entirely similar in the two cases. The mesoblast appears as
solid mesoblastic somites. In the section above Figure 4 this layer is
shown as having split into somatopleur (so.) and splanchnopleur
(spch.). Figure 3 answers to Figure 6 of the frog, and Figure 4 is a
later stage, in which the medullary groove is beginning to close at its
middle part. The clear club-shaped area around the embryo (a.p.) is
the area pellucida; the larger area without this is the area opaca
(a.o.), in which the first bloodvessels arise by a running together and
a specialization of cells. The entire germinal area grows steadily at its
edges to creep over and enclose the yolk.

Section 27. So far, the essential differences between the development
of fowl and frog, the meroblastic segmentation, absence of a typical
gastrula, and the primitive streak, seem comprehensible on the theory
that such differences are due to the presence of an enormous amount
of yolk. Another difference that appears later is that, while the tadpole
has an efficient pronephros, the fowl, which has no larval (free
imperfect) stages in its life history, has the merest indication of such
a structure.

Section 28. Another striking contrast, due to, or connected with, this
plethora of yolk, is the differentiation of a yolk sac (= umbilical
vesicle) and the development of two new structures, the amnion and
allantois, in the fowl. If the student will compare Figure 10 of the frog,
he will see that the developing tadpole encloses in its abdomen all the
yolk provided for it. This is a physical impossibility in the fowl. In the
fowl (Figure 2, Sheet 24) the enormous yolk (Y.) lies outside of the
embryo, and, as the cells of the germinal area grow slowly over
it, umbilical bloodvessels are developed to absorb and carry the
material to the embryo. In the case of an embryo sinking in upon, as
it absorbs, this mass of nutritive material, a necessity for some
respiratory structure is evident. From the hinder end of the fowl's
intestine, in a position corresponding to the so-called, urinary bladder
of the frog, a solid outgrowth, the allantois, which speedily becomes
hollow, appears. Early stages are shown in Figures 1 and 2, Sheet
24 (al.); while the same thing is shown more diagrammatically on
Sheet 23, Figure 6 (all.). This becomes at last a great hollow sac,
which is applied closely to the porous shell, and the extent of which
will be appreciated by looking at Figure 5, Sheet 24,