FIEST PERIOD OF DEVELOPMENT. 



THE SEGMENTATION (Fias. 2-20). 



THE segmentation was previously followed by Kowa- 

 levsky in its several features. The first stages 

 apparently lie had described very accurately, namely 

 the division of the egg into two, and afterwards into 

 four cells, [which then, by an equatorial furrow, are 

 divided into eight. A sixteen-cell stage was after- 

 wards described, as well as the formation of a hollow 

 sphere occasioned by succeeding divisions, this hav- 

 ing a large cavity and a thin wall constructed out 

 of a single layer of cells. 



According to this account of Kowalevsky, the 

 segmentation of the Amphioxus was always treated as 

 the type of an equal segmentation leading to a bias- 

 tula without any well-defined main axis. My own 

 observations complete our knowledge of the Amphi- 

 oxus to this extent that we recognise that the 

 segmentation is by no means of exact equality, but 



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