240 



ELEMENTS OF BIOLOGY 



the embryonic heart long before any nervous connections are 

 estabUshed. 



A somewhat different aspect of coordination through tissues other 

 than nervous is shown during the development of the embryo. The 

 form and pace of development appear to be regulated by influences 

 arising in a series of dominant regions that exercise control over the 



fate of adjacent regions long before any nerv- 

 ous connections are established. A similar type 

 of coordination is shown in the relation be- 

 tween the terminal bud and the lateral or 

 axillary buds of a growing plant. Ordinarily 

 the lateral buds are quiescent unless the 

 terminal bud is pinched oflF or otherwise iso- 

 lated. Then the lateral buds, freed from the 

 dominance of the terminal bud, start develop- 

 ing (Fig. 165). The role played by this type 

 of neuroid transmission, this action-at-a-dis- 

 tance, in regulating form during development 

 constitutes one of the most important, and at 

 the same time one of the least understood 

 problems of Biology. 



Effectors: Muscles. The effector mem- 

 ber of the series receptor-transmittor-effector 

 may be a muscle or a gland. The most promi- 

 nent effect of a stimulus is the response of 

 muscles; glandular responses are usually more 

 obscure. The most primitive of the contractile cells are the contrac- 

 tile elements in the ectoderm of the sponges and the epithelio- 

 muscular cells in the ectoderm of the ccelenterates. But in general 

 the muscle cells of tripoblastic animals are of mesodermal origin. 

 One finds them functionally highly developed very low in the scale 

 of animal complexity; a long neck clam withdraws its siphon with 

 great rapidity; an oyster holds its shell shut with considerable firm- 



vima 



Fig. 166. — A muscle 

 cell from the wing 

 muscle of a wasp. Only 

 a portion of the whole 

 cell is shown and a 

 strand has been separated 

 to show its fibrous char- 

 acter. (After Jordan.) 



