NORTH AMERICAN ODONATA. 



191 



placed just beyond the gelatinous mass. Both membrane and mass 

 increase the facility of entrance of the spermatozoa into the egg. 

 Within the very thin vitelline membrane are mostly yolk spheres 

 and fat drops with, of course, the egg-nucleus. 



The blastoderm does not apparently arise as a continuous layer, 

 but in isolated patches, which afterwards unite and form a single 

 layer of cells over the entire surface of the egg, but within the egg 

 membranes. Six to twelve hours from the time of oviposition were 

 required for its complete formation. The blastoderm then thickens 

 slightly at both poles and also on the ventral surface near the poste- 

 rior pole. This latter ih\cken'm g (the ventral ])late — Bauchplatte — 

 bp, fig. 33A) is shield-shaped and consists of two or more layers of 



S 



Fig. 33. 



Five diagrammatic median longitudinal sections of the egg of CalopUryx (after Kors- 

 chelt and Haider, based on Brandt). 



A — E, successive stages in development. The egg-membranes are omitted. 



a, anterior pole of egg. d, dorsal side of egg. 

 of, amnion fold. do, yolk. 



ah, amnion cavity. k, head end of the germinal band. 



am, amnion. k^, tail end of the germinal band. 



b, posterior pole of egg. kh, beginning of the invagination. 

 bl, blastoderm, J, serosa. 



bp, ventral plate. v, ventral side of egg. 



cells, but fades, without distinct boundaries, into the rest of the blas- 

 toderm. An invagination now takes place in the ventral plate in 

 the form of a tube-like insinking into the central part of the egg 



TKANS. AM. ENT. SOC. XX. AUGUST, 1893. 



