111 THE ALIMENTARY CANAL 195 



assumed a ventral position being overhung by the bulging tail 

 rudiment, in D, E, F, G the tail rudiment is seen to be extending 

 actively past the position of the anus, the specially actively growing 

 tissues bein^ indicated by the darker shading. 



In Fig. 80, G, a feature is well shown which occurs in the 

 embryos of most Vertebrates the postanal gut (pa.g). It was 

 shown in Chap. I. how a connexion the neureuteric canal existed 

 in some Vertebrates between the cavity of the enteron and that of 

 the neural rudiment at their posterior ends. Here, in the postanal 

 gut, we have such a connexion still persisting in a drawn-out form 

 though, as in the present case, it may be a solid strand of yolky 

 cells and not a hollow tube. The postanal gut is a purely transitory 

 structure which at a relatively early period of development dis- 

 integrates completely. 



In endeavouring to determine the morphological significance of 

 the postanal gut it is necessary to bear in mind that the Vertebrate 

 in early stages develops from before backwards and that the growth 

 in length by the addition of new segments takes place at its hinder 

 end where there is a mass of actively growing embryonic tissue 

 forming a kind of "growing point." The tissue of this, although 

 to the eye quite undifferentiated, contains the elements which form 

 all the various tissues such as nerve cord, notochord, myotomes, 

 alimentary canal, etc. As growth goes on these gradually become 

 differentiated out, the differentiation always proceeding from before 

 backwards. If we now look at such a young Vertebrate as that 

 shown in Fig. 80, G, we see the typical Vertebrate structure, includ- 

 ing alimentary canal (pa>.g) extending right back practically to the 

 tip of the tail : it is only at the extreme tip that the various organs, 

 merge together into undifferentiated embryonic tissue. The only 

 striking peculiarity is that the communication of the alimentary 

 canal with the exterior, the anus, is not in the midst of the growing 

 tissue of the tip, as it would be, for example, in a young Chaetopod 

 worm, but well forwards on the ventral side. 



This peculiarity, in the writer's opinion, finds its explanation in 

 the development from before backwards already alluded to. The 

 appearance of the anus at a point relatively far forwards means that 

 it and the organs related to it such as the excretory ducts complete 

 their development at an earlier period of time. As it is of 

 functional importance that the organs in question should do so, in 

 contradistinction to the purely motor arrangements farther back, 

 we see a physiological reason why evolution should have brought 

 about a development of the anal opening in its anterior position 

 from the beginning, and the elimination of those stages in which it 

 was situated farther back. 



As regards the phyletic evolution of this part of the enteron, we 

 may sum up probabilities as follows : that the alimentary canal with 

 its surrounding splanchnocoele originally extended to the hind end 

 of the body : that the anal opening came to be shifted on to the 



