1907.] NATURAL SCIIOXCES OF IMIILADKr.IMI I A. 531 



cells correspnml closely with the "apical rosette" of annelids. In 

 Dinophilus (Nelson, 04) these cells arise by exactly the same division 

 as in Planocera. In Nereis (Wilson, 92) they arise one division earlier, 

 i.e., by the division of la*-ld^ In many moUusks, cells of this same 

 lineage la*' *-lrf'-'-^ are not formed until much later in the develop- 

 ment, e.g., in Crepidula at the eighty-eight-cell stage, in Fiona at the 

 sixty-four-cell stage, etc. The formation of these small apical cells in 

 the polyclad embryo is an interesting and significant phenomenon, 

 marking in another detail the close resemblance between the early 

 cleavage of these platodes and that of annelids and moUusks. The 

 fate of the apical rosette in the majority of annelids is the formation of 

 an apical sense organ with an apical tuft of cilia. In CapiteUa (Eisig, 

 98) and Dinophilus (Nelson, 04) the apical plate does not bear a tuft 

 of cilia. Conklin (97) finds that these cells form an apical sense plate 

 in the Crepidula veliger, without, howe-ver, bearing a bunch of large 

 cilia. 



As to the ultimate fate of these cells in Planocera, I must again 

 disagree with the previous students of polyclad embryology. Both 

 Selenka (81) and Lang (84) state that these cells sink down and are 

 finally covered over by the other ectoderm cells. Thus Lang (p. 337) 

 says : '' Ihre Abschniirung geschieht nicht ganz gegen den aboralen Pol 

 zu, sondern etwas nach innen, gegen die Furchungshohle, so dass sie, 

 wie auch Selenka bemerkt, den Bod en einer napfartigen A'ertiefung 



am aboralen Pol bilden Da spater in der Nahe der Stelle, an 



der sich die Scheitelzellen gebildet haben, in besonderen Zellen des 

 Ectoderms die Augen entstehen, so ware es moglich, dass aus ihnen 

 Theile des Nervensystems, vielleicht der sensorielle Theile des Gehirns 

 (oberes Schlundganglion?) entstiinden." Again, I believe the decep- 

 tive conditions of the living egg have led both Selenka and Lang 

 astray. It is true that by reason of the small size of these apical cells 

 the surrounding cells somewhat overtower them and form a "bowl-like 

 depression" (PI. XXXVII, fig. 21). However I do not find, as Lang 

 states, that by the further division of the first quartet cells the apical 

 cells are covered over and so sink beneath the surface of the ectoderm. 

 In focusing on an egg from the animal pole in stages shown in PI. 

 XXXVIII, figs. 22, 23 and 24, the first nuclei to come into view are 

 those of the apical cells. Furthermore these cells tend to move out 

 from the animal pole and remain on the surface (figs. 24, 29, 31). In 

 the later stages of segmentation they cannot be definitely distinguished 

 from other cells of the same size which come to lie near them. The 

 further diA'ision of the ectoderm cells in this region do not show any 



