TRACK EAT A. 



403 



becomes very thin, and 

 forms a covering corre 

 spending with the amnion 

 of the more ordinary types. 

 The remainder of the blas- 

 toderm covering the yolk 

 (se) forms the homologue 

 of the serous membrane 

 of other types. The ven- 

 tral surface of the ventral 

 plate is turned towards 

 the dorsal side (retaining 

 the same nomenclature as 

 in ordinary cases) of the 

 egg, and the cephalic 

 extremity is situated at 

 the point of origin of the 

 infolding. 



The further history is 

 however somewhat pecu- 

 liar. The amnion is at first 

 (fig. 182 C) continuous with 

 the serous envelope on the 

 posterior side only, so that 

 the serous envelope does 

 not form a continuous sack, 

 but has an opening close 

 to the head of the embryo. 



A 



FIG. 182. THREE STAGES IN THE DEVELOPMENT 

 OF THE EMBRYO OF CALOPTERYX. (After Brandt.) 



The embryo is represented in the egg-shell. 



A. Embryo with ventral plate. 



B. Commencing involution of ventral plate. 



C. Involution of ventral plate completed. 



ps. ventral plate; g. edge of ventral plate; am. 



amnion ; se. serous envelope. 



In the Hemiptera parasita this opening (Melnikow, No. 422) remains per- 

 manent, and the embryo, after it has reached a certain stage of development, 

 becomes everted through it, while the yolk, enclosed in the continuous mem- 

 brane formed by the amnion and serous envelope, forms a yolk sack on the 

 dorsal surface. In the Libellulidae however and most Hemiptera, a fusion of 

 the two limbs of the serous membrane takes place in the usual way, so as to 

 convert it into a completely closed sack (fig. 183 A). After the formation of 

 the appendages a fusion takes place between the amnion and serous enve- 

 lope over a small area close to the head of the embryo. In the middle of 

 this area a rupture is then effected, and the head of the embryo followed by 

 the body is gradually pushed through the opening (fig. 183 B and C). The 

 embryo becomes in the process completely rotated, and carried into a 

 position in the egg-shell identical with that of the embryos of other orders of 

 Insects (fig. 183 C). 



Owing to the rupture of the embryonic envelopes taking place at the 

 point where they are fused into one, the yolk does not escape in the above 

 process, but is carried into a kind of yolk sack, on the dorsal surface of the 

 embryo, formed of the remains of the amnion and serous envelope. The 



26 2 



