110 Biology in America 



vealing new links in the chain of life, it is likely ever to 

 remain so. How arose the vertebrate mouth and jaws, through 

 what steps has the skull evolved, how have the paired fins and 

 their successors, the limbs, developed? These are some of 

 the problems with which the student of vertebrate develop- 

 ment has to struggle. 



Is the vertebrate mouth a modified invertebrate one, and if 

 so is it derived from the dorsal mouth of the tunicate or the 

 ventral one of Balanoglossus, or was it made anew when the 

 vertebrate was fashioned in Nature's work shop? Are the 

 jaws new structures, sui generis, or are they second-hand gill 

 bars employed by Nature for the purpose because they were 

 the handiest structures she could find? 



A possible clue to this question is found in the peculiar 

 development of the anterior end of Amphioxus. When the 

 mouth of this animal first appears it is not in its final position 

 in the median plane of the body, but tilted far up on the left 

 side, while vice versa the first gill slits make their appearance 

 on the right side and secondarily are shifted over to the left. 

 This larval asymmetry would be produced by any force twist- 

 ing the anterior end from left to right; i. e., in the opposite 

 direction to that in which the clock hands move. But if, as 

 the result of such a twisting, the mouth is carried over to the 

 left side its original position must have been dorsal. Now 

 what evolutionary change could have produced such a twist- 

 ing? In the early stages of the larva the notochord does not 

 reach the tip of the body, only later extending there. Further- 

 more, neither in the tunicate larva nor in Balanoglossus does 

 it extend to the tip of the body. If, in correlation with the 

 burrowing habit of the animal, the notochord extended forward 

 in the course of evolution, thereby stiffening the anterior end 

 of the body, and aiding the animal to wriggle through the 

 sand, the tendency would be to displace the anterior organs, 

 and among them the mouth. Such an assumption seems very 

 reasonable. If correct, then the mouth of Amphioxus must 

 originally have been dorsal, and if this animal represents a 

 primitive vertebrate type, then the vertebrate mouth must 

 originally have had this position, secondarily migrating to 

 the ventral side. But what of the origin of the jaws which 

 are such characteristic features of all vertebrates above the 

 cyclostomes ? 



As we have already seen a characteristic feature of all 

 higher animals is their segmentation. This is seen in many 

 parts of the body, muscles, nerves, blood vessels, gills, etc. 

 "We have further seen that one phase of advance is the reduc- 

 tion in number of these segments and their specialization, or 

 modification for the performance of other work than that 



