EXPLANATION OF PLATE 15. 441 



The notochord cannot be traced in the living embryo so far forward as it is repre- 

 sented. 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 (i) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the 

 brain, (i) 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 (r) 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 and 5^. Two sections through the head of a Torpedo embryo with 3 

 visceral clefts. Zeiss A, ocul. i. 



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. i. 



The section shews (i) 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. i. 



The section illustrates (i) the formation of the pituitary body (pt) from the mouth 

 involution (m), and proves that, although the wall of the throat (a!) 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 (i pp, ipp). (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 

 (ipp). 



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. i. 



9 a is taken through the median line, but is reconstructed from two sections. It 

 shews (i) The divisions of the brain The cerebrum and thalamencephalon in the 

 fore-brain ; the mid-brain ; the commencing cerebellum in the hind-brain. (2) The 

 relation of the mouth involution to the infundibulum. (3) The termination of the 

 notochord. 



B. 2 9 



