OTIC AND OPTIC PRIMORDIA IN MAN 219 
division is wider, the optic sulcus has deepened so that the optic 
anlage is sharply delimited on all sides from the rest of the area. 
Mesial and rostral to it cells are migrating out from the primor- 
—_> 
Fig. 10 A photomicrograph of the seventeenth section of the twelve-somite 
embryo H 197 (Univ. of California). % 100. On the right the section passes 
near the middle of the optic anlage (op.sul.); on the left through its rostral end 
(op.prim.). Medial to the optic evagination the thickened neural fold is pro- 
liferating mesectoderm (mesec.). The section passes horizontally through the 
heart. pe.c., pericardial cavity; am., amnion; ph., pharynx; y.s., yolk sac. 
dium as mesectoderm. Caudally the primordium continues 
without a break into the hindbrain where, as figure 8 (mesec.) 
shows, the mesectoderm is clearly differentiated from the rest 
of the mesenchyme by its deeper stain. The optic sulcus appears 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 2 
