VIVIPAROUS FISH. 117 
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a most complicated and strangely beautiful vas- 
cular arrangement—a network of vessels, the 
use of which is clearly to convey the lifegiving 
fluid to the infant fish, and carry it back again, 
after having served its destined purpose, to be 
revivified for future use. The way this sac is, 
as it were, folded, and the different compart- 
ments made for the accommodation of the embry- 
onic fish, is most singular, and very difficult to 
describe clearly. 
The best illustration I can think of is an 
orange. You must imagine the orange divided 
into its regular number of little wedge-shaped 
pieces, and each piece to represent a fish; that 
the rind of the orange is a delicate mem- 
brane, having a globular shape, and easily com- 
pressed or folded. You now desire to fit the 
pieces together again in the original orange- 
shape, but you must begin on the outside of the 
globular membrane, pressing in with each sec- 
tion a fold of membrane (remember that each 
represents a fish) ; when each piece is in its place, 
you will still have the sac in its rounded form, 
but the rind or membrane has been folded in 
with the different pieces. If I have made 
myself understood, it will be seen that there 
must be a double fold of membrane between 
