OTIC AND OPTIC PRIMORDIA IN MAN 207 
Here we find the summit of the cranial flexure marked by a 
boss, the midbrain, m.b., limited fore and aft by slight constric- 
tions of the neural folds. Caudal to it the two hindbrain sub- 
divisions are obvious. Cross-section pictures of the otic region 
mmm nelG. TAC, 
GANG. ; “ 
Fig. 3 A geometric projection of the head end of a model of H87 (Univ. of 
Chicago Coll.), drawn by J. F. Didusch and reduced to 100 diameters. Upon it 
the otic and optic-crest primordia are plotted and represented by stippling. The 
densest areas indicate the position of sulei; ac. fac. gang., acousticofacial ganglion; 
cr. sul., neural-crest sulcus corresponding in this case to the position of the future 
Vth ganglion; f.b., forebrain; h.b., hindbrain; m.b., midbrain; op.cr.prim., optic- 
crest primordium; ot.pl., otic plate; som.J, first somite. 
are given in figures 5 and 2b which are taken from successive 
sections, the level of which is indicated in the two general views. 
The photomicrograph (fig. 5) brings out clearly the apparent 
evagination of the neural fold at the site of the ganglionic an- 
age, an earlier stage of which we saw in the eight-somite embryo 
