JOURNAL OF MICROSCOPICAL SCIENCE. 



DESCRIPTION OF PLATES V AND VI, 



Illustrating Prof. Milnes Marshall's paper on " The 

 Head Cavities and Associated Nerves of Elasmobranchs." 



Each figure has been drawn from a single sectien by the aid of a Hart- 

 nack camera. The numbers attached indicate, in diameters, the magnifying 

 power employed. The majority of the figures are of a semi-diagramma- 

 tic nature, the mesoblast being in nearly all cases omitted for the sake 

 of clearness ; the outlines are, however, strictly accurate in all cases. 



Alphabetical List of References. 



a. Artery ; al. alimentary canal ; al'. outgrowth of alimentary canal to 

 form visceral cleft ; and. auditory vesicle ; b. c. body-cavity or coelom ; 

 i?r.i first branchial arch ; c. g. ciliary ganglion ; d. r. ramus dorsalis ; 

 ep. epiblast \ f.h, fore brain ; g. gills or branchiae; h. b. hind brain; hy. 

 hypoblast ; Hy. hyoid arch ; i. m. investing mass ; i>f. infundibulum ; 

 m. muscle ; m. b. mid brain ; mes. mesoblast ; Mu. mandibular arch ; 

 «. notochord ; o. i. obliquus inferior ; olf olfactory pit ; o. s. obliquus 

 superior; o. y. optic vesicle ; ^;e"«. pineal body; ])it. pituitary involution 

 from mouth ; r. e. rectus esternus or posterior ; r. i. rectus internus ; 

 r. inf. rectus inferior ; r. s. rectus superior ; sp. spiracular or hyoraandi- 

 bular cleft ; v. vein ; 1. first head cavity ; 2. second head cavity ; 3. third 

 head cavity ; i. olfactory nerve ; ii. optic nerve ; iii. third or oculomotor 

 nerve ; in a. anterior branch of third nerve, or ramus ophthalmicus pro- 

 fundus ; III 3. main branch of third nerve to rectus inferior a.nd obliquus 

 inferior ; v. fifth or trigeminal nerve ; v a. ophthalmic branch of fifth 

 nerve ; v b. maxillary branch of fifth ; v c. mandibular branch of fifth ; 

 V d. communicating branch between fifth and third nerve ; v e. branch to 

 obliquus superior ; ^f. anterior non-gangliouic roots of fifth ; vi. sixth 

 or abducens nerve ; vii, seventh or facial nerve ; vii a. ophthalmic 

 branch of seventh ; vii b. palatine branch of seventh ; vii c. hyoidean 

 or post-spiracular branch of seventh ; vii d. mandibular or pre-spiracular 

 branch of seventh; vni. eighth or auditory nerve; ix. ninth or 

 glosso-pharyngeal nerve ; x. tenth or pneumogastric nerve. 



Fig. 1, — Transverse section through the head of a Scyllium embryo of 

 stage D, showing the mesoblast divided into two layers, which have 

 not yet separated to form the coelom. x 90. 



Fig. 2. — Transverse section through the head of an embryo of stage H, 

 showing the head cavities, hind brain, facial nerves, fore gut, and noto- 

 chord. X 90. 



Fig. 3. — Somewhat oblique section through the hinder part of the head of 

 an embryo of stage i, showing the head cavities and the obliteration 

 thereof caused by the commencing visceral clefts ; also the auditory 

 pits and hinder part of the auditory nerve. X 90. 



Fig. 4. — Longitudinal and vertical sections through the head of an embryo 

 of stage K, showing the dorsal ends of the first three head cavities, the 



