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. 3a, 3b, 3c, 3d. Four sections of the lateral line from an embryo belonging to stage L. 3a is the most anterior. In 3a the lateral nerve (nl) is seen to lie in the mesoblast at some little distance from the lateral line. In 3b and 3c it lies in immediate contact with and partly enclosed by the modified epiblast cells of the lateral line. In 3d, 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) 3d (46), 3c (64), 3b (84), 3a (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.