336 PROCEEDINGS OF THE ACADEMY OF [April, 



is radially symmetrical (PI. 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, l^ut 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 {Dreissensia, 

 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, Limncea, Limax) 

 and Lamellibranchs ( Unio, Cyclas, Dreissensia), Chiton and Ischnochiton 

 among the Amphineura, Trochus 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 di\nsion of the first 

 quartet — "con fusi distintamente dessiotropici." Such is, however, 

 not the case, as his figures show. Carazzi has evidently, in some 



