s 



THE FROG 131 



the cranium below. The joint is also attached to the max- 

 illa by a little bone called the quadrato-jugal; Fig. Z), q. 

 Although in the adult frog these chains of bones are firmly 

 attached to the cranium, they are at first free from it, and are 

 really the upper parts of the arches below, rather than a part 

 of, the cranium proper. The second arch is very rudimentary, 

 only a small part of it being left in the frog. It is called the 

 hyoid arch. Although in some animals this is also a well- 

 developed V-shaped arch, all that is left of it in the frog is a 

 flat plate, made partly of bone and partly of cartilage (Fig. 

 E), which is so loosely attached to the skull that it is usually 

 lost in prepared skulls. In the living frog it lies underneath 

 the larynx, to which it gives support and rigidity. It is attached 

 to the skull only by ligaments, without any bony connection. 



When the skull begins to form in the young frog the parts 

 are soft, and only, as development proceeds, does true bone 

 form. Part of the skull forms first as cartilage, a material 

 that is harder than membrane but softer than bone. Later 

 within this cartilage the mineral matter is deposited, forming 

 true bone, and the bones thus formed are consequently called 

 cartilage bones. These are the occipitals, palatines, pterygoids, 

 and the mandibles. The other bones are formed first as mem- 

 branes rather than cartilage. Within the membrane the mineral 

 bony matter is laid down, and bones developing in this manner , 

 are known as membrane bones. The membrane bones are 

 thefrontals, parietals, parasphenoids } squamosals, nasals, vomers, 

 premaxilla, and the maxilla. 



At its posterior end the skull is articulated with the first 

 vertebra by means of two rounded, smooth surfaces which fit 

 into two corresponding smooth depressions on the upper sur- 

 face of the first vertebra. The articular projections are called 

 the occipital condyles; Fig. C, con. 



Appendicular skeleton. Each appendage consists of a girdle 

 and the appendage proper. The shoulder girdle is a girdle 

 of bones surrounding the body just back of the head, and 



