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.