400 Julia B. Platt 
bed, although sufficiently reduced to distinguish the ectoderm sharply 
from both endoderm and mesoderm. Near the upper margin of the 
figure, a row of deep ectoderm cells are pushing their way inwards 
to form the posterior (ventral) lining of the buccal cavity. Between 
these ectoderm cells and those likewise pushing inwards to form 
the anterior (dorsal) epithelium of the mouth, a thin layer of endo- 
derm extends to the surface of the head. To the layer of endoderm 
thus enclosed, the yolk granules which bound the upper surface of 
the figure bear witness. The line where the ectodermic cells of the 
epithelium lining the mouth meet the endodermie cells of the branch- 
ial cavity is sharply marked by yolk differentiation. The yolk-laden 
tissue at the bottom of the figure is from the wall of the pericar- 
dium, and is continuous with the mesothelium of the mandibular 
arch, which occupies the centre of the figure. Between the wall 
of the pericardium and the endoderm of the mouth one or two 
mesenchymal cells are found. 
It will be noticed that a few yolk granules are scattered through 
the mesectoderm, the tissue which nearly surrounds the mesothelium 
of the mandibular arch. It is possible that these granules belong 
to cells which have not yet absorbed their primitive supply of yolk, 
but it is more probable that the granules have been washed onto 
the mesectoderm from a neighboring tissue, since large yolk granules 
are apt to float off when the slide is possed through aqueous solu- 
tions, and may then become again attached to the slide as it is 
withdrawn from the vessel in which the solution is contained. 
However this may be, the difference between ectoderm and endoderm 
is not more sharply marked than that between mesoderm and mesecto- 
derm. It should be recalled that the yolk differentiation becomes 
evident when the mesectodermic cells are still at that stage of 
development in which they are recognized, described, and figured, 
by every embryologist who presents a study of the development 
of the Vertebrate nervous system, as composing a tissue of eetodermie 
origin, the Anlage of the cranial ganglia. From that earlier stage 
until the present, the sharp yolk distinction between the two kinds 
of median tissues is always found. After the present stage, however, 
the yolk differentiation of the median tissues rapidly disappears through 
the reduction of the yolk granules contained in the mesoderm. 
The first series of figures ends with fig. 1 7. The branchial 
region has nearly passed out of .section, and with it the region im 
which the trabeculae first appear. After a few sections, the right 
