THE VEETEBRAI/ COLUMN AND THE SKULL. 201 



ctord : posteriorly, it extends back as a flattened epichordal 

 plate above the notochord, and a thick median kypocliordal 

 rod below it. During the shortening of the tail, on the com- 

 pletion of the metamorphosis, the e})ichordal and hypochordal 

 cartilages extend and meet, so as to completely surround the 

 notochord. Later still, the cartilage becomes in great part 

 replaced by bone ; but the hinder end persists as a terminal 

 plug of cartilage throughout life. 



The anterior end of the notochord, imbedded in the base of 

 the skull, is gradually encroached upon by the cartilage and 

 bone around it ; and ultimately, in half-grown frogs, is completely 

 absorbed. 



2. The Skull. 



a. T he skull of the tadpo le. The skull of the tadpole consists 

 almost e ntirel y of cartilage ; none of the bones of the skull, 

 with the exception of the parasphenoid, appearing until a short 

 time before the metamorphosis. 



In the cartilaginous skull of the tadpole two main elements 

 may be distinguished : 



(i) The cranium. _ witli the olfactory and auditory capsules, 

 which may conveniently be taken together. 



(ii) The visceral skeleton , wliich consists of a series of 

 cartilaginous bars developed in the visceral arches, and en- 

 circling the pharynx. 



Speaking generally, the skull as a whole develops from' 

 befoi'e backwards ; the first formed parts being at the anterior 

 end, in connection with the jaws ; and the hinder end of the 

 skull being very imperfect in the early stages of tadpole life. 



True cartilage does not appear until the tadpole is about 10 

 to 11 mm. in length; i.e. when the opercular covering of the 

 gills is almost completed, and the hind limbs are visible as 

 jninute buds at the base of the tail. Lmio- before true '''nirt"'"'°rf° 

 i s present , however^ the skeletal elements can be readily distin - 

 guishe d, as tr acts of condensed and modified mesoblast . 



The first stage in the histological differentiation, by which 

 cartilage is formed, consists in a number of mesoblast cells 

 becoming more closely compacted together ; these cells have 

 large nuclei and very scanty protoplasm, and are further 

 distinguished from the surrounding mesoblast cells by having 



