600 



ARTHUR WILLEY. 



region of the embryo at a later stage. According to Wheeler's 

 perfectly clear account (12), the indusial thickening becomes 

 overgrown by the surrounding serosa, sinks below the surface, 

 becoming entirely separated from the serosa of which it is a 

 derivative, and gradually spreads over the entire egg until its 

 growing edges meet on the dorsal side of the vitellus and fuse 

 together. The indusium then forms a complete envelope round 

 the egg below the serosa, and it completely usurps the functions 

 of the latter. Why this substitution of an indusium for the 

 serosa should take place in Locustid embryos is a question not 

 easily answered. At present all that can be said is that it 

 happens so and there's an end. 



In the course of its development, Wheeler found that the 



Fig. 5. — Median section of egg of An u rid a maritinia, showing dorsal 

 organ with vacuolar cell contents, as a thickening of the blastoderm. 

 (After Wheeler.) 



indusium was subject to great time variations, growth varia- 

 tions, and even numerical variations. In two young embryos 



