i8 7 



flat upper surface and a deeply convex lower surface. 

 One end which contains the head of the embryo is 

 blunter and broader than the other. During the act 

 of hatching this end is forced open by the escaping 

 larvae. 



1. The Upper Surface. Observe that the upper 

 surface is generally granular or tuberculated in appear- 

 ance. At either extremity it is continuous with the 

 pointed ends of the ovum, and in this position there 

 are usually several small polygonal areas. The width 

 of the upper surface and the extent to which it is 

 encroached upon by the floats varies in different 

 species. 



2. The Lower Surface. The lower surface is 

 generally smooth and dark grey. In damaged ova a 

 silvery membrane will be seen partly detached, shewing 

 a deep shiny-black surface beneath. The silvery 

 membrane is the outer covering of the egg, and formed 

 by the layer of follicular epithelium (Fig. 47). In 

 some species the lower surface is marked with silvery 

 lines forming a reticular pattern. 



3. The Floats. Occupying about the middle 

 third of the side of the ovum is a remarkable structure 

 the float. This consists of a very delicate membrane 

 continuous with the chitinous cuticle covering the 

 whole ovum and containing air cells. 



The floats are generally oval in shape and shew 

 regular transverse corrugations. The shape and 

 position of the floats vary considerably in the different 

 species. 



4. The Frill. Around the margin of the upper 

 surface (forming the gunwale of the boat) there is in 

 some species a gleaming white frill-like structure. 

 This is striated in appearance, but portions of it may 



