DEVELOPMENT OF ELASMOBRAXCH FISHES. 91 



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 arrange- 

 ment 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 

 Plate IX. 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 confiaed 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^ in Amphibians and by Kowalevsky, Ows- 

 jannikow and Wagner'^ in the Sturgeon (Acipenser). The same 

 arrangement is also stated by Kowalevsky^ to exist in Osseous 

 Fishes and Amphioxus. The same investigator has shewn that 

 the alimentary and neural canals communicate in larval Asci- 

 dians, and we may feel almost sure that they do so in the 

 Marsipobranchii. 



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 vertebrates, by the positive embryonic character that 

 their neural and alimentary canals at first communicate pos- 



1 EiitwicJclungsgeschichte der JJiike. 



2 Melanges Biologiques de VAcademie Petersbourg, Tome vii. 



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

 Kowalevsky states that in Elasmobranchs the neural and alimentary canals 

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

 arranpremeut. 



