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 membrane, having a globular shape, and easily compressed 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 section 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 each portion of orange. This is exactly the way the fish are packed in this novel placental sac. If it were practicable to remove each fish from its space, and the sac retain its normal shape, there would be twelve or fourteen openings (depending upon the number of young fish), the wall of each division being a double fold of membrane—the double edges wrapping or, as it were, folding over the fish. Now make a hole in the end of this folded bag, and _blow_ it full of air, and you get at once the globe-shaped membranous sac I have likened to an orange. The fish are always arranged to economise space: when the head of a young fish points to the head of its mother, the next to it is reversed, and looks towards the tail. I am quite convinced that the young fish are packed away by doubling or folding the sac in the way I have endeavoured to describe. I have again and again dissected out this ovarian bag, filled with fish in various stages of development, and floating it in saltwater, have, with a fine-pointed needle, opened the edges of the double membranous divisions that enwrap the fish—(the amount of overlapping is of course greater when the fish is in its earlier stages of development). On separating the edges of the sac, out the little fishes pop. I have obtained them in all stages of their growth,—but sometimes (and this not once or twice, but often) have set free the young fish from its dead mother. Thus prematurely cut loose from its membranous prison, the infant captive, revelling in its newly-acquired liberty, swam about in the saltwater, active, brisk, and jolly, in every particular, as well able to take care and provide for itself as its parent. The female external genital opening is situated a little posterior to the anal opening; the orifice is at the apex, and in the centre of a fleshy conical protuberance, which is in fact, a powerful sphincter muscle, _moored_, as it were, in its place by two strong muscular ropes, acting from and attached to the walls of the abdomen.