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. 4a and 4b. 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 4b, the posterior of the two sections, no trace of an auditory nerve is to be seen.
Figs. 5a and 5b. Two sections through the head of a Torpedo embryo with 3 visceral clefts. Zeiss A, ocul. 1.