VIII 



AETHEOPODA 



249 



germ 



rise to the ectoderm. A little later the area of the blastoderm with 

 sparse and large nuclei is seen to have spread out, so as to completely 

 cover the dorsal surface and also the sides of the egg, while in front 

 it encroaches on the ventral surface ; this change is effected, partly 

 at any rate, by the modification 

 of the cells of the blastoderm 

 from the previous columnar to 

 a flattened form. 



All that is left of the 

 original columnar cells is a 

 median streak occupying the 

 hinder part of the ventral 

 surface, and reaching forward 

 to about a distance of one- 

 fourth the length of the egg 

 from the front end. At the 

 posterior end it curves over a 

 little on to the dorsal surface, 

 but between the columnar 

 cells of the streak and the flat 

 pale cells of the " sheafti ecto- 

 derm " ordinary flat cells inter- 

 vene; they form, as we shall 

 see directly, the amnion or 

 inner embryonic membrane. 

 The streak constitutes the 

 germinal disc or embryonic 

 area, for on it the first organs 

 of the embryo appear. On it 

 are the two curved ridges 

 alluded to above, which divide 

 it into median and lateral 

 plates, the last-named being 

 very much encroached on by 

 the extension of the modified 

 ectoderm destined to give rise 

 to the embryonic membrane. 



Soon after this stage, at 

 the posterior end of the egg, 

 the fold appears which is 

 destined to cover in most of 

 the germinal area. This is termed the posterior amniotic fold. 



^-^/. ';.'i;ii|'!ii i::;nto"'^^ 



emb 



germ 



ser 



am 



FIG. 197. Diagrams to illustrate the formation 

 of the posterior amniotic fold in the egg of 

 Donacia crassipes. (After Hirschler.) 



Letters as in previous figure. A, youngest stage. 

 B, intermediate stage. C, oldest stage. 



Its 



outer limb, termed the serosa, involves almost exclusively the sheath 

 ectoderm ; its inner limb, termed the amnion, is composed of ordinary 

 blastoderm cells which pass without any break into the germinal 

 disc or embryonic area. A small part, however, of the embryonic area 

 itself is arched up into the amnion fold behind, and this is regarded 

 by Hirschler as a proof that the whole of the amniotic ectoderm is 



