No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 403 



it has a peculiar and most interesting history. At first it lay 

 at the base of the anterior arm of the cross. When the cells 

 J ^1.2.1.2.1 ^^^ J ^1.2.1.2.2 ^jyj(^g^ |.j^jg ^,g|| comes to lie between the 



cells produced by these divisions, and soon it is seen further 

 forward, between the posterior pair of cells arising from the 

 third cell of the arm, \b^-'-^. PL XX, Fig. 42, shows this cell 

 where the anterior cells are in contact, and PI. XX, Fig. 48, 

 shows it pushed still further forward, until it has forced the 

 cells i<$i"-^'' and i(^'"=-' to either side and come into contact 

 with the tip cells. Its journey does not end here, but it appar- 

 ently pushes aside the tip cells as well, and comes in contact 

 with the cells of the second quartette, which lie below them 

 (PI. XX, Fig. 46). TJiis cell has, therefore, pushed its way 

 through the split anterior a^'ni from the base to the tip. In 

 later stages it becomes much elongated transversely, and forms 

 a part of the prototroch. The origin of this upper median cell 

 of the prototroch was for a long time a puzzling problem. I 

 have fortunately found all stages of the process by which it 

 travels through the middle of the anterior arm of the cross to 

 its definitive position. 



The further history of the cells of the cross is very difficult 

 to follow. The cells in the center enlarge unequally in differ- 

 ent cases, and the position of the cells is altered considerably 

 by this process, which makes it very difficult to follow their 

 lineage. I have observed a bilateral division of the cells 

 i^''^'"'' and irt:'"^"'"*'^ the lines of division converging anteriorly. 

 Both products of this division then divide at right angles to the 

 preceding cleavage. 



The Second Quartette. 



The cleavage of the cells of the second quartette has already 

 been traced to a stage in which there are four cells in each 

 quadrant. The two middle cells in each quadrant, which are 

 larger than the upper and lower cells, are the first to divide ; 

 the division of both cells in each pair is laeotropic, the cleav- 

 age of the right cell occurring a little before the left. These 

 divisions occur between the fifty-two and the sixty-four cell 



