318 MISS E. A. ERASER ON THE DEVELOPMENT 



semicircle, and the eyelid has begun to develop as a fold of the 

 ectoderm on the ventral and posterior sides of the optic cup. 



The premandibular cavity is now represented by a . mass of 

 mesoderm in which are situated a few cleft-like spaces lying 

 obliquely on each side of the fore-brain (text-fig. 16, l.h.c). On 

 this side the cells have lost almost all traces of their former 

 epithelial character and the spaces are bounded by a thin 

 degenerating layer only a few cells thick ; the remaining meso- 

 dermal mass, except those parts which are giving rise to the 

 eye muscles, is also breaking down. 



Text-figure 14. 

 " . .^- r.s. 



c 



v-C.a. ,/ 





K 



.0« O' 



c'li-t 



V.O.I' 



THchosurvs vulpectda. G.L. 8"5 mm. ('97). 



Longitudinal section (S 7-1-8), showing tlie ciliary ganglion {cil.g.), the anterior 

 portion (r.e.) of the abducens muscle-mass and the primordmm of the m. rectus 

 superior (r.s.). X 110. 



s.a.=arteria cerebri anterior. ,9'.T'. = Gasserian ganglion. op.c.=optic cup. 

 '«.c.a.=V('na cerebralis anterior. v.c.m.^vewA capitis m^edialis. ?>.o.i.=veua 

 orbitalis inferior. I II. = oculomotor nerve. 



The dorso-lateral proliferation is now well marked and its 

 cells are undergoing distinct differentiation to form the future 

 m. rectus superior. It has moved dorsally and anteriorly as 

 compared with the last stage, and now lies medial to the dorsal 

 region of the bulbus (text-figs. 14-16, r.s.) and is showing signs 

 of becoming independent. 



