464 Description of a 4-mm. Human Embryo 
Between this anterior part of the fore brain and the mid brain, not 
marked distinctly from either, is found a narrow strip of less caliber 
than the anterior portion, with no special prominence and no connec- 
tion with the ectoderm of the skin. A cross section of this is shown in 
Fig. 2. 
The mid brain, oceupying the head bend, is also without special in- 
terest, except that it is of smaller caliber than might be expected. It 
merges without line of demarcation into the hind brain. The hind brain 
is again divisible into two parts; one posterior, smooth, of nearly even 
caliber, merging gradually at the neck bend into the spinal cord; and 
one anterior, showing a further development. The roof of this anterior 
part is thin, has become shrunken and crumpled. As yet, however, there 
is no sign of a Varolian bend. Attached to the side, ventrally, are two 
Lt.um.art. 
Fig. 8. Section through body of embryo, near the cloaca. X 60. 
ganglia, one for the Trigeminal nerve, and one for the Acoustico-facial 
complex, in which there is only a faint indication of separation into 
distinct nerves. Behind this second ganglion, applied closely to it, hes 
the otocyst, a rounded hollow vesicle, close to the brain, but not touching 
it, and still attached externally to the ectoderm, a patch of which can 
be seen on its outer surface. Of the other cranial ganglia nothing is 
shown in the model, as they are represented only by diffuse groups of cells 
on each side of the hind brain, not yet divided into separate ganglia, 
and not attached to the brain wall. There is a peculiar notch in the 
floor of this part of the hind brain, shown in Fig. 1, between the two 
ganglia, bounded by two rounded prominences seen better in Fig. 6, 
the significance of which I do not know. 
On taking off the top part of the model, we find that the cavity of 
the hind brain is marked by a median ventral furrow. The sides are 
