1904.] NATURAL SCIENCES OF PHILADELPHIA. 349 



its component cells are neither among themselves distinguishable nor 

 may they be definitely separated from the surrounding blastomeres. 

 Of course, this is largely due to the multiplication of the trochoblasts 

 and the similarity in size of most of the cells upon the upper surface of 

 the egg, yet the Iseotropic twist given to the basal elements at their 

 initial cleavage is largely responsible for that irregularity of contour 

 which so early marks the outlines of the cross. The peripheral ends 

 of the arms of the cross of Fiona become strongly twisted to the left, 

 and as the structure becomes older the ends tend to bend around in 

 that direction to a marked degree, greatly confusing their component 

 cells with those arising by multiplication of the trochoblasts. Up to 

 the stage shown in fig. 53 the cross has, with the exception of a slight 

 tendency toward variation in the first division of the basals, been 

 radially symmetrical, but at the next cleavage of the basals the cell 

 of this series in the posterior arm divides so that its daughter cells 

 lie transverse to the longitudinal axis of this arm. In the anterior 

 quadrant this division produces cells which lie radially, while in C 

 quadrant the cleavage is Iseotropic, in A dexiotropic. 



The first indication of transverse splitting of the arms is thus seen 

 to occur in the basal cell of the posterior quadrant. In Crepidula the 

 reverse is the case, the anterior and lateral arms alone increasing in 

 width, while the posterior later elongates by radial cleavages. In Fiona 

 all the arms become longitudinally split at a later period. The inner 

 and outer rosettes have not A'et arisen in Crepidula when the splitting 

 begins in the cells, la-b-c^-^, while in Fiona they are present and the 

 egg contains many more cells, the basal cells of the anterior and lateral 

 arms having again divided in such a manner that these arms are length- 

 ened before increase in breadth occurs. The same is true of Planorbis. 

 The early splitting of the arms of the cross in Crepidula is probably in 

 part due, as Holmes suggests, to the fact that, through pressure, they 

 have become much wider and tend to divide in a direction opposite to 

 this elongation. It might also be suggested that the extreme breadth 

 of the cross of Crepidula and the early transverse division of its anterior 

 and lateral arms may be correlated with the presence of a large amount 

 of yolk which must be covered b}' the ectoblast, while in the posterior 

 region the extensive multiplication of the elements of the second quar- 

 tet obviates the necessary broadening of the arm which reaches in that 

 direction. 



The transverse cleavage of the anterior and lateral arms of the cross 

 of Fiona occurs shortly after the initiation of a similar process in the 

 posterior arm, but it has been found impossible to trace the lineage 



