THE SPIRAL TYPE OF CLEAVAGE. 



249 



would be if the regular alternations of the spiral period had 

 continued. 



In the formation of the two dorsal paratroch cells (Fig. 14, 

 dp^"", dp"^), the last two divisions which they and their sister 

 cells undergo are, so far as direction and alternation are con- 

 cerned, true spiral divisions, although they give rise to a bilat- 

 erally symmetrical group of cells. The first of the two is 

 laeotropic (Fig. 12, dp, Fig. 13, dp\ dp"), the second dexio- 

 tropic (Fig. 14, dp'', dp'\ dp'\ dp''). The cell (Fig. 12, dp) 

 from which the four are formed is a cell of the tenth gener- 

 ation, and if all the preceding divisions had been spiral it 

 should divide laeotropically as it does. But in the ninth gener- 

 ation the spindle was inclined sinistrally, though approaching 

 horizontal (Fig. 11, dp, xl), while in the eighth and seventh 

 generations the spindles were both inclined dextrally. 



Under this head the fact may also be mentioned that in 

 Arenicola some spindles, which are meridional or horizontal in 

 later stages of mitosis, are sometimes at first distinctly spiral. 

 One of the best examples of this kind is the first somatoblast 

 after its first two divisions. The third cell (Fig. 10) lies, 

 when formed, directly anterior to the large stem cell, and the 

 spindle in later stages is perfectly meridional, but often, even 

 up to the equatorial plate stage, it is distinctly dexiotropic, as 

 it normally would be in spiral cleavage. 



Conklin has given a table of the reversals of cleavage in 

 Crepidida, of which there are a considerable number. Some of 

 these, however, are peculiar and possess especial interest with 

 respect to the relation of spiral and bilateral cleavage to each 

 other. These are described as follows : " In a few cases which 

 are classified here as reversals, the nuclear spindle does not 

 indicate that the cleavage is to be reversed, and even the 

 daughter-nuclei may occupy the same relative positions as in 

 the quadrants in whiqh there is no reversal, while at the same 

 time the lobing of the cytoplasm and the subsequent rotation 

 shows that the cleavage is reversed ; the first division of 3^ 

 (Figs. 25, 26, 29) is a case in point. In such cases the con- 

 ditions which influence the direction of the cleavage are not 

 manifested until after the nuclear division is completed. 



