J. L. Bremer 465 
smooth, except in the anterior portion, where there are distinct traces 
of neuromeres. ‘Toward the cord the cavity is compressed laterally, but 
there is no sign of a groove between the zones of His. 
Fig. 6 shows the wide open anterior neuropore, through which one 
can look into the hollow optic vesicles, and into the median cavity of 
the fore brain, which is seen to be a narrow slit, compressed laterally. 
The edges of the opening are broadly curved or rolling, with no demarca- 
tion to indicate where the future line of closure is to be. This is shown 
in section in Fig. 5. 
MODEL OF PHARYNX AND AORTIC ARCHES. 
The pharynx, of which the model is a cast, the shape of the cavity 
being represented, is shown in Fig. 1, in its proper relation to the brain. 
It consists of a broad body opening anteriorly into the mouth, the lateral 
extension of which, between the maxillary and mandibular processes, is 
represented as a cut surface. On the dorsal surface of this body, near 
the fore brain, are irregularities, chiefly two rounded ridges, one on each 
side of the median line, representing the out-pocketings for the pharyn- 
geal lobes of the hypophysis, as shown in Fig. 3, and explained above. 
Behind these, at the angle made by the roof of the pharynx correspond- 
ing to the curve made by the brain at the head bend, is a median ridge 
or point which is continuous with the notochord, and marks the anterior 
end of the latter. Toward the cesophagus, into which the body of the 
pharynx merges, the cavity becomes more and more compressed laterally, 
until it is an antero-posterior slit. From the sides of this main cavity 
three lateral out-pocketings project, the most dorsal one being divided 
into two smaller projections near the end. These represent the gill 
pouches, and diminish in size from before backward. The first two 
rapidly become compressed antero-posteriorly and end in blunt, more or 
less vertical ridges, while the third is more tapering, and the fourth ends 
in two distinct pointed branches. 
In the tissue between the opening of the mouth and the first gill 
pouch, and also between the first and second, and second and third gill 
pouches, run the first, second, and third aortic arches on each side, joined 
dorsally by the dorsal aortee, which make an impression on either side 
of the median line of the body of the pharynx, and then continue down- 
ward, beside and behind the esophagus. On the model, the aortic 
bulb is represented as cut off just above the standard; a cup-shaped 
chamber leads from this forward, and gives off on each side two vessels ; 
one pair of vessels forms the first aortic arches, a second pair divides 
34 ; 
