560 



TEXT-BOOK OF ENTOMOLOGY 



During the invagination of the middle plate and its transforma- 

 tion into the gastrula-tube a change takes place in its histological 

 character (Fig. 539, A and B). While it originally consists of a 

 high cylinder epithelium, which after farther changes becomes 

 divided into several layers, since the wedge-shaped single cells 

 push themselves over each other, the cells in later stages become 



more and more cubical or irregularly 

 polygonal (Fig. 539, B), and are irregu- 

 larly arranged. At the same time the 

 gastrula-tube is compressed in a dorso- 

 ventral direction. While it in this way 

 spreads out laterally under the side 

 plates (ectoderm), its originally circular 



I 

 ec 



B 



FIG. 537. Two successive stages in the gastrulation 

 of Apis. Cross-section through the primitive band : b, 

 lower (inner) layer; ec, ectoderm. After Grassi, from 

 Korschelt and Heider. 



primitive lumen passes into the form 

 of a horizontal fissure, which in Hydro- 

 philus long remains as the boundary 

 between the two layers of the inner 



side rjoraer tne mnuue piate teage 01 / 



the biastopore) ; m the partly seg- / lower) membrane. (Korschelt and 



merited middle plate (here = rudiment V wl 



of the mesoderm) ; 8, the segmented JTeider ") 



lateral plate (becoming afterwards the 



ectoderm of the primitive band); vf, 



^[f& cS re:'fr'Korsche;S There are numerous variations of the process 

 Heider. of gastrulation, which are by Korschelt and 



Heider divided into three types, as follows : - 



1. Through invagination and formation of a tube (Fig. 539, A, Hydrophilus, 

 Musca, Pyrrhocoris, etc.). 



2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera) . 



3. By an inward growth of cells from a median furrow (Aphides and Tri- 



choptera). 



In Doryphora and Lina (Fig. 524) the hinder end of the gastrula furrow is 



forked. 



