20 WHEELER. [Vol. VIII. 



During or more frequently at the close of the embryo's migra- 

 tion the primary serosa secretes from its whole outer surface 

 a thin chitinous cuticle. In my preliminary notes ('90^' '90°) 

 I wrongly designated this cuticle as the vitelline membrane, an 

 error which is, to a certain extent, pardonable, inasmuch as 

 the layer in question is structurally exactly like the vitelline 

 membranes of other insects. But it certainly cannot be 

 homologized with these membranes since it is secreted during 

 a comparatively advanced stage by an embryonic cell-layer, 

 the serosa, and not by the surface protoplasm of the un- 

 segmented egg. 



As soon as the embryo has taken up its position on the 

 dorsal surface, the yolk segments ; each vitellophag appro- 

 priating as many of the yolk-bodies as the radiating filaments 

 of its cytoplasm can hold together and fashion into a rounded 

 mass. Apparently the process is delayed in order that the 

 passage of the embryo through the yolk may be facilitated, for 

 obviously the embryo will move more easily over a prescribed 

 path through a mass of small mobile particles than between 

 large masses formed by the aggregation of such particles. The 

 yolk-masses, at first very distinctly marked, soon fuse with 

 one another so that their boundaries can be traced only by 

 reference to their centres, which coincide with the nuclei of 

 the vitellophags. 



After leaving the ventral face of the ^gg the embryo in- 

 creases greatly in length. Just before burying its tail in 

 the yolk and while still completely on the ventral surface it 

 measured only ./mm.; now it measures 1.7 mm. This in- 

 crease in length, as will be inferred from the foregoing descrip- 

 tion, is due to two causes : an intercalation of new segments 

 in front of the anal plate to complete the abdomen, and a 

 stretching of the segments thus arising. 



A glance at Fig. 6, which represents an embryo in the stage 

 of its greatest elongation on the dorsal surface, shows that 

 many important changes have taken place since it left the 

 ventral surface. The cephalic and thoracic appendages have 

 assumed a more definite character. The labrum {lb}) has sud- 

 denly appeared, the first and second maxillae (inx^, mx~) have 



