INTERNAL ORGANS i6i 



armour on the surface, which contains much fewer elements 

 than that of the Sturgeon, some of the cartilages of the cranium 

 itself have been replaced by bone. In the more highly organised 

 Bony Fishes the amount of cartilage in the cranium of the 

 adult fish becomes less and less, until finally it is entirely absent 

 as in the great majority of the members of this class living to-day. 



As previously explained {cf. p. 107) the bones are of two 

 distinct kinds, each with a diflferent mode of origin, but they 

 are so welded together to form a compact whole that in the 

 adult fish it is often impossible to decide as to which category 

 a particular element belongs. Firstly, there are cartilage 

 bones, so called because the bony tissue develops in the cartilage 

 itself and eventually replaces it. Secondly, there are dermal 

 or membrane bones, not preceded by cartilage, but developing 

 as new structures in the thin membranes of certain regions of 

 the head and forming an investing sheath on the outer surface 

 of the cranium. The bones of the roof, most of those on the 

 floor, as well as those supporting the gill-covers, are of this 

 nature (Figs. 46B, c; 65). The dermal bones are particularly 

 interesting from an evolutionary point of view, representing, as 

 they do, nothing more than much modified scales, which at 

 some time or other sank inwards from the skin and came into 

 intimate connection with the cranium. Many of these bones 

 in the fish's skull persist in the higher land vertebrates, so 

 that in reality the large frontal bones of the human skull have 

 been derived from the bony scales on the head of a fish, which 

 were themselves developed from dermal denticles similar to 

 those of the Selachian skin. 



The development of the skull in such a fish as the Salmon 

 (Salmo) provides yet another interesting example of the manner 

 in which the evolutionary history of any organ or organs is 

 recapitulated during the development of the individual. The 

 first trace of the skull as seen in the unhatched embryo takes 

 the form of two pairs of cartilaginous plates lying below the 

 brain, known as the trabeculae and parachordals. Later, 

 centres of cartilage start to grow in the regions of the organs 

 connected with the senses of smell and hearing, the cartilages 

 in the floor grow up the sides and meet above the brain, and, 

 later still, the capsules round the sense organs fuse with the 

 remainder of the cranium. At this stage, which may occur as 

 late as the second week after hatching, the cranium is still 

 entirely cartilaginous, and is not very unlike that of the adult 

 Dog-fish. Soon afterwards, dermal and cartilage bones begin 



