230 TOWARDS THE HUMAN FORM 



structure. The vertebras are still cartilage impregnated 

 with lime. This is also the case with the earlier Ctenobranch 

 Fishes, whose direct descendants are the present-day Sturgeons. 

 With them, however, calcification is, so to speak, more 

 deep-seated. Abandoning the epidermis, so as to reach 

 the dermis, the process of calcification invades the region 

 of the star-shaped dermal cells, which then constitute the 

 osseous corpuscles. The superficial parts of the dermis, 

 richer in lime and more compact in texture, form at first 

 over each osseous plate a brilliant glaze, to which the name 

 ganoin has been given. Fishes having such scales were called 

 by Louis Agassiz Ganoids. At first they preserved a 

 cartilaginous skeleton, and resembled the Elasmobranchs 

 in the peculiar shape of the tail, the extremity of which 

 curves up. Underneath this erect part the caudal fin 

 develops into a triangular blade, which gives the tail the 

 appearance of being divided into two unequal lobes. On 

 account of this dissymmetry in their tails, Sharks and Ganoids 

 are called heterocercal. Franz Eilhard Schulze, Professor at 

 the University of Berlin, has shown that this arrangement 

 aided the Elasmobranchs, which have no swim-bladder, to 

 rise in the water. It persists as a simple inherited 

 character among such of the Ganoids as possess it, and 

 tends to disappear in Amia of the North American rivers, 

 which are Ganoids in all other respects. In the other 

 Fish described as homocercal, the caudal fin terminates either 

 in a regular convex curve or is emarginated into a fork having 

 two equal branches. 



Ossification of the skeleton has already begun in the higher 

 Ganoids, and it is definitely osseous in the homocercal Fish 

 or Teleostei. Here the tegumentary skeleton goes deeper 

 even than in the Ganoids, and the fish is protected by plates of 

 bony tissue pure and simple, contained in the dermis itself. 

 These are the true scales. They unite in the region of the 

 head, and form more or less extensive plates which closely 

 fit the cartilaginous cranium, but are still easily detachable 

 in the Salmon and the Pike, for example, but in later forms 

 become incrusted in the cartilage, uniting with the 

 bones of the cranial basis, and constituting, in the higher 

 Vertebrates, the bones of the cranial vault known as the 

 membrane-bones. Their frontal and parietal bones, and their 



