III. 



DEVELOPMENT OF AMPHIOXUS. 



As an introduction to the study of embryology, and as 

 an indispensable aid to a reasonable appreciation of the 

 value of embryological facts, the life-history of Amphioxus 

 provides an object which, for its capability of application 

 to almost every branch of zoological discussion, is perhaps 

 unrivalled. It is alike useful in a text-book of human em- 

 bryology, and in one of invertebrate zoology. 



The reason for this obviously lies in the fact that in 

 Amphioxus everything has its own definite line of de- 

 marcation, all the fundamental structures of the body are 

 laid down with schematic clearness, there are no massive 

 agglomerations of cells forming complicated tissues, but all 

 the organs are of simple epithelial origin and constitution. 



Whereas in many of the higher and lower animals the 

 greatest difficulty is often experienced in deciding to which 

 of the primary layers of the body this or that structure 

 owes its origin, in Amphioxus there is no such difficulty. 

 With these advantages it is, therefore, no wonder that 

 Amphioxus should serve as a refuge to the perplexed 

 embryologist. 



It is not an exaggeration to say that the researches both 

 of KowALEVSKY and of Hatschek, on the development of 

 Amphioxus, will always rank among the classics of embry- 

 ological literature ; while it is a familiar fact that Kowa- 

 levsky's earlier work (1S67) on the development of the 



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