AMPHIOXUS, AND THE MOSAIC THEORY OF 



DEVELOPMENT. ' 



EDMUND B. WILSON. 



In view of certain current discussions regarding the nature 

 of embryological development, the cleavage of the ovum in 

 Amphioxus is of more than ordinary interest on account of its 

 remarkably plastic character, which is shown in two directions. 

 First, it normally exhibits a protean variability, and a study of 

 its various forms yields new data bearing on the origin of 

 cleavage-forms in general. Second, the development is capable 

 in a very high degree of artificial modification through mechani- 

 cal disturbances operating on the early stages. The descriptive 

 portion of this paper is accordingly divided into two parts, the 

 first treating of the natural forms of cleavage, and the second 

 of induced forms caused by the isolation or mechanical dis- 

 placement of the blastomeres. In a third and a fourth part 

 the bearing of these facts on the origin of cleavage-forms and 

 on some of the problems of embryological dynamics is briefly 

 discussed. 



In studying the natural forms of cleavage I have sought, 

 especially, to find a basis, first, for the accurate comparison of 

 the cleavage of the chordates with that of lower forms, and 

 second, for the experimental studies described in Part II. To 

 those who have followed recent inquiries into the significance 

 of the first stages of cleavage I need not apologize for the 

 description of many apparently trifling details, especially since 

 my observations on the early development differ very materi- 

 ally from Hatschek's account.^ 



1 The material for this study was procured at Faro during the months of June 

 and July, 1892, by the following method : The animals, on being removed from 

 the sand of the Pantano, were placed in large shallow glass vessels filled with 

 clean water; here they lie perfectly quiescent and, if mature, discharge the repro- 

 ductive elements within a few minutes. The eggs (which are invariably passed 

 out through the atrial opening) were drawn up, during or immediately after their 

 discharge, into a pipette and transferred to a smaller vessel of clean water to 



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