Albert C. Eyeleshymer and James Meredith Wilson 147 
still formed by a loosely scattered layer of entodermal cells which lie 
above the large yolk masses. 
The last section of this embryo, which we have represented in Fig. 47, 
is taken at the level of the last protovertebra. In this section we see 
that the neural keel of the earlier stages has taken on a cylindrical out- 
line and has acquired a large well defined lumen in which there are no 
traces of cell degeneration. Below the neural tube and in contact with 
it, is the large notochord, and between the notochord and the gut hypo- 
blast, is the sub-notochordal rod (h. ch.) which, from the character of its 
cells, seems to have arisen from the hypoblast of the gut. 
On either side of the notochord, are the large masses of mesoblast which 
form the last protovertebre. At this level, the mesoblast shows no line 
of division between its somatic and splanchnic portions. In that portion 
which must be considered as potentially somatic, there is a shght pro- 
liferation which gives rise to a more or less well defined rod (p. d.) 
which soon becomes the pronephric duct. 
Embryo About One Hundred and Five Hours After Fertilization. 
Embryo Surrounds 220°.—The embryo (Fig. 17) shows a marked ad- 
vance beyond the condition represented in Figs. 15 and 16. The di- 
visions of the brain are more distinct. The hind brain shows a decided 
thinning of its dorsal wall. In front of the anterior end of the fore 
brain there is a slight pocket followed by a projection or median knob. 
On either side of this knob, are the large adhesive organs which are now 
apparent in surface view. The large optic vesicles lie just behind, and 
now show the first beginnings of the lenses. On either side of the medulla 
the auditory vesicles are faintly shown. The pronephric ducts have ex- 
tended both anteriorly and posteriorly. The anterior portion of an em- 
bryo about five hours older is shown in Fig. 18. 
In this stage but few changes are noted beyond those described. ‘The 
lateral walls of the medulla are more widely separated and the roof has 
become thinner. The visceral arches and clefts are more pronounced. 
There is no trace as yet of the olfactory organs. As a result of the up- 
lifting of the embryo through growth, the adhesive organs have assumed 
an oblique position. 
A sagittal section, passing slightly to one side of the median plane of 
an embryo in this stage, is represented in Fig. 48. The lumen of the 
brain is enlarged and its subdivisions more clearly marked. The dorsal 
wall of the fore brain now shows a slight evagination which is the begin- 
ning of the epiphysis. Just opposite in the floor is another evagination 
which is the beginning of the infundibulum. Anteriorly the cavity nar- 
rows down in conformity with its external contour. Just in front of 
