166 



terminal as in the Annelida) must of necessity, cause a repetition of 

 some sort in some of the organs. 



Certain structures may conceivably be prolonged indefinitively by 

 addition in this way without shewing metamerism — such for instance 

 as the alimentary canal or the notochord of a vertebrate. The noto- 

 chord, as a matter of fact, is actually without sign of metamerism in 

 the embryos of all vertebrates long after metamerism is apparent in 

 other organs and in some cases is actually so throughout life. 



On the other hand organs like blood vessels are incapable of con- 

 tinued elongation and at the same time of performing their function 

 of supplying the adjacent tissues with blood. A repetition of the blood 

 supplying channels is sine qua non. The same argument will apply 

 with almost equal force to a nervous system. 



If repetition is an essential consequence of embryonic growth in 

 length in so important a system as the vascular system it would in 

 all probability lead to repetition of other organs as well. 



That a very marked deuterogenetic centre arises in Annelids as 

 well as in Vertebrates is very obvious, and in each case a metameric 

 segmentation of the part of the adult derived from that centre has 

 resulted ; but I submit that metamerism to some extent is an inevitable 

 result of growth in length from a terminal growing point, and meta- 

 merism in no wise implies that there is in these two groups any nearer 

 relationship than possibly through the earher radial or pre -deutero- 

 genetic organism. 



The relation of the deuterogenetic centre to the blastopore is 

 different in the two phyla. In the vertebrate one part of the area 

 (which is radially symmetrical in itself) remains active for a longer 

 period and produces the tail — while the other, which surrounds the 

 anus, dies out early and the anus is left far forwards. 



There is absolutely no hint in any chordate of direct evidence 

 that the mouth is formed from any part of the blastopore. 



In the Annelida, on the other hand, the mouth sometimes actu- 

 ally is formed from the blastopore. 



Here there is a deuterogenetic centre which gives rise to the 

 metamerically segmented part of the body, but this, unlike the verte- 

 brate, involves the anus. Apparently, but it is difficult to determine, 

 the growing point lies more between the anus and mouth than round 

 the anus — though so close to the former as to carry the anus back 

 with it. A comparison of the Figure 8 C with 8 A illustrates the true 

 relationship as derived from embryological evidence, experimental as 

 well as anatomical. The arguments of H^ckel, Lankestee, Balfour 



