310 



MISS E. A. FRASER ON THE DEVELOPMENT 



From the Gasserian ganglion arise a well-developed mandibular 

 nerve, a small maxillary branch and an oj)hthalmic iierve. 



The abdncens is here seen for the first time. It leaves the 

 mid-brain by many fine fibres which unite and run forwards 

 pa,rallel to the side of the brain for a short distance. 



Another example of this stage, which measures 7*5 mm. 

 (XIX. '04) ( = Stage IV.), presents almost the same features. 

 Although the sections are cut in the same direction, that is 

 horizontally through the head, the head-cavity in section is more 

 triangular and less elongated so that its area is larger than in 

 the 7 mm. embryo above described. The postero-medial and 

 postero-lateral proliferations form one thickening and the vena 

 cerebralis anterior appeal's to be only slightly sui-rounded by the 

 anterior poi'tion of the abducens muscle-mass. 



The conditions in this embryo, however, are difficult to make 

 out accurately. 



Embryo of G.L. 6 mm. (II. '01) ( = Stage Y.). 

 (Text-figs. 7 & 8.) 

 The cavity of the optic vesicle is still further reduced and the 

 lens is almost separated off from the ectoderm which is closing 

 over it. 



Text-fisfure 7. 







'^ pm. 'iK 



Trichosurus vulpecula. G.L. 6 mm. (II. '01). 



Horizontal section through the head (S 2-4-2), passing just above tlie middle of the 

 iioad-cavity of the right side {rJi.c), and showing the proliferating- bud on the 

 dorso-lateral wall {d.l.) and the thickening on the postero-mediarwall (p.m ). 

 1 he anterior portion of the abducens muscle-mass (r.e.) lies posterior to the 

 head-cavity and m front of the vena capitis uiedialis (v.c.m.). X IIC and 

 reduced by j. 



a.e.ff.=artcria cerebri anterior. :F_B.=fore-braiii. /;/>. = hypophysis. 

 oj}.v. = optk vesicle. 



