ingling of
protoplasm to divide. Such a type of segmentation in which only part
of the ovum segments is called meroblastic. If we compare this with
the typical blastosphere of the lower type, we see that it is, as it
were, flattened out on the yolk. This stage is shown in section in the
lower figure of Figure 1. b.d., the blastoderm, is from this point of
view, a part of the ripped and flattened blastosphere, spread out on
the yolk; s.c. is the segmentation cavity, and y. the yolk.

Section 25. There is no open invagination of an archenteron in the
fowl, as in the frog--, the gastrula, like the blastosphere, stage is
also masked. But, in the hinder region of the germinal area, a thick
mass of cells, grows inward and forward, and, appearing in the
dorsal view of the egg as a white streak, is called the primitive
streak (p.s.). By a comparison of the figures of frog and fowl the
student will easily perceive the complete correspondence of the
position of this with the blastopore of the frog. The relation of the two
will be easily understood if we compare the fowl's archenteron to a
glove-finger under pressure-- its cavity is obliterated-- and the frog's
to the glove-finger blown out. The tension of the protoplasm,
straining over the enormous yolk, answers to the pressure. The
gastrula in the fowl is solid. The primitive streak is, in fact, the scar
of a closed blastopore. As we should expect from this view of its
homology, at the primitive streak, the three embryonic layers are
continuous and indistinguishable (Figure 2). Elsewhere in the
blastoderm they are distinctly separate. Just as the yolk cells of the
frog form the ventral wall of the intestine, so nuclei appear along the
upper side of the yolk of the fowl, where some protoplasm still
exists, and give rise to the ventral hypoblastic cells. By conceiving a
gradually increasing amount of yolk in the hypoblastic cells in the
ventral side of the archenteron, the substantial identity of the
gastrula stage in the three types, which at first appear so strikingly
different, will be perceived. Carry Figures 4 and 5 of the frog one
step further by increasing the size of the shaded yolk and leaving it
unsegmented, and instead of ar. in 5 show a solid mass of cells,
and the condition of things in the fowl would at once be rendered.

Section 26. Figure 3a of the fowl will conveniently serve for
comparison with Figure 7 of the frog. The inturning of the medullary
groove is