Albert C. Eycleshymer and James Meredith Wilson 145 
marked enlargement which forms the basis of the anterior portion of 
the medulla. In this region, as Keibel has pointed out, the anlage of 
the otic vesicles will later appear. The darkened zone around the anterior 
end of the embryo represents the extent of the mesoblast. Posteriorly 
the lateral boundaries of the mesoblast are poorly defined so that in sur- 
face views it is impossible to indicate them. 
Embryo Ninety Hours After Fertilization. Embryo Covers About 
180°.—Figs. 15 and 16 represent the anterior and posterior portions of 
an embryo in this stage. Many striking changes have occurred. The 
subdivisions of the brain are more clearly defined and are more promi- 
nent. The optic vesicles are better defined. The precerebral portion 
extends forward as a distinct process. The first visceral arch has formed 
and, just behind it, is the first visceral cleft. The protovertebre have 
increased to sixteen or more pairs. The pronephric ducts have extended 
both anteriorly and posteriorly. There are at this time, however, no ex- 
ternal indications of the olfactory, auditory or adhesive organs. 
Fig. 42 represents a section passing close to the median sagittal plane 
of an embryo slightly younger than that shown in Fig. 17. The super- 
ficial ectoblast (s. ec.) consists of two layers of cells which are invagi- 
nated at a point lying between the anterior margin of the fore brain 
(f. b.) and the median portion of the adhesive organs. 
The brain cavities are now well defined. There is, however, as yet no 
indication of the infundibular or epiphysial evaginations. The noto- 
chord extends nearly to the level of the middle portion of the brain, as 
shown in the figure. The gut cavity is well defined beneath the posterior 
portion of the brain; it is greatly reduced in size anteriorly. After reach- 
ing the level of the epiblastic invagination described above, it again ex- 
pands into a wide cavity (g. d.). The walls of the gut show little change 
until the head region is passed when the dorsal wall is greatly thickened 
to form the beginnings of adhesive organs (a. 0.). 
A transverse section through the extreme anterior end of the brain is 
represented in Fig. 43. The superficial ectoblast shows no modification 
in this section. Just beneath this layer the deep epiblast extends over 
the surface, but in the median line it is lost in the mass of cells which 
are radially disposed and which represent the anterior end of the fore 
brain. Below the fore brain is a wide layer of mesoblast (mes.) which 
extends upward on each side. On either side of the median line the fore 
gut is greatly expanded. The layer of columnar cells covering these ex- 
panded portions, even at this early stage, is different from that forming 
the dorsal wall of the gut in other portions of the body. On either side 
the hypoblast extends peripherally, its cells take on the cuboidal form, 
