_al._ Alimentary tract. _ao._ Aorta. _c._ Connective tissue. _cav._
Cardinal vein. _ch._ Notochord. _ep._ Epiblast. _ha._ Hæmal arch. _l._
Liver. _ll._ Lateral line. _mc._ Mucous canal of the head. _mel._ Membrana
elastica externa. _mp._ Muscle-plate. _mp´._ Muscles of muscle-plate. _na._
Neural arch. _nl._ Nervus lateralis. _rp._ Rib process. _sd._ Segmental
duct. _sh._ Sheath of notochord. _spc._ Spinal cord. _spg._ Spinal
ganglion. _syg._ Sympathetic ganglion. _um._ Ductus choledochus. _v._
Blood-vessel. _var._ Vertebral arch. _vb._ Vertebral body. _vcau._ Caudal
vein. _vin._ Intestinal branch of the vagus. _vop._ Ramus ophthalmicus of
the fifth nerve. _x._ Subnotochordal rod.
Fig. 1. Section through the anterior part of an embryo of _Scyllium
canicula_ during stage L.
_c._ Peculiar large cells which are found at the dorsal part of the spinal
cord. Sympathetic ganglion shewn at _syg._ Zeiss A, ocul. 1.
Fig. 2. Section through the lateral line at the time of its first
formation.
The cells marked _nl_ were not sufficiently distinct to make it quite
certain that they really formed part of the lateral nerve. Zeiss B, ocul.
2.
Figs. 3_a_, 3_b_, 3_c_, 3_d_. Four sections of the lateral line from an
embryo belonging to stage L. 3_a_ is the most anterior. In 3_a_ the lateral
nerve (_nl_) is seen to lie in the mesoblast at some little distance from
the lateral line. In 3_b_ and 3_c_ it lies in immediate contact with and
partly enclosed by the modified epiblast cells of the lateral line. In
3_d_, the hindermost section, the lateral line is much larger than in the
other sections, but no trace is present of the lateral nerve. The sections
were taken from the following slides of my series of the embryo (the series
commencing at the tail end) 3_d_ (46), 3_c_ (64), 3_b_ (84), 3_a_ (93). The
figures all drawn on the same scale, but 3 _a_ is not from the same side of
the body as the other sections.
Fig. 4. Section through lateral line of an embryo of stage P at the point
where it is acquiring an opening to the exterior. The peculiar modified
cells of its innermost part deserve to be noticed. Zeiss D, ocul. 2.
Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus
growing towards them. Stage O. Zeiss A, ocul. 2.
Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus
growing towards them. Stage between O and P. Zeiss a a, ocul. 2.
Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. 2.
Fig. 8. Longitudinal and horizontal section through the muscle-plates and
adjoining structures at a stage intermediate between L and M. The section
is intended to shew the gradual conversion of the cells of the somatic
layer of muscle-plates into muscles.
Fig. 9. Longitudinal section through the notochord and adjoining parts to
shew the first appearance of the cartilaginous notochordal sheath which
forms the vertebral centra. Stage N.
Fig. 10. Transverse section through the tail of an embryo of stage P to
shew the coexistence of the rib-process and hæmal arches in the first few
sections behind the point where the latter appear. Zeiss C, ocul. 1.
Fig. 11. Transverse section through the centre of a caudal vertebra of an
embryo somewhat older than Q. It shews (1) the similarity between the
arch-tissue and the hyaline tissue of the outer layer of the vertebral
centrum, and (2) the separation of the two by the membrana elastica
externa[2] (_mel_). It shews also the differentiation of three layers in
the vertebral centrum: vide p. 374.
Footnote 2: The slight difference observable between these
two tissues in the arrangement of their nuclei has been much
exaggerated by the engraver.