Fig. 1. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet
completely lined by cells. The roof of the segmentation cavity is broken.
(Magnified 60 diam.) The section is intended chiefly to illustrate the
distribution of nuclei (_n_) in the yolk under the blastoderm. One of the
chief points to be noticed in their distribution is the fact that they form
almost a complete layer under the floor of the segmentation cavity. This
probably indicates that the cells whose nuclei they become take some share
in forming the layer of cells which subsequently (vide fig. 4) forms the
floor of the cavity.
Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the
time of the first appearance of the medullary groove. (Magnified 300 diam.)
The specimen is taken from a portion of the blastoderm which will form part
of the embryo. It shews two large nuclei of the yolk (_n_) and the network
in the yolk between them; this network is seen to be closer around the
nuclei than in the intervening space. The specimen further shews that there
are no areas representing cells around the nuclei.
Fig. 3. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity is not yet
covered by a complete layer of cells. (Magnified 60 diam.)
It illustrates (1) the characters of the epiblast, (2) the embryonic
swelling (_es_), (3) the segmentation cavity (_sg_). It should have been
drawn upon the same scale as fig. 4; the line above it represents its true
length upon this scale.
Fig. 4. Longitudinal section through a blastoderm at the time of the first
appearance of the embryonic rim, and before the formation of the medullary
groove. (Magnified 45 diam.)
It illustrates (1) the embryonic rim, (2) the continuity of epiblast and
hypoblast at edge of this, (3) the continual differentiation of the lower
layer cells, to form, on the one hand, the hypoblast, which is continuous
with the epiblast, and on the other the mesoblast, between this and the
epiblast; (4) the segmentation cavity, whose floor of cells is now
completed.
N.B. The cells at the embryonic end of the blastoderm have been made rather
too large.
Fig. 5. Surface view of the blastoderm shortly after the appearance of the
medullary groove. To shew the relation of the embryo to the blastoderm.
Fig. 6_a_ and _b_. Two transverse sections of the same embryo, shortly
after the appearance of the medullary groove. (Magnified 96 diam.)
_a._ In the region of the groove. It shews (1) the two masses of mesoblast
on each side, and the deficiency of the mesoblast underneath the medullary
groove; (2) the commencement of the closing in of the alimentary canal
below, chiefly from cells (_na_) derived from the yolk.
_b._ Section in the region of the head where the medullary groove is
deficient, other points as above.
Fig. 7_a_ and _b_. Two transverse sections of an embryo about the age or
rather younger than that represented in fig. 5. (Magnified 96 diam.)
_a._ Section nearer the tail; it shews the thickening of the hypoblast to
form the notochord (_ch´_).
In _b_ the thickening has become completely separated from the hypoblast as
the notochord. In _a_ the epiblast and hypoblast are continuous at the edge
of the section, owing to the section passing through the embryonic rim.
Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the
flattened head (_h_) where the medullary groove is deficient, (2) the
caudal lobes, with a groove between them; it also shews that at this point,
the medullary groove has become roofed over and converted into a canal.
Fig. 8_a_. Transverse section of fig. 8, passing through the line _a_.
(Magnified 90 diam.) The section shews (1) the absence of the medullary
groove in the head and the medullary folds turning down at this time
instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the
head (_pp_); (3) the completely closed alimentary canal (_al_).
Fig. 8_b_. Transverse section of fig. 8, through the line _b_. (Magnified
90 diam.) It shews (1) the neural canal completely formed; (2) the
vertebral plates of mesoblast not yet split up into somatopleure and
splanchnopleure.
Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object
a little older than the embryo represented in fig. 8. (Magnified 20 diam.)
The internal anatomy has hardly altered, with the exception of the
medullary folds having closed over above the head and the whole embryo
having become more folded off from the germ.
The two caudal lobes, and the very marked groove between them, are seen at
_ts_. The front end of the notochord became indistinct, and I could not see
its exact termination. The epithelium of the alimentary canal (_al_) is
seen closely underlying the notochord and becoming continuous with the
epiblast at the hind end of the notochord.
The first visceral cleft (1_vc_) and eye (_op_) are just commencing to be
formed, and the cranial flexure has just appeared.
Fig. 10. Section through the dorsal region of an embryo somewhat older than
the one represented in fig. 9. (Magnified 96 diam.)
It shews (1) the formation by a pinching off from the top of the alimentary
canal of a peculiar body which underlies the notochord (_x_); (2) the
primitive extension of the pleuro-peritoneal cavity up to the top of the
vertebral plates.