270 



INSECTA. 



which covers the surface of the food-yolk. This part of the blasto- 

 derm, after the completion of the envelopes, is usually reckoned as 

 part of the serosa, so that in this sense we can say that the serosa 

 forms a closed sac covering the whole surface of the egg (Fig. 

 133 C, s), one part of it enveloping the surface of the food-yolk 

 and the other that of the germ-band.* 



The inner of the two envelopes covering the germ-band which was 

 derived from the inner layer of the amniotic fold is known as the 

 amnion (Figs. 132 and 133, am). This, at the edges of the germ- 

 band, is continued into the ectoderm of the latter, the transition being 

 in most cases quite gradual. The amnion and the ectoderm (ec) of 

 the germ-band thus together form an epithelial sac closed on all sides, 

 the lumen of which is represented by the amniotic cavity. 



Fig. 134. — Ventral aspect of five stages in the development of Hydrophilus (after Heider, 

 from Lang's Text-book). The anterior end is directed upwards, a and b, points at which 

 the blastopore closes ; af, edge of the amnion-fold ; of, caudal fold ; a/", paired cephalic 

 fold of the amnion ; an, antenna ; es, terminal segment ; g, pit-like invagination (rudiment 

 of the amniotic cavity); k, cephalic lobes; r, groove-like invagination; s, part of the 

 germ-band covered by the amnion. 



The origin of the germ-band is to be sought in a thickening of 

 the blastoderm on the ventral side of the egg (Fig. 133 A, bp). 

 While, as was mentioned above (p. 268), the blastodermic cells 

 originally exhibited the same shape and size over the whole surface 

 of the egg, they soon become differentiated in such a way that the 

 cells of the dorsal side flatten to form a thin pavement epithelium, 



* The fact that Graber (No. 27) observed in Melolontha the secretion of 

 a cuticle from the outer surface of the serosa, after the completion of the 

 development of the embryonic envelopes, deserves mention. A certain parallel 

 may perhaps exist between this process and the development of the blastodermic 

 cuticle in the Crustacea and other Arthropoda. 



