12 WILSON. 



Neither of these observers, it is true, suggests the interpretation 

 given above, Lillie somewhat doubtfully assigning to the super- 

 ficial cells the same fate as I originally did in Nereis^ while 

 Mead leaves the matter undetermined. It seems probable, 

 however, that we may look for the same fate for these cells as 

 in Crcpidiila or Nereis} indeed I venture to think that Lillie' s 

 observations are themselves open to such an interpretation.^ 



These facts, I believe, support the view which has been held 

 by many embryologists from the time of Kowalevsky onwards ^ 

 that the primary mesoblasts, or mesoblastic pole-cells of an- 

 nelids and mollusks must be regarded as derivatives of the 

 archenteron. In both these groups the primary mesoblasts are 

 derived from the posterior cell of the fourth quartet of '* micro- 

 meres," the lateral and anterior cells of which are, so far as we 

 know, strictly and always entoblastic. The facts indicate, fur- 

 ther, that a progressive process of differentiation in cleavage has 

 been going forward, through which the posterior cell of this 

 quartet has become more and more strictly given over to the 

 formation of mesoblast. The vestigial cells of Aricia, Spio, 

 AmpJdtritc and P/<r^;<?<9;'/;^" would seem to represent the last traces 

 of such archenteric origin of the teloblasts ; and it is possible, 

 indeed probable, that there are cases in which even these traces 

 have disappeared, the posterior cell of the fourth quartet being 

 strictly mesoblastic from the first. "^ 



^ Conklin has fully considered [Crepidula, p. 72) the apparently contradictory 

 case of Umbrella, as described by Heymons (1893), where cells exactly corre- 

 sponding to the "posterior enteroblasts " of Crepidiila are described as giving rise to 

 mesoblast. Despite Heymon's careful account, I venture to think that the case de- 

 mands re-investigation in the light of Conklin's work. In a recent account of the 

 mesoblast in Physa (1897), Wierzejski finds that small cells ( " mesoderm-micro- 

 meres' ' ) are budded forth not only from the ' ' primary mesoblasts ' ' but also from the 

 larger lateral cells derived from them. All these cells are assumed to be meso- 

 blastic, though their fate was not followed out (1897, p. 391). 



2 Unio, Fig. 67. 



3Cf. Kowalevsky, 1871, p. 30; O. and R. Hertwig, 1881, p. 47. Hatschek, 

 1888, p. 76; Rabl, 1889, p. 207, and earlier literature there cited* 



*This point must remain doubtful until renewed investigation shall show 

 whether the superficial budding is ever entirely suppressed ; for we cannot safely 

 infer its absence from existing accounts, and I am not convinced that my own state- 

 ment of their apparent absence in Polyinnia {^Nereis, p. 458) may not have rested 

 upon an oversight. 



