No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 405 



261"'' differs in shape and in the direction of its cleavage from 

 the other cells of the same tier. Before it divides, it becomes 

 elongated in a vertical instead of a horizontal direction. It 

 divides laeotropically, its daughter-cells lying obliquely side by 

 side, with their long axes nearly radial. Later, owing to the 

 approach of the large cells 3^^ and 3^^", the cell 2^'-'-' becomes 

 pushed upwards, losing its connection with the entoderm when 

 these cells meet in the middle line. 



The next divisions in this quadrant occur in the cells 2(7''^'^, 

 2a^''^''^, etc. The direction of the cleavage is nearly radial, but 

 slightly dexiotropic. The corresponding cleavages in the b 

 quadrant are delayed until a later period. The upper pair 

 of cells in the quadrants a, c, and d next divide, the right 

 cell laeotropically, the left dexiotropically. The cells from the 

 adjacent arms of the cross push under the trochoblasts from 

 either side and often meet each other, thus separating the 

 trochoblasts entirely from the cells of the third quartette 

 (PI. XIX, Fig. 32). Owing to the forward rotation of the 

 apical cap of cells, the cells 2<5'''^' and 2<^^'' become pushed 

 apart, so that they come to lie on either side, instead of above 

 the cells 2^'-" and 2b'-'-' {see PI. XX, Fig. 39). The cleavage 

 of the tip cell 2<5'''' has already been mentioned ; its two daughter- 

 cells, owing to the rotation of the apical pole, come to lie side 

 by side and in contact with the cells 2b'^'''' and 2b'-'''. The cells 

 2^'-'', 2b'-'-\ 2b'-'-\ 2b''\ 2b'-'-% and 2b'-'-' all become large and 

 clear and enter into the formation of the prototroch. In the 

 anterior quadrant the three upper cells of the four, therefore, 

 enter into the prototroch as in the annelids, and their products 

 undergo, I believe, no further divisions beyond the stage just 

 described. The cell 2b'-''' divides horizontally ; this is the last 

 cleavage in this quadrant that could be observed. The cleavage 

 of the second quartette has been followed to a stage in which 

 there are eleven cells in each of the quadrants a, c, and d, and 

 ten in the anterior, or b, quadrant. After this stage the divi- 

 sions of this quartette become very difficult to follow. I have 

 seen a nearly horizontal cleavage of 2c'-''''', and have been able 

 to recognize the cells of this quartette when there are fifteen 

 cells in each quadrant, but only hypothetical derivations of these 



