1904.] NATURAL SCIENCES OF PHILADELPHIA. 347 



ality must be very slight, as the radial symmetry of the upper pole is 

 not disturbed to any appreciable degree. 



By the divisions of the basal cells above described each arm of the 

 cross is composed of four cells — an outer tip cell (2a"-2d"), next to it 

 the "middle" cell (la^^'-kP-^), an inner ''basal" cell (la^^i.^^im)^ ^^^ich 

 is larger than its sister middle cell, and an apical (la"-ld"). 



S3nichronously with the cleavage of the basals occurs that of the 

 turrets, the cell of this series in each quadrant dividing into two of 

 nearly equal size, the outer being the smaller. All divisions are 

 dexiotropic and alternating with those by which these cells arose 

 (fig. 33). 



Comparing the cleavage of the turrets with conditions found in other 

 forms, it will be noted that considerable variation exists. While in 

 Fiona these cells divide when there are about sixty blastomeres in 

 the whole egg, in Umbrella (Heymons) approximately seventy are 

 present; like Fiona all four turrets divide at relatively the same time. 

 In Crepidula the anterior trochoblasts do not divide until there are 

 over one hundred cells in the egg, and Conklin states that he believes 

 the posterior ones never divide. The trochoblasts of Trochus (Robert) 

 arise very early, at the sixteen-cell stage, and have all divided when 

 there are thirty-two cells present. In Planorbis Holmes finds them in 

 division at about forty cells, and Limax (Kofoid) shows a similar con- 

 dition. In Unio (Lillie) there are about fifty cells, while in Ischno- 

 chiton (Heath) but thirty-two, when the "primary trochoblasts" of the 

 latter form divide. Thus Fiona appears to occupy an intermediate 

 position in relation to these and other molluscan forms in which the 

 time of cleavage of these cells has been determined. 



Division next occurs in the cross at a stage of about eighty-four 

 cells and results in the division of the apicals into eight small cells, 

 of which those lying centrally form the "apical rosettes" (la"^-ld"^), 

 while the outer series are the "peripheral rosettes" (la"^-ld"^) of 

 Conklin. Direction of cleavage is Iseotropic, and of the resulting ceDs the 

 outer are the larger (PI. XXVII, fig. 53). Shortly after the rosette series 

 are estabhshed the basal cells of all arms divide again, the posterior 

 one last. In the anterior quadrant the spindle and resulting cells, 

 Ib^-" and Ib^^^^ lie radially in the lateral arms, the division of Ic^'^ is 

 Inotropic, that of la^^^ dexiotropic, again showing bilateral influence, 

 while in Id^^^ the spindle is so strongly turned in Iseotropic direction 

 that the resulting cells lie transversely across the posterior arm (figs. 

 56, 62). While this last cleavage of the basals is being accomplished 

 a similar process is seen in the four inner trochoblasts (la-^-ld-^), result- 



