CELL-LINEAGE. 



27 



three quartets, while the mesoblast-bands arise from the pos- 

 terior cell of the fourth quartet, the other three, with the 

 remains of the primary quadrants, giving rise to the entoblast 

 (Fig. i). The same general result has been reached by sub- 



FiG. I. — Diagram of the typical quartet-formation in an annelid or gasteropod; the quartets 

 numbered in the order of their formation ; A , B, C, D, the basal quadrants. Ectoblast 

 unshaded, mesoblast dotted, entoblast ruled in parallel lines. In many forms, (^.^., 

 Arki'a) a fifth quartet (entoblastic) is formed; in others (i?.,^., iV^r(?/>) only three com- 

 plete quartets and the posterior member of the fourth quartet {d* or M). 



sequent investigators of molluscan cell-lineage, though there 

 are one or two apparent exceptions {e.g., Teredo, according to 

 Hatschek) that demand reinvestigation. The same remarkable 

 fact holds true throughout the annelids,^ the well-determined 



1 See footnote at p. 36 for reference to Eisig's widely divergent account of 

 the development of Capitella. 



