THE ENDOSKELETON 141 



crawls, or even lies passively on the surface of the ground, 

 must be very great. It is evidently to overcome this in part 

 and furnish some protection for the soft parts that there de- 

 velops in this region a series of skeletal elements precisely 

 similar in origin to the primitive sternal pieces of Nectums, 

 formed by the ossification of the ventral portion of the ab- 

 dominal myocommata. Developing along the mid-ventral 

 line also, many of the pieces become connected together and 

 form a system of " abdominal ribs," as they have been called, 

 better known as the parasternum. As these do not appear to 

 be represented in any other Order, they are of no phylogenetic 

 value, but serve to explain the reason for the origin of the 

 archisternum in the salamanders by furnishing an exact phy- 

 siological parallel. Associated with the sternal region, both 

 in Amphibia and in the Amniota, there is a rather problematic 

 element, known as the episternum, of which no continuous 

 history is yet known, so that it is not even certain that the 

 various elements in different animals called by that name are 

 homologous. The typical episternum is a skeletal piece occur- 

 ring in lizards and consisting of a thin cross-shaped or T-- 

 shaped piece lying, as its name denotes, upon (i. e., on the 

 ventral side of) the sternum, and a little anterior to it. 



This part is not clearly present in other vertebrates, but 

 similar pieces occur in several cases, and are often designated 

 by the same name. Thus, in the shoulder-girdle-sternum com- 

 plex of the frog there is a piece extending anteriorly along the 

 mid-ventral line, between the clavicles, and closely resembling 

 the true sternum (archisternum) that extends posteriorly. 

 This has been often called the episternum, but is more likely 

 a portion of the archisternum, formed like the other, from 

 myocommata. Again, the well-known " wish-bone " of birds 

 is formed by a fusion of the two clavicles with a middle 

 piece, the inter clavicle, which forms the " head " and is es- 

 pecially well developed in the common fowl. This element also 

 has been identified with the episternum by some investigators, 

 as have also certain parts of the keel of the sternum, which 

 develop from separate centers of ossification. 



