No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 31 



huge flat cells and nuclei, is being as gradually restricted to a 

 more and more limited yolk surface. In consequence of this 

 restriction its component cells become broader radially and 

 narrower tangentially. In this stage the envelope functionally 

 corresponds to the "dorsal organ" of other insects. It cannot, 

 however, be thus designated without still further increasing 

 the number of heterogeneous structures included under that 

 unfortunate term, since the "dorsal organ" of other insects is 

 a thickening of an envelope represented in Xiphidmm by the 

 serosa. 



The thickened inner indusium is soon reduced to a cap of 

 cells on the anterior pointed pole of the ^gg. As the head of 

 the embryo advances to cover more of this pole, the envelope 

 is pushed further forward and finally stripped from the yolk 

 altogether. The anterior cranial walls then close over the 

 pole and thus effectually separate the yolk from the inner 

 indusium. The latter is reduced to a small conical mass, the 

 cells of which soon show unmistakable signs of degeneration. 



Soon after the embryo has thus rid itself of its envelopes 

 and has taken into its mesenteron the whole mass of yolk not 

 utilized in the processes of development hitherto undergone, a 

 chitinous cuticle is shed from its entire surface. This may be 

 designated as the first larval cuticle. It appears first on the 

 ventral abdominal surface and spreads thence headward and 

 dorsad. The progress of cuticularization is readily traceable 

 by staining embryos in this stage, for the parts over which the 

 cuticle is formed will not take the color ; where it is being 

 deposited the stain takes faintly and where it has not yet 

 appeared, the stain, of course, penetrates easily. Ayers ('84) 

 observed in CEcantJms that the secretion of the cuticle 

 began on the ventral surface of the embryo and extended 

 dorsad. This is just what we should expect from the fact that 

 the dorsal hypodermis is ontogenetically a more recent forma- 

 tion than that of the ventral surface. 



The first larval cuticle is about 5 /x thick and consists of 

 three layers. The innermost is apparently homogeneous and 

 stains deeply in Orth's lithium carmine while the middle layer 

 remains clear and vitreous. The outer layer is radially striated 



