194 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



as has been shown in Chap. I., widely open to the exterior in all the 

 lower Vertebrates during early stages and in various cases this 

 opening can be traced either into direct continuity, or into less direct 

 but still clear relationship, with the anal opening. The explanation 

 of this lesser degree of modification of the development of the anal 

 opening as compared with the mouth may probably be associated 

 with the less accentuated delay in the functional need for this 

 opening. At stages long before ingestion or inspiration takes place 

 by the mouth, the formation of waste products during the digestion 

 of the yolk necessitates an outlet from the enteric canal at its 

 hinder end. Where obliteration does take place during still earlier 

 stages this is probably correlated with the fact that the need of the 

 opening is still non-existent. 



It is of interest to notice that obliteration of the anal opening 

 which is of a directly adaptive significance may take place at a 

 later stage. Thus in Lepidosiren during about the first two weeks 

 of larval life, when large numbers of practically motionless larvae are 

 lying crowded together in the nest, the anal opening, which had 

 been continuously patent in earlier stages, is closed, so as to prevent 

 the poisonous excretory products from finding their way out. So 

 also in the case of the Elasmobranch embryo enclosed within its 

 egg-shell. In the Amniota the perforation of the anus is delayed to 

 a relatively late period doubtless for a similar reason. 



It is characteristic of the phylum Vertebrata that the anal 

 opening no longer occupies its primitive position at the extreme end 

 of the body but has become shifted forwards along the ventral side. 

 This shifting has probably come about with increased specialization 

 for swimming by lateral flexure of the body, the withdrawal of the 

 alimentary canal with its surrounding splanchnocoelic cavity from 

 the hinder portion of the body, leaving the space they occupied free 

 for increased development of the lateral muscles. This shifting 

 forwards of the anus, leading to the differentiation of a distinct 

 postanal or tail region, has occurred in all Vertebrates, least 

 markedly in the more archaic groups. It reaches its maximum 

 in some members of that group of Vertebrates which is above all 

 others highly specialized for active swimming, the Teleostei, in 

 some families of which the anus has actually assumed a jugular 

 position. 



During the actual ontogeny of the Vertebrate the process by 

 which the anus comes to occupy a position more or less distant from 

 the tip of the tail region is somewhat modified from that which 

 probably occurred during phyletic evolution. We do not find that 

 the anus remains at the tip of the tail during the growth in length 

 and that it then gradually shifts forwards along the ventral side. 

 What happens is that the opening at an early stage assumes a 

 ventral position and that the tad region proceeds to sprout out 

 dorsal to it. The process will be understood from an inspection of 

 Fig. 80 (p. 146). In B the anus is at the hinder end, in C it has 



