314 



MISS E. A. FRASER ON THE DEVELOPMENT 



indicated in the 7 mm. and 6 mm. embryos, develops into the 

 future m. obliquns inferior. 



On the medial side of the primorditim of the m. obliqims 

 inferior at the lower end of the cavity is a darker patch of cells 

 (text-fig. 10, 7\inf.), the first indication of the m. rectus inferior, 

 which therefore apparently arises from the lateral side of the 

 postero-ventral wall just medial to the m. obliquus inferior. 



Text -figure 10. 

 a c a. 



yji-'^ "-"^ ', ,'" ' ' ' "'; \'. ,1s*^^ r D 



-op.c 



W^'/S' "'1, "K"'-'.^' » 



p. inf. \ ^ /, 



y.c.m., r.inf. Lh.c. 



TricJiosuriis vulpecula. G.L. 7'25 mm. (III. '01). 



Horizontal section through the head (S 5-1-2), passing through the optic cup {op.c.) 

 above the optic stalk and through the extreme ventral end of the left cavitj- 

 {IJi.c). to show the primordium of the m. obliquus inferior {o.inf.) and, just 

 medial to the latter, the region from which the m. rectus inferior {r.inf.) takes 

 its origin. X 110 and reduced by J. 



a.c.a. -^arteria cerebri anterior. ^5.=fore-brain. 7ij3.=hypophysis. 

 ■y.c.m.= vena capitis medialis. 



The muscle-bud from the dorso-lateral wall, representing the 

 primordium of the m. rectus superior, is less conspicuous than 

 the other proliferations. It is present on both sides, projecting 

 behind the dorsal part of the optic cup (text-fig. 9, dl.) ; on the 

 right side in (c) the bud is less distinct, probably owing to the 

 further thickening of its posterior wall and to the commencing 

 separation of the' dorso-lateral portion of the cavity from the 

 rest. 



A distinct epithelial layer can only be distinguished on the 

 side next the fore-brain. 



The abducens muscle-mass still consists of two portions. The 

 dorsal portion (text-fig. 11, r.l.) stretches still further back 

 on the medial side of the vena capitis medialis as a narrow 



