OF THE EYE MUSCLES IN MARSUPIALS. 



311 



The premandibular head-cavity is large and conspicuous on 

 each side ; although at its widest it is only slightly smaller than 

 that of the 7 mm. embryo, the ventral portion is reduced, 

 perhaps owing to further proliferation of the walls. 



The proliferation of the postero-medial wall is more extensive ; 

 it begins more dorsally and, as the cavity narrows ventrally, it 

 extends round on to the postero-lateral wall, the two thickenings 

 forming one mass (text-fig. 8, pm.-\-pl.). The thickened area 

 is continued on to the extreme ventral wall from which a solid 

 mesodermal mass, the m. obliquus inferior, hangs down below 

 the cavity posterior to the optic cup and tapers at its lower 

 end. 



Text-figure 8. 



^^- ,.'-,■ 



'/V 



^ o> ^0<l 8 5 5 









v.o.h 





Tricliosurus viilpecula. G.L. 6 mm. (II. '01). 



Horizontal section through the head (S 2-4-8), passing through the dorsal region 

 of the optic vesicle (ojj.'y.) and the ventral portion of the left head-cavity (l.h.c.), 

 to show the now united postero-medial and postero-lateral proliferations from 

 the wall (pm. +pl.). X 110 and reduced by 5. 



a.c.a.=arteria cerebri anterior. i?'i?.=fore-hrain. 7ij9.= hypophysis. 

 ■y.o.i. = vena orbitalis inferior. 



At this stage for the first time, on the dorso-lateral region of 

 the cavity on both sides, the wall shows a distinct, though not 

 very extensive thickening which originates as a bud growing out 

 of the cavity (text-fig. 7, d.l). This bud is the primordium of 

 the m. rectus superior. 



The cavity is lined by a single layer of cells except where the 

 wall is proliferating. 



The abducens muscle complex has increased considerably in 

 size and is more distinct than in our earlier stages. The much 

 larger anterior portion, still connected on its lateral side with 



