CHAPTER XI 



HINTS BEGABDING THE PEACTICAL STUDY OF "THE 

 EMBBYOLOGY OF THE VABIOUS TYPES OF LOWEB 

 VEBTEBBATES 



Amphioxus. — The interest and importance of Amphioxus to the 

 student of Vertebrate morphology are due to the fact of its position 

 near the base of the Vertebrate phylum. It is true that in its adult 

 structure Amphioxus is intensely specialized in correlation with its 

 burrowing habit. Further, it is necessary to recognize that a 

 burrowing like a pelagic mode of life, in which the environmental 

 conditions are comparatively uniform, is likely to lead to a kind of 

 fixing of the organization which will be fatal to its adaptability to 

 new sets of conditions and consequently to its capacity for evolving 

 along new lines. We must therefore regard it as improbable that 

 the Vertebrata passed through an ancestral condition of specialization 

 for a burrowing habit and the specialized features of the later stages 

 of the life history of Amphioxus cease on that account to have 

 a phylogenetic interest. The main interest to the Vertebrate 

 morphologist lies therefore in the earlier stages before the specializa- 

 tion of the adult has developed — in such features as segmentation, 

 gastrulation and the origin of the main systems of organs. And the 

 interest of these stages is heightened by the fact that food yolk — 

 that potent disturbing factor — is present to a far smaller extent in the 

 egg of Amphioxus than in that of any other of the lower Vertebrates. 

 , Unfortunately the known localities in which fresh embryological 

 material of Amphioxus can be obtained in abundance are still few, 

 and in most laboratories recourse must be had to preserved material 

 purchased from supply stations such as the Naples aquarium. 



The best locality so far known for obtaining developmental stages 

 of Amphioxus is the pantano or shallow lagoon at Faro near Messina. 

 Here the spawning takes place each evening, when conditions are 

 favourable, during the summer months from April to July. The 

 eggs pass to the exterior through the atriopore. If in a dish on 

 board a boat the eggs are liable by its movements to become 

 distributed through the water and they are then apt to become drawn 

 by the inspiratory current in amongst the buccal cirri. When the 



558 



