24 PROFESSOR A. M. MARSHALL. 



central canal of the nervous system and the hinder end of the 

 alimentary 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 Amphioxus. He drew special attention to 

 the occurrence of a stage in both Ascidians and Amphioxus in which the 

 larva is free swimming and in which the sole communication between 

 the alimentary cavity and the exterior is through the neurenteric canal 

 and the central canal of the nervous system ; and suggested x 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 extravagant form. 



A passage of food particles into the alimentary cavity through the 

 neural tube has not yet been seen, and probably does not occur, as the 

 larva still possesses sufficient food yolk to carry it on in its develop- 

 ment. It is therefore permissible to hold that the neurenteric canal 

 may be a mere embryological device, and devoid of any deep morpho- 

 logical significance. 



The question of variation in development is one of very great 

 importance, and has perhaps not yet received the attention it deserves. 

 We are in some danger of assuming tacitly that the mode of develop- 

 ment of allied animals will necessarily agree in all important respects 

 or even in details, and that if the development of one member of 

 a group be known, that of the others may be assumed to be similar. 

 The more recent progress of embryology is showing us showing us that 

 such inferences are not safe, and that in allied genera or species, or 

 even in different individuals of the same species, variations of 

 development may occur affecting important organs and at almost any 

 stage in their formation. 



Great individual variations in the earliest processes of development, 

 i.e., the segmentation of the egg, have been described by different 

 writers. 



1 A. Kowalevsky, ' Weitere Studien iiber die Entwickelungs-Geschichte 

 des Amphioxus lanceolatus ; ' Archiv fur mikroskopische Anatomie, Bd. xiii. 

 1877, p. 201. 



