CLEAVAGE AND DEVELOPMENTAL PATTERN 



549 



give rise to ectoderm and mesoderm, and the fourth izd) is entirely ecto- 

 dermal (Conklin, 1897). In Unio three are wholly ectodermal, only one 



Fig. 174, A-D. — Cleavage stages of mollusks. A, Unio, first and second quartets, 2d 

 largest (after F. R. Lillie, 1895); B, Crepidula, 2d no larger than other second quartet cells 

 (after Conklin, 1897); C, D, Fiilgur, first and second quartets, including 2d, all relatively very 

 small (after Conklin, 1907). 



{2h) ectomesodermal (F. R. Lillie, 1895); while in polychetes all are ap- 

 parently wholly ectodermal, except for the doubtful origin of mesoderm 

 from cells of the second or third quartet of Aricia (Wilson, 1898). If the 



