OTIC AND OPTIC PRIMORDIA IN MAN 205 
whole rostrocaudal extent of the otic plate. In most sections it 
fades off gradually into the surrounding ectoderm so that its 
limits are difficult to determine. 
Both the ganglion and the otic plate are present in the six- 
somite ‘Klb.’ of Keibel, but our tracings are not sufficiently 
detailed to permit of an accurate description. At the time they 
were made the presence of the anlagen was not suspected. They 
can be found in the eight-somite embryo first described by Dandy 
(10) (Mall, No. 391). Figure 2a is taken from a section through 
the middle of the otic plate of this embryo and shows the gan- 
glionic primordium clearly on the right side. This appears as an 
outpouching of the neural fold with the characteristic cap of 
overlying ectoderm. 
The ‘otic sulcus’ is manifest in a model of this region made at 
400 diameters. It is a broad shallow pit near the dorsal edge 
of the fold extending through four of the 10 uw sections. In this 
case also it belongs to the second hindbrain segment and lies 
just caudal to the first visceral pouch.! 
The depression to the left in the figure is not the beginning 
of the otic pit, but a chance wrinkling of the ectoderm. 
With the illustrations at hand it will be easier to visualize these 
relations in the other eight-somite embryo, H87. Figure 3 
represents the dorsal aspect of a model and figure 4 is from a 
projection reconstruction of the embryo cut in the mid-sagittal 
plane and viewed from the right. The sensory anlagen are 
plotted in from a detailed study of the sections and indicated by 
stippling. In the former figure we see the broad expanse of 
forebrain, still in the neural-plate stage. The deep neural groove 
has progressed as far forward as the midbrain, behind which are 
two large hindbrain subdivisions. From the second of these 
the acousticofacial ganglion is arising and laterally in the ectoderm 
is the otic plate reaching back as far as the third hindbrain seg- 
ment. The primary brain segments are better shown in figure 4. 
1 Manifestly our interpretation of the nervous system of this embryo does not 
agree with that of Dandy (’10). Having identified the cranial flexure, the otic 
plate and its associated ganglion throughout our series, it has been possible to 
interpret correctly the subdivisions of the neural folds. What he has termed the 
second brain vesicle is in reality the second division of the hindbrain. 
