332 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



portionally reduced, and the trabecular mass becomes the compact 

 substance of the adult tooth. In the functional tooth the tips of 

 the original denticles have completely disappeared. 



In Lepidosiren and Protopterus the separate denticle phase of 

 development is not so distinct as in Geratodus but a reminiscence of 

 it is seen in the pointed cusps which are present on the teeth in early 

 stages (Fig. 164, 0). 



The Bones in General. — The view is now accepted by many 

 morpholpgists, following Hertwig and Gegenbaur, that the true bony 

 skeleton has come about in evolution by the spreading inwards of 

 bone-forming activity from the skin, where it arose in association 

 with the coating of placoid scales which occurs in the lowest Gnatho- 

 stomata. The probability of this view being correct is rendered 

 apparent by a survey of the phenomena of development of some of 

 the bones in the lower Vertebrates. Both in Lung-fishes and in 

 Amphibians the bones of the skull which carry teeth are found to 

 arise in development in the form of more or less trabecular bony 



tissue which spreads outwards from 

 the tooth-bases in the same way as 

 has already been described as occur- 

 ring in the development of the com- 

 pound tooth in Lung -fishes (Fig. 

 164). 



0. Hertwig (1874*) found for 



example that the vomer, palatine 



and opercular of Urodele Amphibians 



are developed in this way, forming 



* Axlu7x°4 n 5! °\lZr S" Plated bony plates studded with 



1874*.) conical teeth (Fig. 165). In the case 



of dentary, maxilla, and premaxilla, 

 part of the bone arises in exactly the same way, while part on the 

 other hand spreads through the mesenchyme without having teeth 

 on its surface. It is to be noted that these bones at first, as 

 frequently happens in the development of bony tissue, have no 

 cells actually enclosed in the calcified substance. Later on the teeth 

 in some cases disappear, leaving behind merely the basal plate of 

 bone which gradually increases in thickness. On turning to the 

 Anura it is found that the bony trabeculae develop precociously 

 and form the basal plate of bone while the teeth belonging to it are 

 delayed in their appearance and may even be omitted. 



The embryology of the Amphibia then teaches us (!) that typical 

 bones may be developed from the basal trabeculae connected with 

 placoid elements and (2) that a secondary modification may arise 

 in which the tooth formation is delayed or suppressed, the trabecular 

 basal plate simply developing by itself and becoming converted into 

 the definitive bone. 



The facts as narrated by Hertwig for Amphibia do not stand alone. 

 On the contrary an exactly similar mode of development is seen in 



