CHAPTER XI 



HINTS REGARDING THE PRACTICAL STUDY OF THE 

 EMBRYOLOGY OF THE VARIOUS TYPES OF LOWER 

 VERTEBRATES 



A.MPHIOXUS. 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 emhrynlo^ieal 

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

 and in most laboratories recourse must he had in preserved material 

 purchased from supply stations such as the Naples aquarium. 



Tim best locality so 1'ar known i'..r obtaining developmental stages 

 of Amphioxun is the pantano or shallow lagoon at l-'aro near Messina. 

 Here tin- spawning takes place ,-ach evening, when condition 

 favourahle, during the summer months from April to .lulv. The 

 eggs pass to tin- exterior through tin- atriopmv. It' in a dish on 

 . a hual, the Bggfl aW liahle l.y its movements to hecome 



.hutrd i hrou^h the water and t h-\ axe i hen apt, t<> become dra\\ n 



hy the inspiratory ciim-nt in amount, tin- hueeal cirri. When the 



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