334 



INSECT A. 



The ventral 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 peculiar. 

 The amnion is at first (fig. 

 182 C) continuous with the 

 serous envelope on the pos- 

 terior side only, so that the 

 serous envelope does not 

 form a continuous sack, but 

 has an opening close to the 

 head of the embryo. In 

 the Hemiptera parasita this 

 opening (Melnikow, No. 

 422) remains permanent, 

 and the embryo, after it 

 has reached a certain stage 

 of development, becomes 

 everted through it, while 

 the yolk, enclosed in the 

 continuous membrane form- 

 ed by the amnion. and 

 serous envelope, forms a 

 yolk sack on the dorsal 

 DEVELOPMENT surface. In the Libellulidje 

 however and most Hemi- 

 ptera, a fusion of the two 

 limbs of the serous mem- 

 brane takes place in the 

 usual way, so as to convert 

 it into a completely closed 

 sack (fig. 183 A). After 

 the formation of the ap- 

 pendages a fusion takes place between the amnion and serous envelope 

 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 

 walls of the yolk sack either assist in forming the dorsal parietes of the 

 body, or are more probably enclosed within the body by the growth of the 

 dorsal parietes from the edge of the ventral plate. 



In Hydrophilus and apparently in the Phryganiclse also, there are 

 certain remarkable peculiarities in the closure of the dorsal surface. The 

 fullest observations on the subject have been made by Kowalevsky (ISTo. 416), 



FIG. 183. THREE STAGES IN THE 

 OF CALOPTEEYX. (After Brandt.) 



The embryo is represented in the egg-shell; B. 

 and C. shew the inversion of the embryo. 



se. serous envelope ; am. amnion; ab. abdomen; 

 v. anterior end of head; at. antennae; md. mandible; 

 ma; 1 , maxilla 1 ; mx~. maxilla 2 ; p l p 3 . three pairs 

 of legs ; oe. oasophagus. 



