346 PROCEEDINGS OF THE ACADEMY OF [April, 



after the formation of the basals occupy positions just peripheral to 

 them and shghtly to the left. These four small second quartet ele- 

 ments are the "tip" cells of the cross, 2a"-2d", and together with the 

 basals and apicals form the ectoblastic cross. 



From the time of its formation and until a late period of cleavage 

 the cross of Fiona is a distinctly dexiotropic structure, the apicals of the 

 four arms lying to the right of their respective tips. The cross is thus 

 at the time of its formation (fig. 23) composed of twelve cells, of which 

 the apicals are the central, is radially symmetrical and its anterior and 

 posterior arms lie very near to, if not exactly in, the median plane of 

 the future embryo. In the future history of this structure the tip 

 cells will for convenience be described in connection with the rest of the 

 cross, since they are so closely connected with it. 



Before further cleavage occurs in the first quartet the second and 

 third quartets and the macromeres show marked karyokinetic activity, 

 the number of cells in the egg having increased to nearly sixty. The 

 basal cells and the turret cells or trochoblasts then divide simultaneously 

 (fig. 33), though considerable variation in time occurs in different eggs 

 and in different quadrants, it being, however, universally observed 

 that kP^ divides last of the basals. It may be noted in this connection 

 that in all species of Crepidula examined except C. achmca the division 

 in the basal cell of the posterior arm is delayed for a much longer period. 

 The direction of cleavage of the basals kP- and Ib^^ is Iseotropic and 

 so alternating with the last, those of the other two doubtful; W- 

 usually shows a laeeotropic to radial position of spindle, while in U-^- 

 variations are present all the way from Iseotropic to dexiotropic. After 

 examining a large number of eggs the occurrence of this irregularity 

 was more strongly confirmed, and it thus appears that in this cell, 

 Ic^-, there is a strong tendency, more marked in some case's than in 

 others, toward non-alternation with resulting bilaterality of cleavage 

 in relation to its opposite cell, la^'. In Crepidula, Planorhis and Ncri- 

 tina the cleavage of all these basal cells is non-alternating, while in 

 Umbrella it is regularly alternating. 



In Fiona it would appear that we have an intermediate condition in 

 which, though regular alternation is found in the anterior and posterior 

 basal cells, the two lateral, particularly Ic^-, show a tendency toward 

 non-alternation under the influence of approaching bilaterality. It 

 is just at this time that the first distinctly bilateral cleavages occur in 

 two cells of the third quartet in the two posterior quadrants, 3d^ and 

 3c^ (figs. 31, 32), and this suggestion of bilateral divisions of the cross 

 may be correlated with them. How^ever, the influence toward bilater- 



