190 GEORGE W. TANNREUTHER 
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Fig. 19 Four-cell stage of Unio, upper pole (after Lillie). 
Fig. 20 Four-cell stage of Bdellodrilus, upper pole. 
Fig. 21 Four-cell stage of Dinophilus, upper pole (after Nelson). 
Fig. 22 Four-cell stage of Clepsine, upper pole (after Whitman). 
occurs at nearly right angles, while in Unio and Clepsine it is 
inchned at an angle of about forty-five degrees to the sagittal 
plane of the future adult. In Dinophilus the direction of the 
first cleavage is in doubt. The second cleavage plane in Unio, 
Clepsine and Bdellodrilus occurs at an angle of about forty- 
five degrees to the sagittal axis. The origin of the ectoderm, 
the entoderm and the mesoderm is approximately the same in 
each form. 
1. THE FIRST SOMATOBLAST 
The first somatoblast in each instance is derived from the 
large posterior macromere D (text figs. 23-26). The cell d? 
(X) is extremely large and occupies a median posterior position. 
In Clepsine (Whitman) d? (X) is called the ‘neuro-nephroblast.’ 
