S. IIATTA. THE FATE OF THE PERISTOMAL 



extending from its junction with the ectodcrmic proctodaeum to the 

 curvature, at which this vertical section of the enteric canal passes over 

 into the horizontal section. 



On the other hand, the distal connection of the postanal gut with the 

 tail but is not secondary, but primary, that is to say, it is retained ever 

 since it was found round the blastoporic lips, which arc drawn together so 

 as to meet dorsal of the anus, when the proctodaeum is formed. It follows 

 that the postanal gut is a new formation, in particular, secondarily grown 

 out of the hind wall of the anal gut, which is morphologically the dorsal 

 wall of the enteric canal. 



This fact suggests that the dorsal series of organs in the tail, which 

 are formed behind the blastopore in connection with the postanal gut, are 

 also of a new formation, wc are, therefore, justified in concluding that the 

 tail which is in the Lamprey in length not beyond one eighth of the total 

 body length, constitutes the section of the body, which was not represented 

 in the ancestral form, but is formed secondarily. The fact particularly 

 interests us that the formation of the tail stands in connection with that of 

 the proctodaeum, which causes the blastoporic lip to be produced to the 

 rudiment of 'the tail. Now, the tail is to be regarded as the dorsal out- 

 .groiutJi of the blastoporic lip. 



The assumption above given is supported by the two distinct stages of 

 development, which follow one after the other in such a way, that the 

 development of the tail takes place only when the embryonic organs are 

 established in the remaining portion of the body. The tail which is formed 

 .secondarily only in the postembryonic larval stages, can, therefore, 

 chronologically be distinguished from the primary section of the body, the 

 development of which occupies a very long lasting, principal period of the 

 embryonic development. 



My deepest indebtedness is due to Professor MacBride in the Imperial 

 College of Science and Technology, London, for the courtesy which he 

 has shown me in several ways. 



