338 



MISS E. A. FRASER ON THE DEVELOPMENT 



intermediate mass is also united with tiie lateral side of the 

 primordium of the m. rectus superior, thus recalling the con- 

 nection of the intermediate mass with the postero-lateval wall of 

 the head-cavity in other marsupials. The central region of the 

 cell group is compact and as one passes further ventrally, below 

 the section figured, the primordium of the m. obliquus inferior 

 is seen to grow out downwards from this region, its approximate 

 position being marked in the diagram by a broken circle 

 (text-fig. 26). In still younger embryos it is possible that 

 vestiges of a cavity may be present, but if so they becoxne 

 early obliterated. 



Text-fio'ui^e 26. 



^•ca. 



Didelphys marsupialis. G.L. 8'5 mm. 



Horizontal section tliyoiigh the bead (S 2-4^11), showing tlie mass of mesenchyme 

 dorsal to the optic cup, from which the eye muscles are developing. The broken 

 circle indicates approximately the area from which, further ventrally, the 

 primordium of m. rectus inferior grows out. X 101) and reduced by i. 



a.c.a. = arteria cerebri anterior. FB. = fore-brain, int.m. = intermediate mass- 

 es. = m. obliquus superior, r.e. = anterior portion of abducens muscle-mass. 

 r.s.==m. rectus superior. i).e.a. = vena ' cerebrab's anterior, v.c.m. = veua. 

 capitis medialis. v.o.i. = vena orbitalis inferior. III. = oculomotor nerve. 



The conditions in Didelphys are interesting, as they may 

 possibly form a clue to the more accurate identification of the 

 group of mesenchyme cells from which the eye muscles arise in 

 higher mammals. Renter ('97), as already mentioned, regards 

 the eye muscles in the pig as originating from a single mass 

 of mesenchyme cells. In the description of his earliest stage 

 (G.L. 10 mm.) he says (p. 384) :— " Sie [die allererste Anlage 

 der Augenmuskeln] hat die Form einer gestielten Sichel und 

 umgreift mit den beiden nach vorn gerichteten Schenkeln den 

 Augenstiel, wahrend der dritte hintere Schenkel vom JST, abdu- 

 cens fortgesetzt wird. Die Spitze des oberen Schenkels bildet 



