1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal
vein. _al._ Alimentary canal. _ao._ Aorta. _au._ Thickening of epiblast to
form the auditory pit. _aun._ Auditory nerve. _aup._ Auditory pit. _auv._
Auditory vesicle. _b._ Wall of brain. _bb._ Base of brain. _cb._
Cerebellum. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._
Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External
gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal
nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._
Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base
of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._
Olfactory pit. _op._ Eye. _opn._ Optic nerve. _opv._ Optic vesicle.
_opth_V. Ophthalmic branch of fifth. _p._ Posterior root of spinal nerve.
_pn._ Pineal gland. 1, 2 etc. _pp._ First, second, etc. section of
body-cavity in the head. _pt._ Pituitary body. _so._ Somatopleure. _sp._
Splanchnopleure. _spc._ Spinal cord. _Th._ Thyroid body. _v._ Blood-vessel.
iv._v._ Fourth ventricle. v. Fifth nerve. _Vc._ Visceral cleft. _Vg._
Vagus. vii. Seventh or facial nerve.
Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent
object.
The points which deserve special attention are: (1) The sections of the
body-cavity in the head (_pp_): the first or premandibular section being
situated close to the eye, the second in the mandibular arch. Above this
one the fifth nerve bifurcates. The third at the summit of the hyoid arch.
The cranial nerves and the general appearance of the brain are well shewn
in the figure.
The notochord cannot be traced in the living embryo so far forward as it is
represented. It has been inserted according to the position which it is
seen to occupy in sections.
Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K,
viewed as a transparent object.
The figure shews the condition of the brain; the branches of the fifth and
seventh nerves (v. vii.); the rudiments of the semicircular canals; and the
commencing appearance of the external gills as buds on both walls of 2nd,
3rd, and 4th clefts. The external gills have not appeared on the first
cleft or spiracle.
Fig. 3. Section through the head of a Pristiurus embryo during stage G. It
shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal
summit of the brain. (2) The optic vesicles not yet constricted off from
the fore-brain.
Figs. 4_a_ and 4_b_. Two sections through the head of a Pristiurus embryo
of stage I. They shew (1) the appearance of the seventh nerve. (2) The
portion of the body-cavity belonging to the first and second visceral
arches. (3) The commencing thickening of epiblast to form the auditory
involution.
In 4_b_, the posterior of the two sections, no trace of an auditory nerve
is to be seen.
Figs. 5_a_ and 5_b_. Two sections through the head of a Torpedo embryo with
3 visceral clefts. Zeiss A, ocul. 1.
5_a_ shews the formation of the thin roof of the fourth ventricle by a
divarication of the two lateral halves of the brain.
Both sections shew the commencing formation of the thyroid body (_th_) at
the base of the mandibular arch.
They also illustrate the formation of the visceral clefts by an outgrowth
from the alimentary tract without any corresponding ingrowth of the
external epiblast.
Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo.
Zeiss A, ocul. 1.
The section shews (1) the attachment of a branch of the vagus to the walls
of the hind-brain. (2) The peculiar form of the hind-brain.
Fig. 7. Transverse section through the head of a Pristiurus embryo
belonging to a stage intermediate between I and K, passing through both the
fore-brain and the hind-brain. Zeiss A, ocul. 1.
The section illustrates (1) the formation of the pituitary body (_pt_) from
the mouth involution (_m_), and proves that, although the wall of the
throat (_al_) is in contact with the mouth involution, there is by this
stage no communication between the two. (2) The eye. (3) The sections of
the body-cavity in the head (1_pp_, 2_pp_). (4) The fifth nerve (v.) and
the seventh nerve (vii).
Fig. 8. Transverse section through the brain of a rather older embryo than
fig. 7. It shews the ventral junction of the anterior sections of the
body-cavity in the head (1_pp_).
Figs. 9_a_ and 9_b_. Two longitudinal sections through the brain of a
Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss
A, ocul. 1.
9_a_ is taken through the median line, but is reconstructed from two
sections. It shews (1) The divisions of the brain--The cerebrum and
thalamencephalon in the fore-brain; the mid-brain; the commencing
cerebellum in the hind-brain. (2) The relation of the mouth involution to
the infundibulum. (3) The termination of the notochord.
9_b_ is a section to one side of the same brain. It shews (1) The divisions
of the brain. (2) The point of outgrowth of the optic nerves (_opn_). (3)
The sections of the body-cavity in the head and the bifurcation of the
optic nerve over the second of these.
Fig. 10. Longitudinal section through the head of a Pristiurus embryo
somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of
the nerves and the junction of the fifth, seventh, and auditory nerves with
the brain.
Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo
of stage K, slightly to one side of the middle line. It shews the deep
constriction separating the thalamencephalon from the cerebral hemispheres.
Fig. 12. Longitudinal section through the base of the brain of an embryo of
a stage intermediate between I and K.
It shews (1) the condition of the end of the notochord; (2) the relation of
the mouth involution to the infundibulum.
Fig. 13_a_. Longitudinal and horizontal section through part of the head of
a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.
The figure contains the eye cut through in the plane of the choroid slit.
Thus the optic nerve (_opn_) and choroid slit (_ch_) are both exhibited.
Through the latter is seen passing mesoblast accompanied by a blood-vessel
(_v_). _Op_ represents part of the optic vesicle to one side of the choroid
slit.
No mesoblast can be seen passing round the outside of the optic cup; and
the only mesoblast which enters the optic cup passes through the choroid
slit.
Fig. 13_b_. Transverse section through the last arch but one of the same
embryo as 13_a_. Zeiss A, ocul. 1.
The figure shews (1) The mode of formation of a visceral cleft without any
involution of the external skin. (2) The head-cavity in the arch and its
situation in relation to the aortic arch.
Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13,
considerably magnified. The specimen was prepared by removing the nasal
pit, flattening it out and mounting in glycerine after treatment with
chromic acid. It shews the primitive arrangement of the Schneiderian folds.
One side has been injured.
Figs. 15_a_ and 15_b_. Two longitudinal and vertical sections through the
head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.
15_a_ is the most superficial section of the two. It shews the constitution
of the seventh and fifth nerves, and of the intestinal branch of the vagus.
The anterior branch of the seventh nerve deserves a special notice.
15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_)
continuous with the dorsal commissures of the posterior root of the spinal
nerves.
Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus
embryo belonging to the end of stage K. Zeiss a, ocul. 1.
16_a_ passes through the median line of the brain and shews the
infundibulum, notochord and pituitary body, etc.
The pituitary body still opens into the mouth, though the septum between
the mouth and the throat is broken through.
16_b_ is a more superficial section shewing the head-cavities _pp_ 1, 2, 3,
and the lower vagus commissure.