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TEXT-BOOK OF ENTOMOLOGY 



that it appears somewhat depressed or sunken in under the upper 

 surface of the yolk. While the amnion-folds are extending from all 

 sides over the primitive band, there becomes formed under it, by the 

 invagination of the outer surface of the egg, a cavity, the amnion- 

 cavity (a/*), which, when the amnion-fold has completely overgrown 

 the primitive band and united together (Fig. 509, C), appears com- 

 pletely closed from without. 



Formation of the embryonic membranes. The amnion-folds finally 

 completely overgrow the primitive band (Fig. 509, B and 0), and 



FIG. 508. Two schematic median sections through an insect-embryo to represent the develop- 

 ment of the embryonal membranes. In A the primitive streak is not wholly overgrown by the amnion- 

 fold. In B the amnion-folds have united with each other and completely overgrown the primitive 

 streak: a, fore, b, hind, egg-pole; v, ventral side; d, dorsal side: aj\ amnion-folds; <th, ainnion- 

 cavity ; am, arnnion ; do, yolk ; ec, ectoderm ; k, head-end, A-.',' hinder-end, of the primitive 

 streak; , the part of the serosa arising from the amnion-fold ; ', the part of the serosa arising 

 from the unaltered blastoderm ; u, lower layer. After Korschelt and Heider. 



form the embryonal membranes. The primitive band is seen after 

 its completion to be overgrown by a double cellular epithelial 

 membrane. The outer of these two membranes, that which arises 

 from the outer leaf or layer of the amnion-fold, is the serosa (Figs. 

 508, B; 509, C, s; 510). This passes continuously into the unchanged 

 part of the blastoderm, which has no part in the formation of the 



