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 
