2^8 HOLMES. [Vol. XVI. 



of over four cells. The radial symmetry of the cross is not de- 

 stroyed until the ^g^ contains over one hundred blastomeres. In 

 Neritina the cross becomes a bilaterally symmetrical structure, 

 by the lengthening of the posterior arm, when the egg contains 

 only about fifty cells. In Crepidula the cross may be said to be 

 bilaterally symmetrical from the very beginning of its formation, 

 owing to the smaller size of the basal cell and the larger size of 

 the tip cell in the posterior arm. At about the forty-eight-cell 

 stage there are only two cells in the posterior arm and three in 

 the others ; the divisions which increase the number of cells in 

 the posterior arm to four occur when the ^^g contains sixty-seven 

 cells, at the same time that the first transverse division in the 

 other arms is taking place. The appearance of bilateral sym- 

 metry in the cross in Umbrella, Heymons did not describe. 



The striking differences in the history of the cross in Planorbis 

 and Crepidula are most interesting. It is natural to seek for 

 some explanations of the problems which these differences pre- 

 sent to us. Why is it that the cleavage of the cell, which in 

 Crepidula results in a splitting of the arm of the cross, produces 

 in Planorbis a lengthening of the arm of the cross } Why does 

 the posterior tip cell in Crepidula divide before the others, and 

 in a different direction, while it does not divide at all in Planor- 

 bis .-* Why is the radial symmetry of the cross retained longer 

 in Planorbis than in Crepidula 1 And what is the cause of the 

 very different behavior of all of the tip cells in the two forms } 

 These are a few questions which suggest themselves when we 

 compare the history of the cross in these forms. A complete 

 solution of these problems is at present impossible, but we 

 may, perhaps, determine some of the proximate causes of these 

 differences of behavior. The transverse division of the cells, 

 i«'"^'', etc., in Crepidula is doubtless the result of the fact that 

 the long axes of these cells are transverse to the arm of the 

 cross. In Planorbis the basal cells in the arm, at the time they 

 begin to divide, have their longitudinal axes in the contrary 

 direction. Hence it is natural that their division should be 

 radial and not transverse. Now the different shape of these 

 cells in Planorbis is apparently due to the growth of the trocho- 

 blasts, which, as they increase in size, crowd the adjacent cells 



