308 



MISS E. A. FRASER ON THE DEVELOPMENT 



and is now composed of several layers of cells (text-fig. 4, p.m.) ; 

 immediately ventral to this and on the postero-lateral side occurs 

 a much more extensive proliferation, which extends down as far 

 as the end of the cavity but does not involve the extreme ventral 

 wall (text-fig. 5, p.L). At its lower end, however, a part of this 

 thickening stretches out laterally behind the bulbus and forms 

 the first indication of the m. obliquus inferior (text-fig. 5, o.hif.). 

 On the left side, the two proliferations appear to be separate 

 from each other, but on the right the more dorsal one extends 

 round at its lower end to join the larger ventral. Except where 

 proliferation is proceeding the wall of the cav^ity is very thin ; 

 on the side next the brain it consists of a single but perfectly 

 definite epithelial layer, but on the side next the optic cup it is 

 much less definite (text-figs. 4 & 5). 



Text-figure 5. 



a.c.a 



Triehosurus vulpecula. G.L. 7 mm. (a '97). 



Horizontal section through the head (S 3-3-9), passing through the dorsal portion 

 of the optic vesicle {op.v.) and the ventral portion of the head-cavities {l.h.c. & 

 r.li.c.), to show the proliferation from the postero-lateral wall {p-l.)- The 

 primordium of m. obliquus inferior (p.inf.) is seen on the left side. X 75 and 

 reduced by \. 



<z.c.a.=arteria cerebri anterior. J'jB.= fore-brain. /ij9. = hypophysis. 

 j3A.=pharynx. ■y,ci.i. = vena orbitalis inferior. 



The primordium of the abducens muscle-mass consists of a 

 postero-dorsal portion which lies along the medial side of the 

 vena capitis medialis (text-fig. 6, r.h.) and which, at its anterior 

 end, runs ventrally outwards to join with the larger anterior 

 portion of the mass (text-fig. 6, r.e.). The latter, whose dorsal 

 side surrounds the root of the vena cerebi-alis anterior, lies 

 anterior to the vena capitis medialis and extends half-way down 



