348 EMBRYOLOGY OF THE LOWEE VEETEBBATES ch. 



Harrison (1893), in the form of a localized thickening of the basement 

 membrane underlying the ectoderm. This thickening becomes more 

 and more marked and eventually separates off round its edges in 

 the manner shown in Fig. 167, B, mesenchyme insinuating itself all 

 round between the ray and the basement membrane, so that the 

 former eventually lies free from the basement membrane (or in 

 some cases still connected with it by narrow bridges) deep down in 

 the mesenchyme. The ray soon becomes calcined. New layers are 

 deposited on its inner and outer surfaces, mesenchyme cells become 

 included within its substance and it becomes a plate of ordinary 

 bone. The rays are elongated structures which develop from the 

 fin base towards the tip. They often become jointed, either by 

 calcification being interrupted at intervals (Goodrich) or by a 

 secondary solution of continuity (Harrison). 



Rays are formed in the manner described on each surface of 

 the thin membranous fin. The rays of opposite sides correspond 

 exactly in position and become later on fused across the mesial 

 plane so as to form a single unpaired ray whose paired origin is 

 indicated only by its forking at its inner end to embrace the tip of 

 the median radial, the process of fusion between the two elements 

 not taking place at this proximal end. In the dorsal and anal fins 

 and in the ventral part of the caudal fin the lepidotrichial fin-rays 

 correspond segmentally with the true median skeletal elements 

 the tips of which they embrace as indicated above. 



In the more primitive Teleostomes the identity in nature of these 

 fin-rays with the scales which cover the rest of the body is still 

 more obvious. In Polypterus and Lepidosteus they develop a coating 

 of ganoine and even bear distinct small denticles on their surface. 

 It is also interesting to notice that in the anal fin and ventral part 

 of the caudal fin of Polypterus the fin-rays at their proximal ends 

 merely pass in beneath the edges of the body scales and do not 

 take on any relation to the true median skeletal elements. The 

 palaeontological fact may be recalled in passing that in some of the 

 extinct fishes a perfect gradation can be traced between the fin-rays 

 and typical body scales (see Goodrich, 1904). 



In addition to the fin-rays just described, the homology 

 of which with scales may be taken as well established, it is 

 very usual to find another type of fin-ray in which this homo- 

 logy is not so obvious. This is exemplified by the horny fin-rays 

 (see Goodrich, 1904) which occur in the fins of Elasmobranchs 

 (including Holocephali), in the "adipose" fin of Salmonids and 

 Siluroids, and towards the margins of the fins generally in adult 

 Ganoids and Teleosts. These horny fin-rays develop either as 

 thickenings of the basement membrane or at least in immediate 

 contact with it. Mesenchyme cells insinuate themselves between 

 the ray and the basement membrane as the ray separates off. The 

 ray gradually becomes farther removed from the basement membrane 

 and mesenchyme cells collecting round it deposit fresh layers on its 



