No. 3.] SERPULORBIS SQUAMIGERUS CARPENTER. 121 



Recent researches render it probable that the cell 4 d is not 

 typically a purely mesoblastic cell. As Conklin has pointed 

 out, the divisions of 4 d in Umbrella are strikingly like those in 

 Crepidula, and strongly indicate, as Conklin maintains, that 

 this cell contains both mesoderm and entoderm. The same 

 cell in Cyclas was held by Stauffacher 1 to produce both meso- 

 derm and entoderm. In Unio, Lillie 2 found that the pole cells 

 budded off small cells at the surface before giving rise to the 

 mesoblastic bands, and among annelids similar phenomena have 

 been observed by Mead 3 in Amphitrite, by Wilson 4 in Nereis 

 and Aricia, and by Treadwell 5 in Podarke. In Planorbis 6 I 

 have found that a minute cell is budded off from each of the 

 pole cells before they divide to form the mesoblastic bands. 

 It has been suggested by Wilson that these minute cells corre- 

 spond to the small entomeres found in Nereis, but they are 

 budded off in a different direction and lie in the cleavage cav- 

 ity instead of on the surface. This does not prove, however, 

 that they are not homologous with entodermic cells, as a slight 

 change in the direction of division of the pole cells would bring 

 them in the wall of the blastula. And as they are probably 

 not functional they may represent the last vestige of the ento- 

 dermic portion of the mesentomeres. 



1 Jen. Zeit. Bd. xxviii. 1893. 



2 Journ. Morph. Vol. x. 1895. 



3 Journ. Morph. Vol. xiii. 1897. 



4 Ann. N. Y. Acad. Sci. Vol. xi. 1898. 



5 Biological Lectures, delivered at Woods Roll, Session of 1898, 1899. 



6 Zobl. Bull. Vol. i. 1897. 



