$4 ]'El!iK\NSO'^, Germinal Layers of Vertebrates. 



behaves from the first as an entire ovum, segmenting 

 normally and producing a whole embryo. It may be 

 remarked at this point that Hertvvig^" failed to obtain the 

 half-embryos of the frog which Roux produced by killing 

 one of the first two blastomeres ; that Schulze''' has made 

 double monsters by inverting the Qgg in the two-celled 

 stage ; and, most satisfactory of all, that Herlitzka"'^ has 

 succeeded in isolating the first two blastomeres of the 

 newt, each of which then developed into a complete larva. 



In Amphioxus Wilson*^" was unable to obtain a 

 gastrula from a ^ blastomere, though a ^ blastomere 

 readily gastrulated. But in Hydroids Zoja*'^ found that 

 even a xV blastomere, a cell which is probably either 

 ' ectodermal ' or ' endodermal,' would give rise to a normal 

 embryo. 



In the Echinoderms ^^, however, although eventually a 

 normal whole larva results, the segmentation of an isolated 

 blastomere is partial, a curved plate of cells being produced 

 which only gradually rolls up to form a hollow blastula. It 

 is only from |, \, or ^ blastomeres that perfectly normal 

 gastruljE can be obtained ; but the | gastrulae are of course 

 developed from either animal or vegetative cells. In this 

 connection — the point which for us is of especial interest — 

 Driesch has shown that a normal pluteus can be formed 

 from the eight animal cells alone, the eight vegetative cells 

 alone, or even the four macromeres alone of the sixteen- 

 celled stage. 



The Nemertines, as Wilson''" has recently stated, 

 resemble the Echinoderms ; segmentation of the isolated 



8 Loc. cit. 



8^ Arch. Ent.-mech., vol. i, p. 269 — 303, 1895. 



*" Arch. Ent-mech., vol. 2, p. 352 — 366, 1896 ; vol. 4, 624 — 654, 1S97. 



^'^ Joiirn. Morph., vol. 8, p. 579 — 638, 1893. 



"* Arch. Ent.-iJiech., vol. I, p. 578—594, 1S95 '■> ^o'- 2, p. i — 31, 1896. 



'^° Driesch, 11., Zeitschr. wiss. ZooL, vol. 53, p. 160 — 184, 1892 ; vol. 55, 

 p. I — 62, 1893; Arch. Ent.-mech., vol. 4, p. 75 — 124, 1897; Mitt. Zool. 

 Stat. Neapel, vol. il, p. 221 — 253, 1895. 



