ORTHOPTEROIDEA (PANORTHOPTERA) 



227 



The round concave germ disk composed of long prismatic cells starts 

 forming as a thickening of the primary epithelial cells lying at the pos- 

 terior pole of the egg slightly on the ventral side (Fig. 138). The cells 

 of the extraembryonic primary epithelium are more or less spindle- 

 shaped with large nuclei. The germ disk afterward begins to elongate 

 at the ventroanterior end along the ventral side of the egg. The proto- 

 corm rapidly elongates and at about the 46-hour stage is twice as long as 

 the protocephalon. Four hours later the inner layer begins to show 

 segmentation, especially in the thoracic regions, and the rudiment of the 

 stomodaeum is in evidence. A little later the germ band has divided 

 into four primary segments: a protocephalic and three protocormic 



Fig. 145. — Locusta. Longitudinal section of 40-hour embryo, (am) Amnion, (aw. 

 cav) Amniotic cavity, {antr) Anterior end. (c/i) Chorion, {ect) Ectoderm. (t7) Inner 

 layer, (post) Posterior end. {ser) Serosa. ((/) Yolk, (yc) Secondary yolk cell. 



elements, i.e., four macromeres or macrosomites. Rudiments of three 

 jaw and three thoracic appendages are apparent at the 52-hour stage. 

 Definitive segmentation of the inner layer preceded that of the ectoderm. 

 By the 75-hour stage the entire abdomen becomes externally segmented 

 into 11 segments, thus establishing the definitive body segmentation. 

 Eye pigment first makes its appearance shortly before the beginning of 

 blastokinesis. 



After the primary segmentation of the inner layer into four macro- 

 meres the definitive segmentation of the inner layer takes place. At 

 the 52-hour stage the inner layer in the head and thorax is divided into 

 segments corresponding to the three gnathal (jaw) segments and the three 

 thoracic segments. The somites are also separated on the median line 

 (Fig. 146) and lie somewhat obliquely, the anterior inner end of each 

 meeting its fellow on the opposite side in the intersegmental region in 

 front. The segmental mesoderm masses develop coelomic cavities, and 



