No. 2.] VERTEBRATE CEPHALOGENESIS. 245 



unfrequently, or the cells lie in the axial line of the nerve conl 

 or brain. By a gradual process of development, it came about 

 that the majority of fibres for the right side of the body found 

 their central ends in the left side of the brain, for some as yet 

 unknown cause, although this transposition was probably inti- 

 mately connected with the development of the system of asso- 

 ciation fibres. 



T. The explanation of the increased number of gill slits of 

 Amphioxus over those of other vertebrates {which certainly shozv 

 traces of considerable reduction in number) is to be found in the 

 habits of the Amphioxus, which is not a free swimming animal, 

 afid cajijwt be a predatory one. It depends, for its food, upon the 

 size and power of its branchial apparatus to create currents and 

 keep moving a sufficient volume of water to supply it with the 

 requisite amount of food, which is contained in only limited quan- 

 tities therein. 



As the animal grows larger its needs are greater ; the bran- 

 chial chamber and apparatus must increase to allow a larger 

 volume of water to pass. 



U. The branchial apparatus of Amphioxus is thett not merely a 

 respiratory apparatus, but m.ore an apparatus for the collection of 

 food and for the transfer of such collected store to the pharyngeal 

 opening for deglutition. A much smaller organ than the bran- 

 chial basket of the adult animal would suffice for the adequate 

 respiratiofi. 



We have only to call to mind the peculiarly inactive life that 

 the animal leads, to become satisfied that its so-called respiratory 

 apparatus is much more important as a food collector and 

 strainer than as a respiration organ. 



This modification of the branchial apparatus, for food collec- 

 tion, is paralleled in higher vertebrate by the production of 

 mandibulo-maxillary region for the same purpose. 



The Lake Laboratory, Milwaukee, Wis. 

 April 21, 1890. 



