PI .\V\TH >\ < ! I'l VI I B I 5, l'>. 17. 



lion ihiDiii'li i lu- r!..r-.i] iyo slightly older 



than stage I, \\ith (hire viMvral clefts. (Zeiss CC, ocul. i.) The section shews the 

 !in of a pair >! la ventral nerve rudiment (a r). 



The l.utcr i> >he\vn In .1 distinctly cellular. 



ti'Mi iliiMU-li the dorsal region mbryo slightly younger 



than stage K. (Zdn CC, -.nil. 2.) Tin- connective-tissue cells are The 



rudiment of tin- ganglion (sfg) on the posterior root has appeared, and the junction of 

 pi Mi-ri >r root with the cord is difficult to detect. The anterior root forms an clonga- 



llulnr structure. 



Fig. 6. Section through the dorsal region .f >f stage K. 



s CC, ocul. 2.) The section especially illu mcnt of the posterior 



root to the spinal cord. 



Fig. 7. Section through the same embryo us fig. <V C, ocul. i.) The 



section contains an anterior root, which takes its origin at a point opposite the interval 

 between two posterior roots. 



Fig. 8. A series of posterior roots with their central ends united by a dorsal 

 commissure, from a longitudinal and vertical section of a Scyllium embryo belonging 

 to a stage intermediate between I, and M. The embryo wa> hardened in a mixture 

 of osmic and chromic acids. 



Fig. 9. The central end of a posterior nerve-root from the same embryo, with the 

 commissure springing out from it on either side. 



EXPLANATION OF PLATKS 15, 10, .7. (X. p. 



PLATE 15. (THE HEAD DURING STAGES G K.) 

 COMPLETE LIST OF REFERENCE LETTERS. 



iaa, iaa, etc. ist, id, etc, aortic arch. acv. Anterior cardinal vein. al. Ali- 

 mentary canal, ao. Aorta. an. Thickening of epiblast to form the auditory pit. 

 aun. Auditory nerve, aup. Auditory pit. aitv. Auditory vesicle, b. Wall of 

 brain. bb. Base of brain, cb. Cerebellum. ccr. Cerebrum. Ch. Chor> 

 ch. Notochord. com. Commissure connecting roots of vagus nerve, i, i, 3 etc. 

 eg. External gills, ep. External epiblast. fb. Fore-brain, gl. Glossopharyngeal 

 nerve, hb. Hind-brain, ht. Heart, hy. Hyaloid membrane. In. Infundibulum. 

 /. Lens. M. Mouth involution. ///. M the base of the brain. //; 



brain, mn. v. Mandibular branch of fifth. <>/. ( Mfactory pit. op. Eye. opn. Optic 

 nerve, opv. Optic vesicle, opth v. Ophthalmic branch of fifth. />. Posterior root 

 of spinal nerve, pn. Pineal gland. 1,1 etc. //. First, second, etc. section of body- 

 cavity in the head. // Pituitary body. so. Somatopleure -f/. Splanchnopleure. 

 spc. Spinal cord. Th. Thyroid body. v. Blood-vessel. i Mh ventricle. 



v. Fifth nerve. Vc. Visceral cleft. \'g. \"a;; i-. vii. Seventh or facial nerve. 



Fig. i. Head of a Pristiurus embryo of stage K viewed as a transparent object. 



The points which K i.il attention are: (i) The sections of the body- 



cavity in the head (/>/>) : the first or premandibular section l>eing 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. 



