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DEVELOrMENT OF THE BLASTODERM. 555
epiblast—now composed of one or more layers of cells—^is seen to have
separated from the mass of proliferated cells which lie beneath it. It
is at this juncture that the viesoblast may be said to have assumed a
separate entity.
The mesoblast is composed, as we have seen, of cells derived from
both hypoblast and epiblast, but chiefly from the latter. About the
twentieth hour, if the cells of the mesoblast are examined with a high
power (Zeiss, ^^ oil im.), it will be seen that they are stellate in form.
They take the stain similarly to the cells of the epiblast, and it is only
in carefully-prepared specimens—studied with high powers—that we are
able to detect any difference. They lie in a bed of protoplasm, and it
is from this fluid—with which they are constantly bathed—that they
derive their nourishment. They are simply nucleated structures, each
containing numerous granular particles. They are the bioplasts of Beale,
the nuclei of the future cells of the connective-tissue group. They have
no cell-body, and consequently no cell-limit or wall. As they advance
in age they gradually accumulate around themselves formed material,
probably the undigested or unassimilated portion of the surrounding
protoplasm. They thus assume distinctive and characteristic forms.
Their processes gradually become thicker and more pronounced, so as
to be visible even with low powers. The changes in the cells can now
Fro. 302.
Porcine Embryo {'2J cm. X 250) : c.t, embryonic connective tissue of mesoblast.
be seen in the lower jaw of a fcjetal pig two and a half centimeters in
length (Fig. 302).
In the development of the oral cavity and associate parts we do not
have to do with the hypoblastic layer of the blastoderm, so we M'ill
hereafter confine ourselves to the head of the embryo.
The epiblast in an embryo pig 1 cm. in length is composed of one or
more layers of nuclei or bioplasts lying in a bed of protoplasm. With
low powers they appear in no manner different from the underlying
mesoblastic cells, except that the nuclei are closer together ; and in
sections stained with hfemotoxylon and eosin assume a darker hue than
those of the mesoblast. The epiblast now constitutes the " infant
layer of the epithelium, being the deepest layer of the rete Malpighii.
DEVELOrMENT OF THE BLASTODERM. 555
epiblast—now composed of one or more layers of cells—^is seen to have
separated from the mass of proliferated cells which lie beneath it. It
is at this juncture that the viesoblast may be said to have assumed a
separate entity.
The mesoblast is composed, as we have seen, of cells derived from
both hypoblast and epiblast, but chiefly from the latter. About the
twentieth hour, if the cells of the mesoblast are examined with a high
power (Zeiss, ^^ oil im.), it will be seen that they are stellate in form.
They take the stain similarly to the cells of the epiblast, and it is only
in carefully-prepared specimens—studied with high powers—that we are
able to detect any difference. They lie in a bed of protoplasm, and it
is from this fluid—with which they are constantly bathed—that they
derive their nourishment. They are simply nucleated structures, each
containing numerous granular particles. They are the bioplasts of Beale,
the nuclei of the future cells of the connective-tissue group. They have
no cell-body, and consequently no cell-limit or wall. As they advance
in age they gradually accumulate around themselves formed material,
probably the undigested or unassimilated portion of the surrounding
protoplasm. They thus assume distinctive and characteristic forms.
Their processes gradually become thicker and more pronounced, so as
to be visible even with low powers. The changes in the cells can now
Fro. 302.
Porcine Embryo {'2J cm. X 250) : c.t, embryonic connective tissue of mesoblast.
be seen in the lower jaw of a fcjetal pig two and a half centimeters in
length (Fig. 302).
In the development of the oral cavity and associate parts we do not
have to do with the hypoblastic layer of the blastoderm, so we M'ill
hereafter confine ourselves to the head of the embryo.
The epiblast in an embryo pig 1 cm. in length is composed of one or
more layers of nuclei or bioplasts lying in a bed of protoplasm. With
low powers they appear in no manner different from the underlying
mesoblastic cells, except that the nuclei are closer together ; and in
sections stained with hfemotoxylon and eosin assume a darker hue than
those of the mesoblast. The epiblast now constitutes the " infant
layer of the epithelium, being the deepest layer of the rete Malpighii.
