162 HISTORY OF THE HUMAN BODY 



incomplete, and are frequently obtained by a very indirect 

 and circuitous route, and although many an animal form and 

 many myriads of individuals have succumbed as the result 

 of some mechanical disadvantage which the skill of a simple 

 human mechanic could have remedied, there is never the least 

 sign or indication of such an interference. Everything de- 

 velops as an inevitable result of natural law, as a part of 

 the general plan which is broad enough to include the entire 

 universe and which is willing to sacrifice countless hecatombs 

 of lives rather than submit to a single exception to its 

 laws. 



In about the same degree as the visceral skeleton of the true 

 vertebrates is suggested by the gill armature of Amphioxus, 

 so do its fin-folds and the rows of spines enclosed by them 

 suggest a simple condition from which may be derived the 

 appendicular or limb skeleton. In Amphioxus a continuous 

 though very low fin-fold begins near the anterior end, runs 

 the entire length of the animal, and is continuous around the 

 tail with a median ventral one as far forward as the atriopore, 

 where it divides into two, which, as the metapleural folds, 

 continue almost to the mouth. 



This fold system is supported throughout its entire length 

 by a skeleton of gelatinous fin-spines, thus forming an ap- 

 paratus, which, if developed slightly more than in Amphioxus, 

 would be very serviceable in retaining the equilibrium while 

 swimming, serving as dorsal and ventral keels. The skeletal 

 fin-rays would become developed as well as the external folds, 

 and the general appearance would be not unlike that sug- 

 gested by Fig. 42, a. It will be noticed that in Amphioxus 

 certain parts of the caudal fin are wider than the rest, showing 

 how responsive this fold is to a localized increase of function, 

 and this allows one to draw the hypothetical ancestor with 

 certain areas of the fin-fold marked by a greater width, corre- 

 sponding to the places where the greatest stress would be apt 

 to come in an actively swimming animal. As a farther de- 

 parture from Amphioxus, the division of the median ventral 

 fin takes place at the anus, and not at an atriopore, since this 

 latter is probably a special organ developed to supply the 



