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Updated: May 24, 2025
We are so much accustomed to see differences in structure between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to look at these facts as necessarily contingent in some manner on growth.
We may see, without further discussion, how the representation given by Claus of the development of the Copepoda may pass almost word for word as the primitive history of those animals; we may find in the Nauplius-skin of the larvae of Achtheres and in the egg-like larva of Cryptophialus, precisely similar traces of a transition towards direct development, as were presented by the Nauplius-envelope of the embryos of Mysis and the maggot-like larva of Ligia, etc.
In the animal world, and partly also in the plant world, the single individuals of higher species in their embryonic development pass through states of development, in the former stages of which not only the individuals of the most different species look confusingly similar to one another, but also the embryos in their organization remind us of the perfected state of much lower classes of beings.
On the following morning I expected to achieve still greater intimacy in the lives of the mummy soldier embryos; but at dawn every trace of nesting swarm, larvæ, pupæ and soldiers was gone. A few dead workers were being already carried off by small ants which never would have dared approach them in life.
So that if you suppose a single polype of this kind settled upon the bottom of the sea, it may by these various methods that is to say, by cutting itself in two, which we call "fission," or by budding; or by sending out these swimming embryos, multiply itself to an enormous extent, and give rise to thousands, or millions, of progeny in a comparatively short time; and these thousands, or millions, of progeny may cover a very large surface of the sea bottom; in fact, you will readily perceive that, give them time, and there is no limit to the surface which they may cover.
We cannot remember what we did or did not recollect in that state; we cannot now remember having grown the eyes which we undoubtedly did grow, much less can we remember whether or not we then remembered having grown them before; but it is probable that our memory was then, in respect of our previous existences as embryos, as much more intense than it is now in respect of our childhood, as our power of acquiring a new language was greater when we were one or two years old, than when we were twenty.
The condition of the embryos of those animals which have lungs, whilst these organs are yet in abeyance and not employed, is the same as that of those animals which have no lungs.
From such special adaptations, the similarity of the larvæ or active embryos of allied animals is sometimes much obscured; and cases could be given of the larvæ of two species, or of two groups of species, differing quite as much, or even more, from each other than do their adult parents.
It may be here worth noticing that in some cases of parthenogenesis, the embryos within the eggs of silk moths which had not been fertilised, pass through their early stages of development and then perish like the embryos produced by a cross between distinct species.
Sometimes a single tapeworm, parasitic in the human body, will produce three hundred million embryos; the fact that this animal is relatively rare diverts our attention from the alarming fertility of the species and the excessive rate of its natural increase. Perhaps the most amazing figures are those established by the students of bacteria and other micro-organisms.
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