316 A. T. MASTERMAN. 



of notochordal value. However, the question of the actual 

 homology of these organs has been carefully investigated, and 

 the balance of evidence seems distinctly in favour of regarding 

 them as the primitive paired representatives of the notochord 

 of Balanoglossus. 



There can be no question of their analogy to notochords. 

 Their histological structure and their relationship to the 

 pre-oral mesentery indicate a function of support, as in the 

 case of the true notochord. They are undoubtedly formed 

 from hypoblastic tissue, and arise normally as diverticula of 

 the primitive gut wall. They also arise in the front end of 

 the collar region, a position identical with that of Balano- 

 glossus. The point of difference, therefore, resolves itself 

 into this, — that there are in Actinotrocha two lateral 

 organs, whereas in all the other Chordata, as far as is known, 

 there is but one median dorsal notochord.^ 



If we can assume a fusion of two lateral organs in the 

 median line as having taken place, the change from lateral to 

 dorsal would follow as a matter of course, so that the difference 

 resolves itself into the difference between '^paired" and 

 " median unpaired.'^ 



To any one who can appreciate the principles upon which a 

 bilaterally symmetrical animal is constructed this difference will 

 not bear any importance. Indeed, it is more difficult to con- 

 ceive in such an animal of the presence of an organ which 

 arises primarily in a strictly unpaired condition; and it is at 

 least a defensible hypothesis that in the higher Vertebrata 

 every asymmetrical unpaired organ arose by atrophy of its 

 fellow, and every symmetrical one either by survival from a 

 radially symmetrical condition or by fusion, in the middle 

 line, of paired rudiments. 



The atrium in Tunicata arises by paired lateral rudiments, 

 in Amphioxus by a median ventral invagination, in spite of 

 which difference we do not find the homology of the two called 

 in question, and one might multiply instances from the study 



» The two lateral notochords of Cep halo discus were found subsequently 



to the writing of this statement. 



