DEVELOPMENT OF RANA 195 



by a cartilaginous capsule which gets attached to the para- 

 chordals on each side. Similarly, nasal capsules surround 

 the olfactory sacs and become attached to the front of the 

 trabecular. The floor of the skull is established in this way, 

 and the sides and roof develop later. 



In each of the visceral arches separating the gill-slits, 

 cartilaginous struts develop. In the mandibular arch, these 

 are the pterygo-quadrate, and Meckel's cartilage which forms 

 the lower jaw. The dorsal portion of the skeleton of the 2nd 

 or hyoid arch forms the columella auris. The cartilages of 

 the remainder of the arches eventually form a plate beneath 

 the floor of the mouth and pharynx, and which by raising and 

 lowering this floor assists in the process of respiration. The 

 skeleton of the limbs and girdles does not appear until a late 

 stage of development. 



This cartilaginous skeleton is later on partly replaced by 

 cartilage-bones, and in addition, membrane-bones are 

 developed. 



Teeth arise late. In their formation, an ingrowth of ecto- 

 derm takes place inside the margin of the mouth, forming the 

 enamel-organs of the teeth. These secrete a cap of enamel 

 beneath which the mesodermal cells produce the body of the 

 tooth which is composed of dentine. Eventually the tooth is 

 pushed up through the surface of the mouth and its base is 

 attached to the bone of the jaw. 



Externals. — By the time that differentiation and the forma- 

 tion of organs have proceeded as far as has just been described, 

 the embryo emerges from its membranes and hatches into a 

 free-swimming larva which is familiarly known as the tadpole. 

 Its ectoderm is ciliated, and just beneath the mouth it has a 

 V-shaped sucker by means of which it can attach itself to 

 objects. Its tail elongates and develops dorsal and ventral 

 extensions or fins, which make it a very efficient organ for 

 swimming. Its food consists of vegetable matter, its stock 

 of yolk being by now used up. Food is seized by the edges of 

 the mouth or lips which are assisted by horny epidermal teeth, 

 which have of course nothing to do with the true teeth. 



From the sides of the head, folds grow back which cover 



