388 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
is merely a layer of cells belonging to t-lre “ mesoblast,” and clothed with the epiblastic 
cells of the epidermis ; dura-mater, skull- wall, skin — all these are, as yet, one stratum. 
This stratum, however, is thickening, and becoming differentiated into secondary 
laminae in the floor of the head. One remarkable fact is that this floor is imperfect at 
one part. Where the fore and hind brain meet below and behind the mid brain, there 
we see a large circular fontanelle, with a clear margin. Through this gap the brain 
can be clearly seen. 
In the gap, however, there is a remarkable structure, the rudiment of the pituitary 
body, which is either formed by an involution of the buccal cavity (epiblast), or cut off 
from the fore-gut (hypoblast). The latter is the view taken by Wilhelm Muller; 
the former by Goette. (See Foster and Balfour, pp. 91-93.) 
Where this remarkable structure passes into the cranial cavity, there the epiblast 
and hypoblast unite ; there the axial notochord ends, and, as on a pivot, the skull first 
bends itself into a hook and then gradually straightens itself again. 
On each side of the cranial notochord (Plate 27, fig. 2, and Plate 29, fig. 1, nc., iv.) 
the floor of the hind brain is thickening to form the investing mass or parachordal 
tracts. The innermost cells of the stratum ultimately become cartilage. 
These bands, however, which correspond to a vertebral tract, although not seg- 
mented, pass forwards on each side of the basal opening or pituitary space ; these 
'prochordal bands are the trabeculae cranii (tr.). 
These are the first, and at present the only, rudiments of skeletal structure in this 
membrano-cranium ; the fore part of the floor, the roof and side walls, are at present 
merely a single cellular lamina (Plate 29, fig. 1). 
But each division of the brain has on each side of it the rudiment of what will soon 
become a highly complex sense-capsule. 
The lowermost swelling of the brain, the rudiment of the hemispheres, has on each 
side, a large, shallow, inverted cup, whose lower margin is sinuous and lobate (Plate 27, 
figs. 1, 2, ol.) ; these rudiments of the nasal sacs grow by their base to the skull wall, 
and look like cup-shaped Fungi ( Stereum ). 
Down on each side, in the hollow between the fore and mid brain, the young eye- 
ball is seen (fig. 1, e.) ; it is an imperfect oval, the reduplication which makes it being 
unclosed below ; the notch widens into a triangle above, and shows the crystalline lens 
as a small seed-like body. 
The forepart of the eyeball comes very near the olfactory cup, and is far from the 
third sense- capsule (or ear) ; its hind part rests against a visceral fold in front of the 
commencing mouth (m.). 
The rudiment of the ear is like a depressed gourd, whose short neck is somewhat 
constricted from the bulb ; this sac lies on each side of the hind brain, behind its 
middle, and is more than its own longest diameter from the eye. 
This oval sac is encrusted with crowded mesoblastic cells below and at the sides, 
but a large oval space above is only covered with a thin gelatinous layer. (For a 
