No. 3.] AMPHIOXUS AND- THE MOSAIC THEORY 



599 



clearly marked types, each of which is characteristic of a par- 

 ticular group or series of animals. Although these types have 

 little phyletic importance, their origin and meaning is of great 

 interest in the study of cell-dynamics, and their careful inves- 

 tigation is an indispensable accompaniment of experimental 

 researches on cell division and differentiation. These types 

 may be designated as (i) bilateral, (2) radial, and (3) spiral, 



B 



Diagram of Cleavage-Forms. 



A. Radial type (Antedon ; modified from Seeliger). 



B. Spiral type {Discocoelis ; after Lang). 



C. Bilateral type {Clavelina ; after Van Beneden and Julin. The upper figure 

 is considerably modified, the inequality between the macromeres and micromeres 

 being exaggerated). 



The upper figures represent 8-celled stages seen from the upper pole. The 

 lower figures are the corresponding i6-celled stages in the same position. In the 

 case of unequal division the arrow points towards the smaller cell. 



and their definition at this point will facilitate the discussion 

 of certain questions suggested by the fact that all three of 

 them normally occur in Amphioxtts (p. 581). It will be con- 

 venient to consider only the 8-celled and i6-celled stages 

 which most clearly show the characters (Diagram above). 



