Figs. 6 and 7 are sections through an embryo rather earlier than the one
drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove
(_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque
streak coming from the end of the primitive groove. The notochord is hardly
differentiated, but the _complete_ separation of mesoblast and hypoblast
under the primitive groove is clearly shewn. Fig. 7 passes through the
anterior end of the primitive groove (_pr_), and shews the fusion between
the mesoblast and epiblast, which is always to be found under the primitive
groove.
Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a
transparent object. Primitive groove (_pr_). Medullary groove (_md_), which
passes off from the anterior end of the primitive groove, and is produced
by the thickening of the mesoblast. Head fold (_pf_).
Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through
the lines 1, 2, 3, 4, 5 respectively.
The first section (fig. 13) passes through the true medullary groove
(_mc_); the two medullary folds (_A_, _A_) are seen on each side with the
thickened mesoblast, and the mesoblast cells are beginning to form the
notochord (_nc_) under the medullary groove. There is no adherence between
the mesoblast cells and the epiblast under the medullary groove.
The second (fig. 14) section passes through the medullary groove where it
has become wider. Medullary folds, _A_, _A_; notochord, _ch_.
In the third section (fig. 15) the notochord (_ch_) is broader, and the
epiblast is raised in the centre, while the medullary folds are seen far
apart at _A_.
In section fig. 16 the medullary folds (_A_) are still to be seen enclosing
the anterior end of the primitive groove (_pr_). Where the primitive groove
appears there is a fusion of the epiblast and mesoblast, and no appearance
of the notochord.
In the last section, fig. 17, no trace is to be seen of the medullary
folds.
Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is
twenty-three hours old; fig. 19 twenty-five hours. They both shew how the
medullary canal arises entirely independently of the primitive groove and
in front of it, and also how the primitive groove gets pushed backwards by
the growth of the medullary groove. _pv_, Protovertebræ; other references
as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were
cut.