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 



z-c 



FIG. 587. Two successive stages in the gastrulation 

 of Apis. Cross-section through the primitive band : 6, 

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

 Korschelt and Heider. 



primitive lumen passes into the form 

 of a horizontal fissure, which in Hydro- 



O30. trastrma stage of the i_ -i i .1.11.1 



(Chaiicodoma), so-caiied flask- philus long remains as the boundary 

 between the two layers of the inner 

 (or lower) membrane. (Korschelt and 

 Heider.) 



FIG. 636. Gastrula stage of the 



shaped stage: /, folds which on each 

 side border the middle plate (edge of 

 the blastopore) ; m, the partly seg- 

 mented middle plate (here = rudiment 

 of the mesoderm) ; ft, the segmented 

 lateral plate (becoming afterwards the 

 ectoderm of the primitive band) ; ve, 

 fore, he, hinder entodermal rudiment. 

 After Carrie-re, from Korschelt and 

 Heider. 



There are numerous variations of the process 

 of gastrulation, which are by Korschelt and 

 Heider divided into three types, as follows : 



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



