CELL-LINEA GE. 



37 



3. In the third line lies the evidence, recently obtained, that 

 the pole-cells or teloblasts of the mesoblast-bands of the anne- 

 lids and mollusks are to be regarded as derivatives of the 

 archenteron, and hence differ wholly from the ectomesoblast in 

 their relation to the primary germ-layers. Kowalevsky, the dis- 

 coverer of these teloblasts, expressed the opinion, more than 

 twenty-five years ago (1871), that they were to be regarded as 

 derivatives of the archenteron ; and a large number of later 



FIG. 5. Diagrams comparing the early divisions of the posterior cell of the fourth quartet 

 (d* or M) in Crepidula (a), Nereis (b), Clymenella (c), Unto (d), and Aricia (e). The 

 numerals show the order of division. Cells destined to form entoblast (their fate as actu- 

 ally observed in Crepidula and Nereis, but only inferred in the other cases) ruled in 

 parallel lines, mesoblast dotted. After the divisions here shown, the symmetrical mesoblast- 

 bands are formed from the dotted cells. 



workers from the time of Rabl (1876) have accepted his view, 

 though only very recently has the full strength of the evidence 

 been developed. In the first place, it was shown through the 

 studies of Rabl, Blochmann, and later workers, that while the 

 posterior cell of the fourth quartet gives rise to the mesoblastic 

 pole-cells the other cells are purely entoblastic. In the second 

 place, the recent studies of Conklin and myself have shown that 

 even the posterior cell of the fourth quartet (d*) may contain 

 entoblastic as well as mesoblastic material. I showed several 



