EXPLANATION OF PLATE 13. XXI 



Fig. 150. Picric acid preparation shewing a primitive ovum partially filled with 

 bodies resembling yolk-spherules. 



Fig. 1 6. Horizontal and longitudinal section of Scyllium embryo belonging to 

 stage K. Zeiss A, ocul. i. Picric acid preparation. The connective-tissue cells are 

 omitted. 



The section shews that there is one segmental tube to each vertebral segment. 



Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a trans- 

 parent object. 



It shews the segmental duct and the segmental involutions two of which are seen 

 to belong to segments behind the end of the alimentary tract. 



Fig. 1 8. Vertical longitudinal section of a Scyllium embryo belonging to stage K. 

 Zeiss A, ocul. i. Hardened in a mixture of osmic and chromic acid. It shews 



(1) the commissures connecting together the posterior roots of the spinal nerves ; 



(2) the junction of the anterior and posterior roots; 



(3) the relations of the segmental ducts to the segmental involutions and the 



alternation of calibre in the segmental tube ; 



(4) the germinal epithelium lining the body-cavity. 



EXPLANATION OF PLATE 13. (X. p. 361.) 



COMPLETE LIST OF REFERENCE LETTERS. 



al. Alimentary tract, ao. Aorta, c. Connective tissue, cav. Cardinal vein. 

 ch. Notochord. ep. Epiblast. ha. Haemal arch. /. Liver. //. Lateral line. me. 

 Mucous canal of the head. mel. Membrana elastica externa. m p. Muscle-plate. 

 //'. Muscles of muscle-plate, na. Neural arch. nl. Nervus lateralis. rp. Rib 

 process, sd. Segmental duct. sh. Sheath of notochord. spc. Spinal cord. spg. 

 Spinal ganglion, sy g. Sympathetic ganglion, um. Ductus choledochus. v. Blood- 

 vessel, var. Vertebral arch. vb. Vertebral body. vcau. Caudal vein. vin. In- 

 testinal branch of the vagus, v op. Ramus ophthalmicus of the fifth nerve, x. Sub- 

 notochordal rod. 



Fig. i. Section through the anterior part of an embryo of Scyllium canicula 

 during stage L. 



c. Peculiar large cells which are found at the dorsal part of the spinal cord. 

 Sympathetic ganglion shewn at syg. Zeiss A, ocul. i. 



Fig. i. Section through the lateral line at the time of its first formation. 



The cells marked n I were not sufficiently distinct to make it quite certain that 

 they really formed part of the lateral nerve. Zeiss B, ocul. 2. 



Figs. 3 a, 3^, 3^, j,d. Four sections of the lateral line from an embryo belonging 

 to stage L. - is the most anterior. In 3 the lateral nerve (nl) is seen to lie in the 

 mesoblast at some little distance from the lateral line. In 3 b and 3 c it lies in 

 immediate contact with and partly enclosed by the modified epiblast cells of the 

 lateral line. In 3 d, the hindermost section, the lateral line is much larger than in 

 the other sections, but no trace is present of the lateral nerve. The sections were 

 taken from the following slides of my series of the embryo (the series commencing at 

 the tail end) 3 d (46), 3 c (64), 3 b (84), 3 a (93). The figures all drawn on the same 

 scale, but 3^ is not from the same side of the body as the other sections. 



