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 is, of course, continuous. 



FIG. il 



Transverse section of germ band of Clytra at gastrulation. g, germ band; f, inner 

 layer. After LECAILLON. 



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. 

 190), while the invaginated portion spreads out as an inner 



FIG. 190. 



ac 



Transverse section of germ layers 

 and amnion folds of Clytra. a, am- 

 nion; e, ectoderm; i, inner layer 

 (meso-entoderm) : s, serosa. Original, 

 based on Lecaillon's figures. 



Transverse section of germ layers and 

 embryonal membranes of Clytra. a, am- 

 nion; ac, amnion cavity; e, ectoderm; i, 

 inner layer (meso-entoderm) ; s, serosa. 

 After LECAILLON. 



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 



