The first visceral cleft (1vc) 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.
Fig. 11a, b, and c. Three sections closely following each other from an embryo in which three visceral clefts are present; a is the most anterior of the three. (Magnified 96 diam.) In all of these the muscle-plates are shewn at mp. They have become separated from the lateral plates in b and c, but are still continuous with them in a. The early formed mass of muscles is also shewn in all the figures (mp´).
The figures further shew (1) the formation of the spinal nerves (spn) as small bodies of cells closely applied to the upper and outer edge of the neural canal.
(2) The commencing formation of the cells which form the axial skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sections b and c are given more especially to shew the mode of formation of the oviduct (ov).
In b it is seen as a solid knob (ov), arising from the point where the somatopleure and splanchnopleure unite, and in c (the section behind b) as a solid rod (ov) closely applied to the epiblast, which has grown backwards from the knob seen in b.
N.B. In all three sections only one side is completed.
Fig. 12a and b. Two transverse sections of an embryo just before the appearance of the external gills. (Magnified 96 diam.)