474 EDWIN S. GOODRICH. 



pendent origin. My own observations on the development of 

 the Teleostean dermal rays have also been chiefly made on 

 young Salmonids, and confirm Harrison's results in most 

 particulars. 



The actinotrichia appear to develop just like the cerato- 

 trichia of Elasmobranchs. They are visible at an eai'ly stage 

 as fine threads, deposited by the secreting mesoblastic 

 '^ scleroblasts " immediately below the basement membrane 

 (fig. 20). This membrane is always present as a continuous 

 layer staining dark blue with nigrosin in Teleosts, as in Elas- 

 mobranchs; and, indeed, in all the fish I have studied. Soon 

 the sclerobhxsts push their way in between the ray and the 

 membrane ; and as they grow older and larger the actino- 

 trichia get carried inwards farther from the epidermis 

 (figs. 20 and 21). In quite young fish, where the lepido- 

 trichia have scarcely developed, the actinotrichia bear much 

 the same relation to the connective tissue and the eudo- 

 skeleton, as do the ceratotrichia in Elasmobranchs. They 

 become surrounded by the secreting cells, and grow in thick- 

 ness by the addition of concentric layers of substance. In 

 the adipose fin (especially of Siluroids) they may reach quite 

 a considerable size, and may branch distally. But in the 

 other fins, where lepidotrichia are formed, the actinotrichia 

 do not increase much in length. Either they get re-absorbed 

 proximally as fast as they are secreted distally, or they get 

 bodily carried outwards in the growing edge of the fin. In 

 the adult Teleostei the actinotrichia are relatively very small, 

 and rarely extend along more than one or two joints of the 

 overlying lepidotrichia. Excepting in the adipose fin, where 

 they form an evenly distributed layer over the web as in 

 Elasmobranchs, the actinotrichia become grouped together 

 more closely in the region where the lepidotrichia develop, 

 and gradually thin out in the intervening spaces. An adult 

 jointed fin-ray thus acquires the deceptive appearance of 

 being frayed out into fine threads at its outer extremity. 



Some considerable time after the actinotrichia have been 

 formed, the definitive dermal rays, or lepidotrichia, put in 



