-cavity of the left side il.h.c), to 

 show the proliferation from postero-medial wall of the cavity (-p.m.). The 

 ventral end of the intermediate mass is seen postero-laterally to the cavity. 

 X 110 and reduced by 5. 



a.e.a.=arteria cerebri anterior. JF_B.=fore-brain. int.m. =intevraedia,te mass. 

 v.o.i.=\eivd, orbitalis inferior. 



The premandibular head-cavity has only increased slightly in 

 size but changes are to be seen in its walls. About half-way 

 down the cavity on the postero-medial side, the wall has thickened 



