232 HAROLD R. HAGAN 



of these appendages as swellings it can be seen that each of the 

 two sternites is composed* of two sclerites. One is adjacent to 

 the median furrow and the other, lateral to it, is composed of 

 the tissue which formerly constituted the appendage. The two 

 are separated at first by a very pronounced suture, but in suc- 

 ceeding stages this is gradually obliterated. 



A description of the development of the appendages of the 

 cephalic and thoracic regions would be simply a record of growth 

 by successive slight modifications to form the nymphal struc- 

 tures. The chordotonal organs are now visible externally in the 

 tibiae as deep pits, although they, too, become obsolete some 

 time before hatching (fig. 4). The appendages entering into the 

 formation of the gonapophyses undergo profound modifications 

 from now on throughout embryonic development. 



The dorsal organ. The growth of the dorsal body wall follows 

 the fusion of the embryonic envelopes and their subsequent 

 rupture immediately over the ventral surface of the embryo. 

 An examination of the serosa at this time shows that a contraction 

 of its cells, particularly in the region of the indusium is taking 

 place. The fused part of the amnion and serosa contract and 

 fold back over the margin of fusion. The continued contraction 

 of the ventral serosa (dorsal to the embryo) pulls the remainder 

 of the amnion back over the yolk, leaving the embryo free in 

 the place between the dorsal yolk and the vitelline membrane. 

 The yolk is now enclosed by the embryo in the postero-dorsal 

 region, by the amnion on the posterior, postero-lateral and antero- 

 dorsal areas, while the serosa covers the remainder. As the body 

 wall now grows down over the yolk, the amnion is drawn, by 

 contraction of the serosal cells, along the future mid-dorsal line 

 of the embryo. Growth of the body wall proceeds from the 

 posterior and postero-lateral edges in advance of the anterior 

 regions. This results in fusion along the mid-dorsal line of the 

 embryo and the completion of the tergites successively from the 

 caudal end forward. 



The contracted serosa lags behind the forward growth of the 

 body walls and a large fold of it covers the proliferating margins 

 of the embryo. Two peculiar organs are now apparent in the 



