CELL-LINEAGE. 



31 



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

 zejski (1897) in the case of Physa, where the two anterior cells 

 of this quartet {c^, b^) give rise to mesoblastic as well as to 

 ectoblastic cells, and exactly similar facts were soon afterwards 



Fig. 3. — Diagrams illustrating the value of the quartets in a polyclade {Leptoplatta), a lamelli- 

 branch (6^«/d)), and a gasteropod (Cr^//i/7</«). Lettering and shading as in Fig. i. (For 

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

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

 mesoblast (from a^, IP-, fi) and entomesoblast (from quadrant D). C, Unio, ectomesoblast 

 formed only from a'^. 



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 c'^ and d^- or 



