222 



flat upper surface and a deeply convex lower sur- 

 face. 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 

 appearance. 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. 39). 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 



