312 MISS E. A. FRASER ON THE DEVELOPMENT 



the intermediate mass, lies very close behind the head-cavity and 

 in front of the Gasserian ganglion, being separated from the 

 latter by the vena orbitalis inferior (text-tig. 7, r.e.) ; it extends 

 down along the upper two-thirds oi^ the cavity to the region 

 where the postero-lateral proliferation begins. The smaller 

 postero- dorsal portion runs back as before on the medial side of 

 the vena capitis medialis. 



The pi'imordium of m. obliquus superior has now grown further 

 forwards from the intermediate mass and reaches antei'iorly 

 beyond the distal end of the ophthalmic branch of the trigeminal 

 nerve, on the outer side of which it lies. The connection of the 

 intermediate mass with the maxillo-mandibular mesenchyme is 

 most probably present but could not definitely be made out in 

 this embryo; the mass is again joined with the dorso-lateral 

 corner of the cavitj^ for about three sections. 



The oculomotor nerve is stronger and, running down ventrally 

 on the medial side of the vena capitis medialis, now reaches the 

 head-cavity and appears as a small nerve between the wall of tJie 

 latter and the anterior portion of the abducens muscle-mass ; its 

 distal end soon disappears and can only be followed for a short 

 distance alongside the cavity. 



The abducens nerve is now well developed and runs forwards 

 on the inner side of the Gasserian ganglion, but it is not veiy 

 much longer than at the last stage. 



Embryo of G.L. 7-75 mm. (XX. '04) ( = Stage YI.). 



As regards the development of the eye and the muscle-iu asses 

 this stage is very similar to the last, and only a few observations 

 need be made upon it. 



The optic vesicle is not so flattened as at 6 mm. and the lens 

 is still joined with the ectoderm. 



The premandibular head-cavity is smaller and pear-shaped, its 

 wider side lying next the eye, and the posterior proliferations 

 are more extensive though the thickening on the ventral wall 

 does not appear to be so marked. No dorso-lateral proliferation 

 was recognised. 



The abducens nerve now runs into the pointed posterior end 

 of the dorsal portion of the abducens muscle-mass. 



Embryo of G.L. 7-25 mm. (^Stages VII. & VIII. a & h). 

 (Text-figs. 9-13.) 



In this next stage there are three embryos, a (III. '01), 

 5 (XII A. '02) and c (XII. '02), each measuring 7*25 mm. at its 

 greatest length. Although in many particulars (ci) is the youngest 

 of the three and (c) is slightly older than (6), in respect of the 

 eye-mnscle primordia very little difference exists between them. 



The invagination of the optic vesicle has advanced still further, 

 the cavity being reduced to a narrow slit, but the oj^tic stalk still 

 contains a wide lumen. The lens is closed off from the ectoderm 



