128 CONK LIN. [Vol. X 1 1 1 . 



alike in Crepidula and Umbrella. These resemblances are so 

 minute and so long continued that they form a fitting climax 

 to the many similarities which have been pointed out hereto- 

 fore. 



The divisions of the third quartette have not been followed 

 by Wilson, Lillie, or Kofoid. 



3 . Organs formed from the Second and Third Quartettes. 



As was indicated in another place (p. 114), all the organs of 

 ectodermal origin lying posterior to the first row of velar cells 

 are derived from the second and third quartettes, and a portion 

 even of the foremost row of velar cells comes from the second 

 quartette ; nevertheless I have not been able, save in a few 

 cases, to trace individual cells of these quartettes directly to 

 the organs which they form. However, many of the organs 

 of this region can be derived, in great probability, from certain 

 groups of cells. 



As is shown in Diagram 8, which is the latest stage to 

 which the lineage of the whole ectoblast was traced, the first 

 quartette occupies the apical region of the &zg, and is surrounded 

 by a broad belt of cells derived from the second and third quar- 

 tettes. The cells of the second quartette lie at the ends of the 

 arms of the cross, and approximately over the first and second 

 cleavage furrows, which are still visible between the macro- 

 meres. The cells of the third quartette alternate with those of 

 the second, and lie approximately halfway between the first and 

 second furrows. In each quadrant the third quartette touches 

 the first by only a single cell, but the cell group grows broader 

 as it extends out toward the periphery of the ^gg ; the second 

 quartette is as broad where it touches the first quartette as at 

 the periphery. 



Since from this stage onward there is no extensive rotation 

 of the cells around the egg axis, it is possible to locate some of 

 the organs within certain of these cell groups. Other organs, 

 particularly those near the middle of the ventral surface, can- 

 not be traced even to these different cell groups with any degree 

 of certainty, as I do not know what part the cells of the second 

 and third quartettes take in the closing of the blastopore. 



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