ON THE DIPLOOHORDA. 317 



of the arteries, veins, central nervous axis, skull, swimming 

 bladder, genitalia, &c. 



Scarcely any process in the phylogeny of the higher animals 

 is more clearly indicated than this, unless it be the ease with 

 which primitively unpaired organs may become impressed with 

 bilateral symmetry. 



In the case of the notochord itself we may remark that in 

 Balanoglossus it tails ofFin the hind region (collar) into paired 

 rudiments, which are more marked in the young stages. The 

 figures of Morgan (17) in this connection are very striking, and 

 we cannot explain the fact that the part of the notochord which 

 is least displaced from its primitive position is found in the paired 

 condition, otherwise than as indicating a paired phyletic origin. 



It may be urged that if the notochord of the Chordata 

 arose as a paired organ there should be indications of this in 

 the ontogeny of the higher types. 



I think there is one reason why the vertebrate notochord 

 does not as a rule show traces of the paired condition, and 

 this is that in the higher Chordata the development of the 

 notochord is greatly hastened. Thus in Amphioxus and in 

 several higher types the rudiment of the notochord is pinched 

 off from the gut at quite as early a stage as are the mesoblastic 

 pouches. A transverse section of a larval Amphioxus at a 

 suitable stage shows the ])air of dorsolateral mesoblastic 

 pouches still in continuity with the gut wall, and in the median 

 line the notochordal rudiment is in the same condition. 



So much is this the case in some Vertebrata, e.g. the 

 chick, that certain observers have claimed that the notochord 

 arises primitively as a median rod of mesoblast. 



A careful study of the more primitive forms, however, 

 clearly shows that the notochord naturally arises from the 

 hypoblast which remains after the mesoblast has been de- 

 veloped from it (secondary endoderm). Without offering any 

 explanation of this precocious development of the notochord, 

 it is plain that, with such ontogenetic conditions, the retention 

 of a phyletic process of development from paired rudiments is 

 impossible. The mesoblastic pouches force the notochordal 



