OP THK TBLBOSTEAN FISH SEBASTES MARIN US. 505 



A remarkable feature which has become evident at this stage is 

 the precocious development of the visceral skeleton. The lower 

 jaw, temporal, ceratohyal, and to seme extent the first two 

 l)ranchial bars are all distinguishable, while the cranium consists 

 only of the simple parallel trabeculse and the barely established 

 parachordals. It is interesting at this point to note in connection 

 with the form assumed by the parachordals that Pilatoff (1916) 

 has shown that if the auditory vesicle of Bufo is transplanted to a 

 position overlying the trabeculse a cartilaginous capsule tends to 



Text-figure 3. 



Aud.c. 



••■•• Mk. ^ 



Stage 2. Lateral view of chondrocranium. 



form round it, while the parachordals from which the vesicle has 

 been removed do not form a capsule. It is pointed out that 

 the formation of the cartilaginous capsule is not an inherent 

 property of the parachordals, but depends upon the presence of 

 the vesicle. 



From now onwards the mouth tends to move forwards and 

 upwards, and Meckel's cartilage follows it by growth at the 

 anterior end. 



c. Stage 3. The 5 "5 vim. larva. 



The trabeculse have still further lengthened and are now joined 

 anteriorly in an extensive but very thin ethmoid plate (text-fig. 5) 

 which as yet shows no sign of any of the upgrowths which later 

 form the investment of the olfactory organs. According to 

 Gaupp it has been shown that in Salnio salar the cartilage of the 

 ethmoid plate is formed from the epithelium lining thereof of the 

 mouth, and is therefore ectodermal in origin. In Sehastes at this 

 stage it can still be seen from the structure of the ethmoid plate 

 that it is formed from a single layer of cells. But a study of the 

 4*5 mm. stage shows that these cells are derived from a local 

 condensation of connective tissue. 



