OTIC AND OPTIC PRIMORDIA IN MAN 229 
' fig. 24) represents a dorsal view of a three-somite specimen, of 
which the rostral end of the neural plate resembles that of H87 
(eight somites). It shows two well-marked converging grooves 
which may well be the optic sulci. The thirteen-to-fourteen- 
somite embryo of Semnopithecus (‘Wa,’ Selenka, ’03, figs. 11 
and 12) has large optic vesicles, but no otic plate is indicated. 
There is a deep otic pit in the twenty-three somite Cercocebus 
embryo, ‘Cc,’ however. 
From the Normentafeln of Tarsius (Huprecht and Keibel, 
07) it would seem that the eight-somite embryo (figs. 8 and 9) 
had optic anlagen somewhat further advanced than those of 
our eight-somite embryo H87 (fig. 9a). The youngest stage 
in which the otic plate could be recognized is that of twelve 
somites (table 5), where the neural folds were closed in the region 
of the optic vesicles. Here, then, as in most other forms, the 
optic primordia precede the otic in development. 
The scanty and inaccurate data of this survey emphasize the 
need of detailed studies of early mammalian embryos both on 
their own account and for the light they will throw on human 
development. 
CONCLUSIONS 
1. The earliest sensory anlage in man is the otic plate which 
can be recognized in an embryo of two to three somites as a 
diffuse thickening of ectoderm in the hindbrain region. A 
four-somite embryo shows the beginning of the associated acous- 
ticofacial ganglion, though its fate is not yet completely known. 
2. The ganglion arises near, but not exactly at the dorsal 
edge of the open neural fold and the outermost part of the appar- 
ent evagination delaminates from the fold before the process of 
tube formation is completed. It is clearly derived from the 
wall of the definitive neural tube. 
3. The otic epithelium differentiates by an elongation of the 
distal ends of the cells and the appearance of a brush border. 
Between ten- and twelve-somite stages invagination begins 
and there is a deep otic pit at sixteen somites. 
