74 ]eis!KINSON, Gervii;m/ Lajers of Vertebrates. 



devoted to the production of mesoderm alone, though 

 Wilson^' has stated that in Leptoplana some ectoderm is 

 formed from them as well. 



Similarly, while the cells of the third quartette are 

 usually devoted to ectoderm formation — ectoderm which 

 may surround the blastopore or provide the lining for the 

 stomodaeum or proctodaeum — there are many cases in 

 which a part, for example 3<; i, yi i in Capitella^'^ "^b i, 

 T,c I in Physa^f derivatives of 3«, 3^, 3c in Podarke^^ and 

 of all four quadrants in Dreissensia^^ "^d in Teredo^^ and 

 Cyclas^\ or the whole, as in the Turbellaria — though again 

 this is denied by Wilson — of these blastomeres contributes 

 to the larval or permanent mesoderm. 



In the fourth quartette the posterior cell ^d is as a rule 

 set aside for the exclusive development of the mesoblast 

 bands ; but in many cases this blastomere contains endo- 

 dermal elements as well {Nereis^^, Crepidiila, Aplysia, 

 Podarke, Dreissensid) ; while in the Turbellaria, and 

 apparently in Teredo^ it is purely endodermal. In 

 Capitella, on the other hand, it gives rise not to the meso- 

 blast bands, but to the larval mesoderm, together with 

 some ectoderm, and in TetJiys it is stated by Viguier to 

 be restricted with the other cells of the same quartette to 

 the ectoderm. 



The remaining cells of this quartette (4^, Afi, 4c) with 

 the residual macromeres (i\, B, C, and D) constitute the 

 endoderm. Here there are fewer exceptions although, as 



3^ Loc. cit. 



'^ Eisig, H., Loc. cit. 



32 Wierzejski, A., Biol. CentralbL, vol. 17, p. 388—394, 1897. 



=>* Treadwell, A. 'L.,Journ. Morph., vol. 17, p. 399—486, 1 901. 



3^ Meisenheimer. Loc. cit. 



»• Hatschek, B., Arb. Zool. Lnst. Wicn, vol. 3, p. 1—42, iSSi. 



2^ Stauffacher, W., Jenaische Zeitschr., vol. 28, p. 196 — 240, 1894. 

 W.'lson, E. '2,., Journ. Morph., vol. 6, p. 361 — 480, 1892. 



