404 



INSECTA. 



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 pari- 

 etes from the edge of the 

 ventral plate. 



In Hydrophilus and 

 apparently in the Phry- 

 ganidas also, there are cer- 

 tain remarkable peculiari- 

 ties in the closure of the 

 dorsal surface. The fullest 

 observations on the subject 

 have been made by Kowa- 

 levsky (No. 416), but Dohrn 

 (No. 408) has with some 

 probability thrown doubts 

 on Kowalevsky's interpreta- 

 tions. According to Dohrn 

 the part of the serous enve- 

 lope which covers the dor- 

 sal surface becomes thick- 

 ened, and gives rise to a 

 peculiar dorsal plate which 

 is shewn in surface view in 

 ventral parts of the amnion 

 and serous membrane have 

 either been ruptured or 

 have disappeared. While 

 the dorsal plate is being 

 formed, the mesoblast, and 

 somewhat later the lateral 

 parts of the epiblast of the 

 ventral plate gradually 

 grow towards the dorsal 

 side and enclose the dorsal 

 plate, the wall of which in 

 the process appears to be 

 folded over so as first of 

 all to form a groove and 

 finally a canal. The stages 

 in this growth are shewn 

 from the surface in fig. 184 

 B and C and in section in 



FlG. 183. THREE STAGES IN THE DEVELOPMENT 



OF CALOPTERYX. (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 ; 

 mx 1 . maxilla i ; mx 3 . maxilla 2 ; /' /^ three pairs 

 of legs; of. oesophagus. 



fig. 184 A, do^ and in section in fig. 185 A, do. The 



FIG. 184. THREE LARVAL STAGES OF HYDRO- 

 I'lIIIIs FROM THE DORSAL SIDE, SHEWING THE 

 GRADUAL CLOSING IN OF THE DORSAL REGION WITH 

 THE FORMATION OF THE PECULIAR DORSAL ORGAN 

 do. (After Kowalevsky.) 



do. dorsal organ ; at. antennae. 



