OF THE EYE MUSCLES IN MARSUPIALS. 



305 



rectangular in cross-section in (a) (text-fig. 1, Z.A.c), where on tlie 

 left side it measures -26 mm. in a dorso-ventral direction, -31 mm. 

 antero-posteriorly and -32 mm. transversely ; but it is more oval 

 in (h), and ta.pers somewhat towards the eye (text-fig. 2, r.h.c). On 

 both sides in (a) a small medial portion is marked off from the 

 main body of the cavity by a constriction, the upper part of which 

 can just be seen in text-fig. 1, n. No such condition is present in 

 (b) or (c), although on the left side of the former there is an 

 incomplete partition across the middle of the cavity ; the division 

 in this case, however, may not have the same significance. 



Text-figure 2. 



Md. 



r.h.c 



HB\ 



Trichosurus vulpecula, G.L. 5 mm. (lA. '01). 



Longitudinal section (S 2-1-13) passing thvougli the premandibular head-cavity 

 of tbe right side. X 75 and reduced by \. 



J'5.=fore-brain. 7f5.=hind-brain. JlfJ?.=mid-brain. o^.y.=optic vesicle, 

 r./i.c. =riglit head-cavity. v.c.m.=vei\a. capitis medialis. 



In embryo (c), lying obliquely along the postero-lateral wall of 

 the cavity, on the inner side of the vena capitis medialis, is a 

 slightly more condensed portion of the mesenchyme, representing 

 the third or hyoidean somite and constituting the primordium of 

 the abducens muscle-mass. It runs forwards and laterally just 

 below the origin of the vena cerebralis anterior into another mass 

 of loosely connected cells, which extends downwards round the 

 lateral side of the vena capitis medialis to become united below 



