482 REIGHARD—PHELPS. [Vor. XIX. 
of the processes which have led up to these conditions. At 
the time of hatching the embryo® is about seven millimeters 
long (Plate, Fig. 4). The most conspicuous change that 
has occurred in the brain is the great increase in the size 
of the midbrain and its division into two lobes. No gill 
slits are visible externally. The prehyoidean slits have dis- 
appeared and the hyoid arches now extend backward as nar- 
row opercula so as to cover the second pair of gill slits and 
all others that have formed behind it. The pigment cells 
which are a conspicuous feature of this stage lie thickest in 
the region of the auditory vesicles. They extend for a short 
distance along the lateral lines posteriorly, and arch over the 
optic vesicles anteriorly. No pigment is observed on the 
adhesive organ or in its vicinity. The stomodaeum and 
pharynx are in communication. 
The adhesive organ at this time consists of two sausage 
or U-shaped ridges which have the same position as in stage 
four (Plate, Fig. 3), Each sidge is made up lof a semies 
of six to ten cups whose cavities open to the surface so that 
they are seen from the exterior as pits. The dorsal ends 
of the ridges touch each other in the median plane while the 
ventral ends, still wide apart, abut on the roof of the stomo- 
daeum, and thus help to form the dorsal border of the mouth 
opening. Indeed the ventral ends of the ridges run back 
along the roof of the mouth for a short distance. The planes 
of the ridges remain at right angles to the frontal plane as 
in stage four. Sections of the embryo at the hatching stage 
show that the lumen of the cesophagus is obliterated and that 
there is no longer any communication between the adhesive 
organ and the foregut. A longitudinal section taken just 
at one side of the median plane cuts through both limbs 
of one of the curved ridges of the adhesive organ. If the 
section falls through the center of one of the cups, a pit-like 
cavity appears which opens at the surface and is surrounded 
by thick walls made of a single layer of exceedingly high 
Por a lateral view of an embryo in this stage, see Beckwith, 1907, 
1 Po) WO) Eel a eb hae 
