332 EMBKYOLOGY OF TIIK LoXVKR VERTEBKATES CH. 



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 Ceratodus but a reminiscence of 

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

 stages (Fig. 164, C). 



THE BONES IN GENERAL. The view is now accepted by many 

 morphologists, following Her twig 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 Giiatho- 

 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 

 andopercular of Urodele Amphibians 

 are developed in this way, forming 



p erforated bon y p lates 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 

 ;tiid form the basil plate of bone while the teeth belonging to it are 

 d'-layed in their appearance and may even be omitted. 



The embryology of tin- Amphibia then teaches us (1) that typical 



may l> developed from the basal trabeculae connected with 



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



in which th<- tooth formation is delayed or suppressed, tin 1 trabecular 



basal plate simply developing by ii^-ir ami becoming converted into 



the definitive hone. 



The f.i> i hy Hertwig for Amphibia do nut stand alone. 



< )n the contrary an exactly similar mode of dr\clopment is seen in 



