534 



TEXT-BOOK OF ENTOMOLOGY 



Hence we have eight layers in the winged insects * during embryonic 

 life: 



FIG. 510. Surface view of fresh serosa from an (Ecanthus, treated with acetic carmine; the 

 blastoderm completely formed, x 500 : p, polar body ; rf, radiating fibres ; nls, nuclear substance ; 

 nlm, nuclear membrane. After Ayers. 



1. Exochorion. (Remains of the epithelium of the ovarian follicle.) 



2. Chorion. (Egg-shell or cuticle secreted in the ovarian follicle.) 



3. Vitelline membrane. (Primary egg-niembrane. Yolk-skin or 



membrane.) 



4. Serous or outer germ-membrane. (Serosa.) 



5. Amnion or inner germ-membrane. 



6. Ectoderm. \ 

 Mesoderm. > Embryo. 

 Endoderm. ) 



7. 

 8. 



Derived from 

 the blasto- 

 derm. 



In the embryo of Xiphidium and Orchelimum Wheeler has found and described 

 with much detail a membranous structure which he calls the indusium. " The 

 organ," he says, "appears to have been retained by the Locustidse, and com- 

 pletely lost by the embryos of other winged insects." It arises in Xiphidium, 

 as a simple circular thickening of the blastoderm, between and a little in front 

 of the procephalic lobes (Figs. 511, 512, A-E}, and afterwards spreads over nearly 

 the whole surface of the egg, leaving the poles uncovered, as in Fig. 513, where it is 

 divided into two further membranes, the inner and outer indusium, the former 

 lying in contact with the amnion. After this the serosa "is excluded from 

 taking any part in the development of the embryo ; both its position and func- 

 tion are now usurped by the inner indusium." 



1 Korsehelt and Heider state that no cellular embryonal membranes are present 

 in Synaptera, Uljanin finding none in the Podurids. In the embryo of Isotoma 

 walker! i we, however, observed a membrane which we compared to the larval skin 

 of many Crustacea, and both Sommcr and Lemoine have detected in eggs of the same 

 group ac-iit.icular larval skin which is provided with spines for rupturing the chorion. 

 The amnion is also wanting in Proctotrupids (Ayers), and is rndimental in Muscidae 

 (Kowalevsky, Graber), in viviparous Cecidoinyidas, according to Metschnikoff, who 

 also states that in certain ants of Madeira the envelopes are represented only by a 

 small mass of cells in the dorsal region. 



