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FIG. 6. View of the posterior side of an 

 egg in a somewhat later stage. There are 

 seen two pairs of mesomeres in the cleav- 

 age cavity. 



little later in Crepidula than the stage shown in Fig. 31, other- 

 wise the two eggs are practically identical. The four cells in 

 Serpulorbis are all on the surface of the egg and are not over- 

 lapped so much by the ectomeres 

 as in Crepidula. The upper 

 pair in Fig. 4 is shown in proc- 

 ess of division. Each cell buds 

 off a small cell into the interior 

 of the egg, the spindles diverging 

 anteriorly. At about the same 

 period an almost exactly similar 

 division occurs in Crepidula, the 

 upper pair of cells budding off a 

 small cell into the interior of the 

 egg in very nearly the same direc- 

 tion. At a somewhat later stage 

 in Serpulorbis, shown in Fig. 6, I have found two pairs of small 

 cells lying entirely within the cleavage cavity which probably 

 represent the descendants of the single pair whose origin has 

 just been described. The paral- 

 lelism with Crepidula extends also 

 to this division as the correspond- 

 ing pair of small cells in that form 

 divides at about the same period. 

 This is as far as the cleavage of 

 these cells was carried. These 

 results were worked out before 

 Conklin's paper appeared, and as 

 I did not follow the further history 

 of these cells, as Conklin has suc- 

 ceeded in doing in Crepidula, it 

 seemed uncertain what interpreta- 

 tion of them should be made. It seemed improbable that all 

 of 4 d should form the mesoblast, as it was supposed to do 

 in several forms. The four large cells showed no signs of 

 passing into the interior of the egg, and it is probable that, 

 after the mesoderm is separated from the upper pair, they 

 enter into the formation of the entoderm, as in Crepidula. 



Fie. 7. Vegetal pole of the same egg 

 shown in Fig. 5. 



