CELL-LIXEAGE, 11 



Nereis. They must, therefore, be regarded as vestiges of fiuie- 

 tional entoblast-cells sucJi as those of Xe?ris, ami vwrpliologically 

 they represent the posterior part of tJie entoblast-plate^ (Cf. Fig. i, 

 B; Fig. 2, A\ 



The foregoing interpretation is entirely in harmony with 

 Conkhn's important discoveries in the gasteropod Crepidida. 

 Conkhn here definitely showed, for the first time in any animal,^ 

 that the so-called " primary mesoblasts " give rise to a group of 

 entoblast-cells before dividing to form the mesoblast-bands. 

 But more than this, Crepidida represents a step in the series 

 which may be regarded as anterior to the condition found in 

 Nereis ; for here each mesentoblast divides off two entoblast- 

 cells, the bulk of which taken together is actually greater than 

 that of the mesoblastic material remaining, " less than half the 

 cell (4d) being destined to form mesoblast."^ The three forms 

 Crepidida^ Nereis, Arieia, thus form a progressive series in which 

 the entoblastic part of the mesentoblast cell is reduced from 

 more than half the bulk of the cell to an insignificant vestige. 

 It is probable that two intermediate steps besides Nereis have 

 been observed by Lillie and Mead respectively. The two cells 

 found by the first named observer, in [//do, are somewhat larger 

 than those of Nereis ;^ while in Clymenella as described by 

 Mead, they are equal in size to the mesoblastic moiety. "^ 



lit would be interesting to determine whether the vestigial cells of Arieia may 

 not be taken into the archenteric wall and thus still retain their functional signifi- 

 cance. I have not thus far been able to determine this point; but Mead's obser- 

 vations on Amphitritc seem to show that in this form such is not the case, for the 

 vestigial cells are here fornied so far from the surface that they pass into the cleavage- 

 cavity and are carried forwards at the tips of the mesoblast-bands. Mead himself 

 concludes that their position in Amphitrite is secondary, being a " reminiscence of 

 a surface division which still persists in many forms'' (1897, p. 295) I would sug- 

 gest that their position in A/iiphitrifevi\a\he due to the early inwanderingof the '* pri- 

 mar}- mesoblasts." It is not surprising that a vestigial cell of this kind should vary 

 somewhat in position ; and it should be recalled that in Xercis the later-formed cells 

 lie at some distance below the surface. In Arieia, too, the vestigial cells do not 

 always reach the surface. 



2 Compare, however, the somewhat similar earlier accounts of Patten tor Patella 

 (1896) and Stauffacher for Cyelas (1893). See Conklin, p. 71. 



^ Crepidtila, p. 69. 



* Unio, Fig. 60. 



51897, Fig. 88. 



