Omphalos: An Attempt to Untie the Geological Knot

by Philip Henry Gosse

PREGNANT ASPLANCHNA.
a. Unborn young.

In the beautiful, comparatively large, and economically singular genus, Asplanchna, the same process of development can be watched with perfect facility through every stage.

In the body of the female parent, as transparent as the clearest glass, the band-like ovary is seen floating in the visceral cavity, with several ova in various degrees of advancement. We trace one of these till it becomes a manifestly living young in the ovisac, lying along at the bottom of the parental cavity, more than one-third of whose volume is occupied by it:—supposing it to be a female infant. All its organs,—the eyes, the jaws, the stomach, the pancreatic glands, the ovary with its nuclei, the muscles, the rotatory cilia, &c. can be traced with the utmost distinctness long before birth, and its motions are strong and voluntary.

Neither in this case, nor in that of Rotifer, does the young animal pass through any metamorphosis; the unborn young has the full development of the parent, in every respect but size. In each case, the visible life-history of the individual commences not at birth, but at a period long antecedent, if indeed it can be said to commence at all, where we see it gradually developed from a nucleus, which was an integral part of the parental ovary, even before that parent's birth.

In the case of the amusing little Water-fleas (Daphnia), we have another example of viviparous generation, which, owing to the same cause as in the Rotifera,—the transparency of the integument, can be followed through all its stages by the eye of the observer. The eggs of this little Crustacean are deposited in a special chamber within the valves of the parent, where they are hatched. The young remain in their receptacle for a period, which varies according to the temperature, but long enough for them to undergo important changes in structure, and to pass their first moult.[96]

Here, again, it is impossible to select a condition which does not take hold of a pre-existence; for the youngest independent stage is dependent on earlier stages; and these are passed in visible connexion with the parent.

It is true there is in this genus, another mode of reproduction, by means of eggs which are thrown off enveloped in an organic covering, called the ephippium. If this condition be selected for the argument of my supposed opponent, I reply that it amounts to nearly the same thing; only the case will then come into the category of those animals whose earliest stages are protected by coverings formed from the body of the parent,—like the Hypogymna, and the Cockroach, already alluded to.

Where then, in these species, can we possibly select a stage of life, which is not inseparably and even visibly connected with a previous stage?

If we come to the vertebrate creatures, the argument becomes assuredly not less convincing. The formidable Shark, which we considered as a well-toothed adult ready for slaughter, let us suppose to have been created in the harmlessness of infancy. It is a slender thing, some ten or twelve inches long, bent upon itself, inclosing in the ring thus made, the vitellus or yelk-bag, the contents of which are in process of being absorbed into the abdomen. But the whole,—Shark, yelk-bag, and all—is imprisoned in a brown horny capsule, that looks like a pillow-case, with long tapes appended to the four corners.