xxxvi lAl'I.AXATION OF PLATES 24, 25, 26. 



sp.c. spinal cord. gr. grey matter, w. white matter, ar. anterior root. /;-. 

 posterior root. x. commissure uniting the posterior roots. 



Figs. M I, M II. Two sections through the head of the same embryo as fig. B. 

 M I, the foremost of the two, passes through the anterior part of the thickening of 

 cpiblast, which becomes involuted as the auditory vesicle. It contains the rudiment 

 of the seventh nerve, vii. Camera. (Zeiss CC, ocul. 2.) 



vn. rudiment of seventh nerve. au. thickening of external epiblast, which 

 becomes involuted as the auditory vesicle. n. c. neural canal, ch. notochord. pp. 

 body-cavity in the head. so. somatopleure. sp. splanchnopleure. al. throat ex- 

 hibiting an outgrowth to form the first visceral cleft. 



EXPLANATION OF PLATES 24, 25, 26. (XII. p. 549.) 



PLATE 24. 

 LIST OF REFERENCE LETTERS. 



d n. Modified nucleus of primitive ovum, d o. Permanent ovum in the act of 

 being formed, dv. Developing blood-vessels, d yk. Developing yolk, e p. Non- 

 ovarian epithelium of ovarian ridge, f e. Follicular epithelium, g v. Germinal 

 vesicle. / str. Lymphatic region of stroma. n n. Nests of nuclei of ovaria'n region. 

 o. Permanent ovum. ov r. Ovarian portion of ovarian ridge. / o. Primitive ovum. 

 ps e. Pseudo-epithelium of ovarian ridge, str. Stroma ingrowths into ovarian epithe- 

 lium, v. Blood-vessel, v str. Vascular region of stroma adjoining ovarian ridge. 

 vt. Vitelline membrane, x. Modified nucleus, yk. Yolk, z n. Zona radiata. 



Fig. i. Transverse section of the ovarian ridge of an embryo of Scy. canictila, 

 belonging to stage P, shewing the ovarian region with thickened epithelium and 

 numerous primitive ova. Zeiss C, ocul. 2. Picric acid. 



Fig. i. Transverse section of the ovarian ridge of an embryo of Scyllium cani- 

 tuta, considerably older than stage Q. Zeiss C, ocul. i. Picric acid. Several nests, 

 some with distinct ova, and others with the ova fused together, are present in the sec- 

 tion (n. .), and several examples of modified nuclei in still distinct ova are also repre- 

 sented. One of these is marked x. The stroma of the ovarian ridge is exceptionally 

 scanty. 



Fig. 3. Transverse section through part of the ovarian ridge, including the ovarian 

 region of an almost ripe embryo of Scyllium canicula,. Zeiss C, ocul. 2. Picric acid. 

 Nuclear nests (n. .), developing ova (d. o.), and ova (o.), with completely formed 

 follicular epithelium, are now present. The ovarian region is still well separated from 

 the subjacent stroma, and does not appear to contain any cells except those of the 

 original germinal epithelium. 



Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to illustrate 

 the relation of the stroma (str.) and ovarian region. Zeiss a a, ocul. 2. Picric acid. 



-ntinn through the ovarian ridge of an embryo of Scyllium canicula, 

 10 cm. long, in which the ovary was slightly less advanced than in fig. 3. To illus- 



