350 PROCEEDINGS OF THE ACADEMY OF [April, 



of all the cells accurately though, after lateral extension has occurred, 

 the structure may be demarkated from the trochoblasts and underlying 

 second quartet cells. In fig. 75 its structure and probably cell deri- 

 vation may be seen. Holmes finds for Planorbis that the tip cells 

 divide in a transverse direction first, while in Crepidula the middle 

 cells are the first to cleave. The tips appear to divide last in Fiona. 

 In the posterior arms after the first transverse division most of the cells 

 divide obliquely across the arms, and in this way the arm becomes longer 

 than the other three. While the cross is increasing in lateral extension 

 the outer turret cells of all quadrants divide, so that the four groups 

 each consist of four cells of equal size (fig. 75) lying in the angles 

 formed by the arms of the cross. 



The apical pole of the egg at this period shows' a sHght depression 

 in the region of the rosette series. It is but transient and disappears 

 with the elongation of the gastrula. A similar depression has been 

 observed in Neritina, Crepidula and Trochus. Whether the structure 

 is normal in Fiona is yet doubtful. Robert insists that such is the case 

 with Trochus. 



The entire formation of the cross of Trochus is peculiar. The basals 

 have arisen and divided before the tips appear, and this division of 

 the basals is so directly laeotropic as to be practically transverse. At 

 the next cleavage these two cells form an oblong group of four in each 

 arm. The tips which lie peripherally to these groups next divide, 

 the cleavages of 2a" and 2c" being bilateral, the first of this nature to 

 occur in the egg. 



From the cases cited above of the manner of formation of the 

 ectoblastic cross of Mollusks, it will be seen that this characteristic 

 structure shows great diversity of details throughout the group, 

 though fundamental similarity is evident. Some of the probable 

 causes of such variation are (1) varying amounts of yolk, leading 

 to early lateral extension of the arms in those forms possessing 

 yolk-ladened entomeres, and (2) differences in the manner and rate 

 of development of the trochoblasts, correlated with the later structure 

 and functional importance of the locomotor organ to which they 

 largely give rise. The radial arrangement of blastomeres around the 

 apical pole of the cleaving egg is primarily the result of successively 

 alternating spiral cleavages, and a similar arrangement may be expected 

 in eggs which exhibit this mode of division. A definitely marked cross 

 does not always arise from such an arrangement of blastomeres, as, 

 for example, in Polyclad cleavage, so that this but suffices as a partial 

 explanation. Regarding the form of the crosses of Mollusks and 



