STAGES B TO G. ALIMENTARY CANAL. 307 



end of the body, they meet there, their walls coalesce, and a 

 direct communication from the neural to the alimentary canal 

 is instituted. The process may be described in another way 

 by saying that the medullary folds are continuous round the 

 end of the tail with the lateral walls of the alimentary canal ; so 

 that, when the medullary folds unite to form a canal, this canal 

 becomes continuous with the alimentary canal, which is closed 

 in at the same time. In whatever way this arrangement 

 is produced, the result of it is that it becomes possible to 

 pass in a continuously closed passage along the neural canal 

 round the end of the tail and into the alimentary canal. A 

 longitudinal section shewing this feature is represented on PI. 

 10, fig. 7. 



This communication between the neural and alimentary 

 canals, which is coupled, as will be seen in the sequel, with the 

 atrophy of a posterior segment of the alimentary canal, is a 

 feature of great interest which ought to throw considerable 

 light upon the meaning of the neural canal. So far as I know, 

 no suggestion as to the origin of it has yet been made. It 

 is by no means confined to Elasmobranchs, but is present in 

 all the vertebrates whose embryos are situated at the centre and 

 not at the periphery of the blastoderm. It has been described 

 by Goette 1 in Amphibians and by Kowalevsky, Owsjannikow 

 and Wagner 2 in the Sturgeon (Acipenser). The same arrange- 

 ment is also stated by Kowalevsky 8 to exist in Osseous Fishes 

 and Amphioxus. The same investigator has shewn that the 

 alimentary and neural canals communicate in larval Ascidians, 

 and we may feel almost sure that they do so in the Marsipo- 

 branchii. 



The Reptilia, Aves, and Mammalia have usually been dis- 

 tinguished from other vertebrates by the possession of a well- 

 developed allantois and amnion. I think that we may further 

 say that the lower vertebrates, Pisces and Amphibia, are to be 

 distinguished from the three above-mentioned groups of higher 



1 Eniwicklungsgeschichte der Unke. 



- Melanges Biologiques de F Acadhnie Ptiersbourg, Tome vil. 



3 Archiv. f. mikros. Anat. Vol. vn. p. 114. In the passage on this point 

 Kowalevsky states that in Elasmobranchs the neural and alimentary canals com- 

 municate. This I believe to be the first notice published of this peculiar arrangement. 



