Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews (1) the constriction of the subnotochordal rod (_x_) from the
summit of the alimentary canal. (2) The formation of the body-cavity in the
muscle-plate and the ventral thickening of the parietal plate.
Fig. 1_a_. Portion of alimentary wall of the same embryo, shewing the
formation of the subnotochord rod (_x_).
Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1.
It shews the bilobed condition of the alimentary vesicle and the fusion of
the mesoblast and hypoblast at the caudal vesicle.
Fig. 3_a_. Sections from the caudal region of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1. Picric acid specimen.
It shews the communication which exists posteriorly between the neural and
alimentary canals, and also by comparison with 3_b_ it exhibits the
dilatation undergone by the alimentary canal in the caudal vesicle.
Fig. 3_b_. Section from the caudal region of an embryo slightly younger
than 3_a_. Zeiss C, ocul. 1. Osmic acid specimen.
Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews the formation of the heart (_ht_) as a cavity between the
splanchnopleure and the wall of the throat.
Fig. 5. Section from the posterior dorsal region of a Scyllium embryo,
belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.
It shews the general features of an embryo of stage H, more especially the
relations of the body-cavity in the parietal and vertebral portions of the
lateral plate, and the early-formed band of muscle (_mp´_) in the
splanchnic layer of the vertebral plate.
Fig. 6. Section from the oesophageal region of Scyllium embryo belonging to
stage I. Zeiss C, ocul. 1. Chromic acid specimen.
It shews the formation of the rudiments of the posterior nerve-roots (_pr_)
and of the vertebral rudiments (_Vr_).
Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than
I. Zeiss C, ocul. 1, reduced 1/3. Osmic acid specimen.
It shews (1) the formation of the anterior and posterior nerve-roots. (2)
The solid knob from which the segmental duct (_sd_) originates.
Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a
stage intermediate between I and K. Zeiss C, ocul. 1. Chromic acid
specimen.
It illustrates the structure of the primitive ova, segmental tubes,
notochord, etc.
Fig. 8_a_. Section from the caudal region of an embryo of the same age as
8. Zeiss A, ocul. 1.
It shews (1) the solid oesophagus. (2) The narrow passage connecting the
pericardial (_pc_) and body cavities (_pp_).
Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul.
1. Osmic acid specimen.
It shews the formation of the liver (_l_), the structure of the anterior
fins (_b_), and the anterior opening of the segmental duct into the
body-cavity (_sd_).
Figs. 9_a_, 9_b_, 9_c_, 9_d_. Four sections through the anterior region of
the same embryo as 9. Osmic acid specimens.
The sections shew (1) the atrophy of the post-anal section of the
alimentary tract (9_b_, 9_c_, 9_d_). (2) The existence of the segmental
tubes behind the anus (9_b_, 9_c_, 9_d_). With reference to these it
deserves to be noted that the segmental tubes behind the anus are quite
disconnected, as is proved by the fact that a tube is absent on one side in
9_c_ but reappears in 9_d_. (3) The downward prolongation of the segmental
duct to join the posterior or cloacal extremity of the alimentary tract
(9_b_).