THE RECAPITULATION THEORY 325 



described as occurring in Amphioxus and in Balano- 

 glossus, but in no other animal ; and the occurrence 

 of which, in apparently closely similar fashion, is 

 one of the strongest arguments in favour of a real 

 affinity between these two forms. It is hardly 

 possible that such a modification should have been 

 acquired independently twice over. 



A much more litigious question is the significance 

 of the neurenteric canal of vertebrates, that curious 

 tubular communication between the central canal of 

 the nervous system and the hinder end of the ali- 

 mentary canal that is conspicuously present in the 

 embryos of lower vertebrates, and retained in a 

 more or less disguised condition in the higher groups 

 as well. The neurenteric canal was discovered by 

 that famous embryologist Kowalevsky in Ascidians 

 and in Amphioxus. He drew special attention to 

 the occurrence of a stage in both Ascidians and in 

 Amphioxus in which the larva is free swimming 

 and in which the sole communication between the 

 alimentary cavity and tire exterior is through the 

 neurenteric canal and the central canal of the nervous 

 system ; and suggested * that animals may have 

 existed or may still exist in which the nerve tube 

 fulfilled a non-nervous function, and possibly acted 

 as part of the alimentary canal ; a suggestion that 

 has recently been revived in a somewhat extra- 

 vagant form. A passage of food particles into the 

 alimentary cavity through the neural tube has not 



* A. Kowalevsky, " Weitere Studien iiber die Entwickelungs- 

 geschichte des Amphioxus lanceolatus," Archiv. Jurmihroskopisch 

 Anatomic , Bd. xiii. 1877, p. 201. 



