56 



INTRODUCTION TO EVOLUTION 



into segmentally arranged muscles of the body wall, the myotomes (Fig. 

 4.6). Although the myotomes of fishes take the form of nested F's or W's, 

 it will be evident that the change from the segmental blocks of the embryo 

 is much less drastic than is the change by which rows of blocks are trans- 

 formed into the muscles of the back and limbs of higher vertebrates, in- 

 cluding man. Rows of segmental somites form a "reasonable" beginning 

 for a muscular system which is to consist mainly of muscles divided into 

 successive segments, as in fishes. 



myotomes 



column 



FIG. 4.6. Locomotor apparatus of a typical fish. (After Gregory and Roigneau, Intro- 

 duction to Human Anatomy, American Museum of Natural History, 1934.) 



At this point we may inquire into the utility to a fish of having muscles 

 arranged as shown in Fig. 4.6. Fishes are propelled forward in the water 

 mainly by undulations of the body which confer upon the large tail fin a 

 sculling motion. Other highly aquatic animals are also propelled forward 

 by undulations of the body. The most complete analysis of the mechanism 

 involved was made by Coghill ( 1 929 ) on the water-dwelling larvae of 

 Amblystoma, a genus of tailed amphibians. Since these aquatic larvae also 

 have muscular systems arranged in segments, they afi'ord quite as good 

 information on the advantages of that arrangement as would fishes. 



Fig. 4.7 presents diagrams of the essential mechanism of swimming in 

 Amblystoma larvae. The myotomes (somites) will be noted in parallel 

 rows on both sides of the spinal cord. Each segment is separated from its 

 neighbors before and behind by membranous partitions, the muscle fibers 

 themselves attaching to these membranes. Thus when the muscle fibers in 

 one myotome contract, the width of the myotome, i.e., the distance be- 

 tween anterior and posterior partitions, decreases. In the middle dia- 

 gram of Fig. 4.7 the first six myotomes on the left side are represented as 



