214 



THE MESODERMAL DERIVATIVES 



The frog skull represents a type intermediate between the skulls 

 of lower fishes and higher reptiles. It is made up of the cranium, 

 which arises from the cartilage and is therefore known as the chondro- 

 cranium; the sense capsules; a portion of the notochord; and a visceral 

 skeleton, which includes the jaws and the hyoid apparatus and mem- 

 brane and dermal bones. There are cartilage (endochondral) bones 



Mental 



Meckelian 



Basibranchial 

 Ceratohyal 



Ceratobrani h 



Hypobronchia 



Lower jaw, Hyoid 

 end Branchials 



Notochord 



Trabeculae cornu 



Ethmoid plate 



luscular 

 process 



-Anterior 

 process 



Trabecula 



Paloto- 

 ~quadrate 



Posterior 

 process 



.4/ Basilar plate 

 Parachordals 



Skull and 

 upper jaw 



Rana pipiens embryonic chondrocranium. Dor- 

 sal view of the 11 mm. stage. Neurocranium is 

 solid, labeled to the right. Splanchnocranium is in 

 outline, labeled to the left. (After Gaup, from 

 Ziegler.) 



which arise by the ossification of cartilage and there are membrane 

 (dermal) bones which develop from (dermis) superficial membranes 

 without the intervention of a cartilage phase. These latter bones are 

 sheet-like and can be stripped from the cartilage of the true cranium. 

 The bony floor of the cranium arises largely from the notochord 

 in conjunction with a pair of cartilaginous (sclerotomal) rods known 

 as the parachordals at about the 7 mm. stage. These are arranged 

 longitudinally on either side of the notochord with which they become 

 fused to form the parachordal plate or floor of the cranium. The 

 basilar plate is immediately articulated with the notochord and is 



