Manchester Memoirs, Vol. I. (1906), No. 3. 75 



just mentioned, 4^, 4/7, 4^ in TetJcys are ectodermal and in 

 Cydas and Teredo the macromeres in these three quad- 

 rants as well ; (in the two latter genera the first division 

 is so unequal, as also the following division of the larger, 

 posterior cell, that A, B, and C were termed by Stauffachcr 

 and Hatschek micromeres in contrast to the larger 

 macromere D| In Clepsine no endoderm is derived 

 from D^l 



There is clearly then no necessary regularity in the, 

 subsequent behaviour of cells which in origin are identical; 

 and if now conversely, we trace back identical organs to 

 their origins precisely the same lack of agreement will 

 become manifest. 



The prototroch, for example, though always ecto- 

 dermal, is not limited to a particular set of cells : for 

 while it is true that certain cells in the first quartette — 

 derivatives of la. 2. — \d.2 — always take part in its forma- 

 tion, it frequently happens that these are reinforced by 

 secondary trochoblasts derived from the second quartette 

 {Crepidula, Trochus,^'^ Ainphitritef' etc.). The stomodaeum 

 again, usually formed from the second, may {Aremco/a/^ 

 arise from the third, or sometimes {Capitella and 

 IscJinochitonY from both second and third quartettes. 

 Nor does the ectoderm itself even escape these variations ; 

 ordinarily derived from the first three quartettes of 

 micromeres its formation may (Turbellaria, though not 

 according to Wilson) be limited to the first quartette, or- 

 on the other hand extended to the fourth ( Tethys) and 

 even to some of the residual macromeres {Cyclas and 

 Teredo). 



^^ Whitman, C. O., Quart. Journ. Micr. Set., vol. i8, p. 215—315, 1878. 



*" Robert, A., Arch. Zool. Exp., ser. 3, vol. 10, p. 270—513, 1902. 



*^ Medid, Journ. Morph., vol. 13, p. 227 — 326, 1897. 



*- Child, C. M., Arch. Ent.-inech., vol. 9, p. 587 — 709, 1900. 



*^ Heath, H., Zool. Jahrb. {Altai.), vol. 12, p. 567—656, 1899. 



