356 PROCEEDINGS OF THE ACADEMY OF [April, 



quadrant cells of Fiona. Moreover, these cells, 3c^ and 3d', divide 

 again before the anterior ones as in Fiona, and these cleavages are the 

 first bilateral divisions described. It would appear from Heymons' 

 figures that the two cells next the median plain lie higher than the 

 outer, and this is the condition found in Fiona. If such be the case, 

 these two forms stand in contradistinction to Crepidula, in which the 

 median pair are the lower. The cells 3c", 3d" are the protoblasts 

 of Heymons' excretory cells, and it will be seen later that 3c" serves 

 a similar purpose in Fiona, It is interesting to note that Conklin says 

 of 3c" and 3d" that they are "large and clear" and "have the same 

 characteristics in Crepidula' \ though he does not know their fate. 

 Heymons describes divisions at a later stage in the anterior quadrants, 

 while in the posterior 3c" and 3c^^, 3d" and 3d'^ give rise by horizontal 

 divisions to small cells which lie next to 3c^ and 3d^ — these latter in 

 exact correspondence with Fiona. 



Of this quartet Holmes says of PZanor6ts; "The first cleavage forms 

 a transition from the spiral to the bilateral type, and subsequent 

 cleavages show a bilateral character in a more marked degree. 

 At nearly the same time the lower pair of cells in the two anterior 

 quadrants and the upper pair of cells in the posterior quadrants divide 

 in a nearly horizontal direction into equal moieties. Later the upper 

 pair of cells in the anterior quadrants divide in the same direction as 

 the lower pair. The lower pair of cells in the two posterior quad- 

 rants remain undivided until a much later stage". These divisions 

 closely follow those of Fiona, and the same may be said of subse- 

 quent ones. 



In Aplysia (Carazzi) the two third quartet cells of each anterior 

 quadrant divide into equal moieties, while in the posterior quadrants 

 small cells are given off toward the vegetative pole; the same is true 

 •of Fiona. At the next divisions of 3c' and 3d' "si dividono con fusi 

 i;ransversali, cioe con divisione bilaterale," while 3a' and 3b' remain at 

 Test. Viguier (1898) for Tethys describes the initial division of all 

 the foiu- quartet cells as "suivant des plans sensiblement radiaux", 

 the resulting two cells in each quadrant being equal. Later cleavages 

 of this quartet in Fiona will be considered under the discussion of 

 o-astrulation and secondary mesoderm formation. Bilaterality appears 

 late in the cleavage of Trochus. The first divisions of this nature do 

 not occur imtil the ninety-seven-cell stage, and are concerned with the 

 cells 2c" and 2a". This is the first violation of Sachs-Hertwig's law 

 of alternatingly perpendicular cleavages. The cleavages of the third 

 quartet are very tardy in this Prosobranch, for when there are as many 



