336 PROCEEDINGS OF THE ACADEMY OF [April, 
is radially symmetrical (Pl. XXIII, fig. 19). At the center of the upper 
pole lie four “‘apical”’ cells, while the ‘‘trochoblasts”’ or ‘turret cells”’ 
extend from them into the angles between the second and third quartet 
cells. The third quartet and first generation of second quartet lie 
between them and the macromeres beneath, but from the nature of the 
cleavages do not form so marked a ring as in Crepidula or other 
Mollusks with large macromeres. The ectoblast has been entirely 
separated from the underlying macromeres, which contain all of the 
entoblast and the greater portion of the mesoblast. A small portion 
of the latter is to be derived, as will be shown later, from the third 
quartet of ectoblast cells. The egg has become somewhat flattened 
along its polar axis and within is a small cleavage cavity, which arose 
during the last few divisions and which later becomes of considerable 
size. Upon the lower surface the polar furrow remains distinct and 
offers a convenient means of orientation. 
The fact that in Mollusks, Annelids and Platodes the entire ectoblast 
is separated from the entoblast by the first three successive divisions 
in which the macromeres participate is a point of similarity of the 
highest importance in considering the question of the possible genetic 
relationships of the groups. With scarcely an exception (Drevissensia, 
Meissenheimer, 1901) this is accomplished by regularly alternating 
spiral cleavages. In most cases the first three quartets of micromeres 
are small protoplasmic cells and differ widely from the yolk-ladened 
macromeres, and this is particularly true of the first series being corre- 
lated with the later history of the cells which compose it, since in all 
cases they form the apical pole and the sense organs of the larva. 
Where much yolk is not present, or the spherules are small, more equal 
cleavage results, so that the macromeres are reduced in size; as exam- 
ples may be cited many Pulmonates (Planorbis, Physa, Limnea, Limaz) 
and Lamellibranchs (Unio, Cyclas, Dreissensia), Chiton and Ischnochiton 
among the Amphineura, 7’rochus for the Prosobranchs and the Opistho- 
branchs Tethys and Fiona. The same is true of many Annelids 
(Podarke, Amphitrite, Clymenella, Arenicola, etc.). 
Both in size of cells and rate and direction of division the egg of 
Tethys (Viguier, 1898) exactly parallels that of Fiona up through the 
twenty-four-cell stage. The same may be said’ of Aplysia (Carazzi, 
1900, and Georgeovitch as corrected by Carazzi, 1900), except for the 
larger size of the macromeres, particularly the anterior ones, and Ca- 
razzi’s statement that the trochoblasts arise from division of the first 
quartet—‘‘con fusi distintamente dessiotropici.” Such is, however, 
not the case, as his figures show. Carazzi has evidently, in some 
