EXPLANATION OF PLATE XIV. 

 The Head during stages G — K. 



Complete list of r^erences. 



€ p. external epiblast. op. eye. op. v. optic vesicle. 



op. n. optic nerve. I. lens. Ch. choroid slit. 



hi/, hyaloid membrane. ol. olfactory pit. au. v. auditory vesicle. 



au. n. auditory nerve. aa. p. auditory pit. 



au. thickening of epiblast to form the auditory pit. 



/ h. fore-brain. cer. cerebrum. pn. pineal gland. 



p>t. pituitary body. In. iufimdibulum. 



in h. mid-brain. h b. base of brain. b. wall of brain. 



h b. hind-brain. cb. cerebellum. iv. v. fourth ventricla 



sp. c. spinal cord. 



v. fifth nerve. ojyth. v. ophthalmic branch of fifth. mn. v. mandi- 

 bular branch of fifth, vii. seventh or facial nerve. gl. glossopharyngeal 

 nerve. com. commissure connecting roots of vagus nerve. 



Vg. vagus. 7>. posterior root of spinal nerve. 



1, 2 etc. pp. first, second, etc. section of body cavity in the head. 



ch. notochord. 



m. mesoblast at the base of the brain. 



ht. heart. V. c. visceral cleft. 1, 2, 3 etc. e.g. external gills. 



al. alimentary canal. Th. thyroid body. 



<io. aorta. 1 a. a. 2 a. a. etc. 1st, 2d, etc. aortic arch. 



a. c. T. anterior cardinal vein. v. blood-vessel. 



M. mouth involution. 



so. somatopleure. sp. splanchuopleure. 



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 {jpp^. 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, 3i*d, and 4th clefts. The external gills have not appeared on 

 first cleft or spiracle. 



Fig. 3. Section through the head of a Pristiurus embyro 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. 



