Xn] EMBRYOLOGY 233 



sides of the cavity, the amnion merges into the ventral plate. 

 The part of the blastoderm not drawn into the cavity, but re- 

 maining surrounding the yolk, may from now on be termed the 

 serosa (se). 



Even before the invagination is completed, the ventral plate 

 begins to change its shape by the upgrowth of its antero-lateral 

 borders, so that the anterior border becomes slightly concave in 

 the middle (fig. 109 c). This marks the beginning of the formation 

 of the head. By the time the invagination is completed, the two 

 large cephalic lobes (cl) are very distinctly formed, lying just inside 

 the blastopore. 



Amnion and Serosa. We may here follow out the further 

 development of these single cell-layers. As the invaginated 

 ventral plate takes on the distinct form of the embryo, the amnion 

 grows out ventrally on either side of it, in two folds. These finally 

 grow completely round the embryo, and unite together to enclose 

 it within the amniotic cavity. The latter is filled with a fluid 

 called the amniotic fluid. The cavity is finally completed in the 

 region of the blastopore, where the amnion fuses with the serosa. 

 The function of the amnion is to protect the early embryo from 

 shocks. Except just at the closed-up blastopore, amnion and 

 serosa are definitely separated by a large amount of yolk. Thus 

 all the embryo except the head now lies well within the egg. This 

 condition is known as the "immersed germ-band." Other insects 

 which share with the Odonata the peculiarity of having an immersed 

 germ-band are the Lepidoptera, most Hemiptera, and a number 

 of Orthoptera. 



The Germinal Layers. 



We must now pause to follow the formation of the three 

 germinal layers, or layers of cells, from which all the embryonic 

 structures are derived. 



The Ectoderm. This is formed directly from the outermost 

 layer of cells of the invaginated ventral plate. These become 

 differentiated off from the cells underlying them at a very early 

 stage, by the assumption of a compact columnar form, due to 

 mutual lateral pressure. The ectoderm covers the whole of the 

 ventral surface of the embryo. At first it passes insensibly, on 



