EXPLANATION OF PLATE IX. 



Fig. 1. Tail of a recently-hatched salmon embryo, x 16 ; ep and hy, epural and hypural 

 cartilages ; ch, chorda ; ma, tip of medulla spinalis ; my my, myotomes 

 (partially aborted and transformed into the caudal muscles posteriorly); 

 en, notch between opisthural and permanent cau dal lobe ; /(/ hf, horny 

 fibers from which the permanent rays r rr are partially formed. 



Fig. 2. Insertion of the vibratores spinse or lateral muscles of the anal fin of a young 

 salmon into the basal sheet in which the horny fibers hf end. x 183. 



Fig. 3. Part of the tip of the tail and tail-fold of a young codfish, ten days old, to 

 show the development of the horny fibers hf from the fusiform cells or 

 pterygoblaats p< ; eft, chorda ; ms, medulla spinalis ; in (/", posterior margin 

 of tail-fold. X 305. 



Fig. 4. Diagram from a vertical transverse section through the basal part of the cau- 

 dal lobe of a young salmon recently hatched, to show the permanent rays 

 pr in section, and the horny fibers hf, both being invested by the meso- 

 blast mes ; ep, epiblast or larval skin, x 96. 



Fig. 5. Diagram from a section still nearer the base of the caudal of a young salmon 

 of the same age, showing three hypural cartilages hy cut across, x 96. 



Fig. 6. Diagram from a section through the tail of a young salmon, nearer the mar- 

 gin of the caudal than Fig. 4 ; vv, vessels ; mes, mesoblast ; pr, permanent 

 fin-rays, cut across; pr, showing traces of the way in which the horny fibers 

 ft/ become invested by a homogeneous material and fused together, x 183. 



Fig. 7. A diagram from a portion of the caudal fold still nearer its margin than that 

 shown in Fig. 6 ; g, unicellular goblet cells of the epithelial layer ep of 

 the epiblast ; mp, Malpighian layer of the skin ; pr, homogeneous rudi- 

 ments of the permanent rays between the epiblast and mesoblast, the lat- 

 ter being thickened between the rudimentary halves of the permanent 

 rays of opposite sides ; v v, vessels ; hf, horny fibers ; mes, mesoblast. 

 X 183. 



Fig. 8. More enlarged view of a section through the left half of the rudiment of a 

 permanent ray of a young salmon, taken from the same plane as the pre- 

 ceding. Letters the same as in the preceding figure, x .365. 



Fig. 9. A similar section from near the edge of the caudal fold of the salmon, to show 

 the cylindrical horny fibers ft/" separate and lodged between the skin ep 

 and mp and the mesoblast mes. x 365. 



Fig. 10. A similar section through the layer of horny fibers between the lower layer 

 of the epiblast vip and mesoblast mes, to show the mode in which the fibres 

 are fused together and enveloped by the matrix of the future permanent 

 rays, x 365. 



Fig. 11. A small fragment of the epiblast of the fin-fold of a recently-hatched salmon, 

 viewed from the surface, to show the perlectly straight horny tin-fibers 

 ft/, partially covered by mesoblastic cells, x 366. 



Fig. 12. Diagram to illustrate the mode in which the matrix of the permanent fin-ray 

 pr envelopes the horny fibers hf, which remain more or less divergent 

 and free at their distal extrenrities. 



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