No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 13 



2. The First Generation of Ectomeres. 



Shortly after the division of AB a small cell d^ is bud'^ed off 

 from D towards the apical pole. We have thus a five-cell 

 stage which has been figured by Rabl and others. The tliiec 

 other cells, A, B, C, next bud forth small cells «i, b^, and c^ 

 likewise towards the apical pole. These three divisions do not 

 take place synchronously, but in the almost invariable order C, 

 A, and B, so that six, seven, and eight-cell stages occur. The four 

 apical cells thus formed are the first generation of ectomeres. 



Fig. 1 3 is a view of the apical pole in the six-cell stage. D 

 and C have already divided, and it will be seen that both A and 

 B are preparing to divide. In both of these cells the aster to 

 the left belongs to the ectomere; the one to the right is sunk 

 deep in the substance of the cell, and hence is represented as 

 being fainter. This figure shows further that the oblique 

 nature of the cleavage is already indicated by the spindles. 

 Thus the spindle in B points to the space between B and C, 

 that in A to the space between A and B. Fig. 14 from the 

 vegetative pole shows the time relationships of the cleavage 

 very well; c^ is already separated from C, though the asters are 

 still visible in both cells; the metakinetic stage of division has 

 been reached in A, while B is in the equatorial-plate stage. 

 Fig. 1 5 is a view from the apical pole of the completed eight- 

 cell stage. It will be seen at once that d^ is the largest of the 

 ectomeres. Fig. 16 is a view of the same stage from the right 

 side, and partly from the vegetative pole. 



Fig. 12 illustrates an interesting condition which occurs but 

 rarely. The four macromeres are of more nearly equal size than 

 usual ; in this case A is larger than D. This may, perhaps, be a 

 reversion towards a more primitive type in which the segmenta- 

 tion was equal. It may not be out of place to mention here that 

 all of the eggs obtained from one individual of Unio complanata 

 divided equally at the start. As it was impossible to keep the 

 eggs alive long enough, I cannot say that normal embryos would 

 have been produced. But from the vigor with which the early 

 cleavages took place I feel convinced that such would have 

 been the case under normal conditions. Watas6 (No. 60) has 



