_al._ Alimentary canal. _ch._ Chorda dorsalis or notochord. _ch´._ Ridge of
hypoblast, which will become separated off as the notochord. _ep._
Epiblast. _hy._ Hypoblast. _lp._ Coalesced lateral and vertebral plate of
mesoblast. _mg._ Medullary groove. _n._ Nucleus of yolk. _na._ Cells formed
around the nuclei of the yolk to enter into the ventral wall of the
alimentary canal. _nc._ Neural or medullary canal. _pv._ Protovertebra.
_so._ Somatopleure. _sp._ Splanchnopleure. _ts._ Mesoblast of
tail-swelling. _yk._ Yolk-spherules.
Figs. 1_a_, 1_b_, 1_c_. Three sections from the same embryo belonging to a
stage intermediate between B and C, of which fig. 1_a_ is the most
anterior. (× 96 diameters.)
The sections illustrate (1) The different characters of the medullary
groove in the different regions of the embryo. (2) The structure of the
coalesced lateral and vertebral plates. (3) The mode of formation of the
notochord as a thickening of the hypoblast (_ch´_), which eventually
becomes separated from the hypoblast as an elliptical rod (1_a_, _ch_).
Fig. 2. Section through the anterior part of an embryo belonging to stage
C. The section is mainly intended to illustrate the formation of the
ventral wall of the alimentary canal from cells formed around the nuclei of
the yolk. It also shews the shallowness of the medullary groove in the
anterior part of the body.
Figs. 2_a_, 2_b_, 2_c_. Three sections from the same embryo as fig. 2.
Fig. 2_a_ is the most anterior of the three sections and is taken through a
point shortly in front of fig. 2. The figures illustrate the general
features of an embryo of stage C, more especially the complete closing of
the alimentary canal in front and the triangular section which it there
presents.
Fig. 3. Section through the posterior part of an embryo belonging to stage
D. (× 86 diameters.)
It shews the general features of the layers during the stage, more
especially the differentiation of somatic and splanchnic layers of the
mesoblast.
Figs. 3_a_, 3_b_, 3_c_, 3_d_, 3_e_, 3_f_. Sections of the same embryo as
fig. 3 (× 60 diameters). Fig. 3 belongs to part of the embryo intermediate
between figs. 3_e_ and 3_f_.
The sections shew the features of various parts of the embryo. Figs. 3_a_,
3_b_ and 3_c_ belong to the head, and special attention should be paid to
the presence of a cavity in the mesoblast in 3_b_ and to the ventral
curvature of the medullary folds.
Fig. 3_d_ belongs to the neck, fig. 3_e_ to the back, and fig. 3_f_ to the
tail.
Fig. 4. Section through the region of the tail at the commencement of stage
F. (× 60 diameters.)
The section shews the character of the tail-swellings and the commencing
closure of the medullary groove.
Fig. 5. Transverse section through the anterior part of the head of an
embryo belonging to stage F (× 60 diameters). It shews (1) the ventral
curvature of the medullary folds next the head. (2) The absence of
mesoblast in the anterior part of the head. _hy_ points to the extreme
front end of the alimentary canal.
Fig. 6. Section through the head of an embryo at a stage intermediate
between F and G. (× 86 diameters.)
It shews the manner in which the medullary folds of the head unite to form
the medullary canal.
Fig. 7. Longitudinal and vertical section through the tail of an embryo
belonging to stage G.
It shews the direct communication which exists between the neural and
alimentary canals.
The section is not quite parallel to the long axis of the embryo, so that
the protovertebræ are cut through in its anterior part, and the neural
canal passes out of the section anteriorly.
Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H.