THE EMBRYONIC MEMBRANES 



535 



Hence in an egg of the Locustidse Wheeler distinguishes, passing from within 

 outward in a median transverse section of the egg, the following envelopes : 



1. The chorion. 



2. The blastoderm-skin-like cuticle secreted by the serosa. 



3. The serosa. 



4. The outer indusium. 



5. A layer of dark granular secretion (probably some urate). 



6. The cuticle secreted by the inner indusium. 



7. The inner indusium. 



8. -The amnion. While envelopes 1-7 invest the whole egg ; layer 8, the 

 amnion, covers only the embryo. 



A BO 



FIG. 511. Diagrams illustrating the movements and envelopes of the embryo of Xiphidium : A, 

 after the closure of the amnioserosal folds. B, during the embryo's passage to the dorsal surface. 

 C. .just after the straightening of the embryo on the dorsal surface ; ind, indusium afterwards form- 

 ing iiul 1 , the inner, and ind z , the outer indusium; ch, chorion; sr, serosa; am, amnion; gb, 

 germ-band ; v, yolk ; bl. c, blastoderm membrane. 



Wheeler further suggests that the so-called micropyle of the Collembola 

 (Anurida), which has been homologized with the "dorsal organ 1 ' of Crustacea, 

 is a possible homologue of the indusium, as also the "primitive cumulus" of 

 spiders, and the "facette" or "cervical cross" of Pentastomids described by 

 Leuckart and also by Stiles. 



The gastrula stage. The primitive band invaginates so as to give 

 the opportunity for the formation of the inner layer. This invagi- 

 nation, which at a certain stage is established along the whole 

 length of the primitive band, forms a median furrow and may be 



