148 ENTOMOLOGY 
Germ Band.—The blastoderm, at first of uniform thick- 
ness, becomes thicker in one region, by cell multiplication, 
forming the germ band (primitive streak, etc.) ; this appears 
in surface view as an oval or elongate area, denser than the 
remaining blastoderm, with which it 1s, of course, continuous. 
Transverse section of germ band of Clytra at gastrulation. g, germ band; 7, inner 
layer.—After LEcCAILLON. 
Gastrulation.—The germ band next infolds along the me- 
dian line, appearing in cross section, as in Fig. 189; the 
two lips of the median groove close together over the inva- 
ginated portion and form an outer layer, or ectoderm (Fig. 
1g0), while the invaginated portion spreads out as an inner 
Fic. 190. Fic. 1ot. 
Transverse section of germ layers Transverse section of germ layers and 
and amnion folds of Clytra. a, am- embryonal membranes of Clytra. a, am- 
nion; e, ectoderm; 7%, inner layer nion; ac, amnion cavity; e, ectoderm; 1, 
(meso-entoderm): s, serosa.—Original, inner layer (meso-entoderm); s, serosa. 
based on Lécaillon’s figures. ° —After L&CAILLON. 
layer, which is destined to form two layers, known respectively 
as entoderm and mesoderm. This formation of two primary 
germ layers by invagination or otherwise is termed gastrula- 
tion; it is an important stage in the development of all eggs, 
and among insects several variations of the process occur. 
Amnion and Serosa.—Meanwhile, the blastoderm has been 
