CELL-LINEAGE. 



third quartet. The first of these cases was observed by Wier- 

 zejski (1897) in the case of P/iysa, where the two anterior cells 

 of this quartet ( 3 , fo) give rise to mesoblastic as well as to 

 ectoblastic cells, and exactly similar facts were soon afterwards 



c 



FIG. 3. Diagrams illustrating the value of the quartets in a polyclade (Leptoplana), a lamelli- 

 branch (Unto), and a gasteropod (Crepidula) . Lettering and shading as in Fig. i. (For 

 comment on these figures see footnote at first page.) A, Leptoplana, showing mesoblast- 

 formation in the second quartet. (Cf. Fig. 4.) B, Crepidula, showing source of ecto- 

 mesoblast (from a 2 , IP, c 2 ) and entomesoblast (from quadrant Z>). C, Unto, ectomesoblast 

 formed only from a 2 . 



observed by Holmes in Planorbis. Simultaneously with these 

 researches I independently discovered in the annelid Aricia 

 two mesoblast cells arising from the two posterior cells of 

 either the second or the third quartet (i.e., from & and d* or 



