484 REIGHARD—PHELPS. iWon, akc 
one of the vesicles, while the cells composing the walls of 
the original diverticulum now form the walls of the vesicles. 
Six to ten such vesicles are formed on each side. 
Second—Subsequently the cavities of the vesicles shift their 
position so that they come to lie excentric, close against the 
ectoderm of the snout. The vesicles are thus converted into 
cups whose mouths are still closed by the ectoderm (Fig. F). 
Third—At the time of hatching, or just previous to it, the 
ectoderm stretched over the mouths of the cups disappears 
and their cavities are put into communication with the ex- 
terior. The edges of the cups become thus continuous with 
the adjacent ectoblast and the cups themselves have the ap- 
pearance of being thickenings of the ectoblast. At first the 
cavities are ovoid or spherical, later they become shallower 
and their openings wider. 
The processes described above as converting the beaded 
diverticula of stage four into the open cups of the hatching 
stage bring the organ to its functional condition, and the 
secretion of mucus enables the young Amia to adhere to 
objects with which it may come into contact. 
b. The Mesoblast and Head Cavities. The head cavities 
are now greatly enlarged and have the form of large sacs 
which lie in the triangular space between the tweenbrain, optic 
vesicle and foregut and conform to it. The cord of cells con- 
necting them is converted into a tube with a wide lumen. It 
lies in the saddle cleft dorsal to the infundibulum. The walls 
of the cavities are of cubical or slightly flattened cells. At 
one point they are greatly thickened and connected with the 
third nerve. 
These head cavities from their position, their connection 
across the middle line by a tube which lies at the front end 
of the notochord and by their relation to the third nerve are 
clearly equivalent to the premandibular head cavities of elas- 
mobranchs, reptiles and birds. Their further history has not 
been traced, but it is to be expected that they give rise to the 
oculomotor muscles as in other vertebrates. They are formed 
in the sheet of mesoblast which is delaminated from the 
