OF THE EYE MUSCLES IN MARSUPIALS. 



313 



and forms a hollow vesicle with thickened walls and a central 

 cavity. 



In (a) the premandibular head-cavity has attained its maximnm 

 size and, at its widest, has roughly the form in horizontal 

 section of a right-angled triangle (text-fig. 9, l.h.c.) measuring 

 •48 mm. transversely X "37 mm. antero-posteriorly X "23 mm. dorso- 

 ventrally. In (h) and (c) the cavity is smaller than in («), but is 

 still large, the diflerence being due mainly to a reduction in the 

 antero-posterior width, which here is only "12 mm. 



Text- figure 9. 





■r-j 



\^S. 





ay '•''%, I Ao.'Vi*- ^ 



/-.e. 



Tricliosimis vwlpecula. G.L. 7'2o mm. (III. '01). 



Hovizontal section througb the head (S 4-4-1), passing- through the dorsal region 

 of the optic vesicle (op.v.) and through the middle of the left head-cavity (l.h.c.). 

 The ventral portion of the proliferating bud on the dorso-lateral wall (dl.) and 

 the thickening on the posteio-medial wall (pm.) are shown, and also the 

 anterior portion (?'.e.) of the abduc-sns muscle-mass lying posterior to the 

 cavit}'. Between the latter and abducens muscle-mass runs the oculomotor 

 nerve (HI.). X 110 and reduced by j. 



o.c.t?. =arteria cerebri anterior. P-B.= fore-brain. 7ijj.= hypophysis. ^. F. = Gas- 

 serian ganglion. w.c.F. = Naso-ciliary branch of the trigeminal nerve. 

 v.c.ni.—vena capitis medialis. 



The thickening of the posterior wall is still more marked than 

 in the preceding stage and, joining with the j)rominent ventral 

 proliferation which now forms a conspicuous outgrowth (text- 

 ligs. 10, 12, & 13, o.inf.), extends out ventro-laterally to taper off 

 behind and somewhat below the eye. This portion, already 



