INVOLUTION OF THE EMBRYONIC MEMBRANES 557 



1. Involution under the formation of a continuous dorsal amnion-serosa-sac 

 (Odonata). 



2. Involution with exclusively dorsal absorption of the ainnion (Doryphora). 



3. Involution with exclusively dorsal absorption of serosa and separation 

 of the ainuion (Chironomus and Trichoptera) . 



FIG. 532. Diagram of the formation of the dorsal walls in Doryphora in cross-sections : <t/, 

 ainnion ; in , serving as a provisional dorsal closure, in C, about to break up ; k, primitive band ; 

 6, serosa. After Wheeler, from Korschelt and Heider. 



4. Involution with separation of both embryonic membranes (Lepidoptera 

 and Hymenoptera, Hylotoma). 



The first type occurs in the most primitive order of winged insects. The 

 second type (Coleoptera) appears to be an independently inherited form of 



am. 



Fin. 533. Involution of the embryonic membranes of Chironomus : am, amnion ; r, dorsal 

 umbilicus; s, serosa, which has withdrawn into the region of the dorsal umbilicus, and in C'has 

 passed into the interior of the embryo. After Graber, from Korschelt and Heider. 



dorsal closure. In the first type, the formation of the amnion-serosa-sac is 

 initiated by a rupture of the two fused embryonic membranes. This rupture 

 in the ventral middle line occurs in Odonata only in the region of the head- 



