CHAPTER II

PROTOZOA TO WORMS:  CELLS, TISSUES, AND ORGANS

The first and lowest form in our ancestral series is the amoeba, a little fresh-water animal from 1/500 to 1/1000 of an inch in diameter.  Under the microscope it looks like a little drop of mucilage.  This semifluid, mucilaginous substance is the Protoplasm.  Its outer portion is clear and transparent, its inner more granular.  In the inner portion is a little spheroidal body, the nucleus.  This is certainly of great importance in the life of the animal; but just what it does, or what is its relation to the surrounding protoplasm we do not yet know.  There is also a little cavity around which the protoplasm has drawn back, and on which it will soon close in again, so that it pulsates like a heart.  It is continually taking in water from the body, or the outside, and driving it out again, and thus aids in respiration and excretion.  The animal has no organs in the proper sense of the word, and yet it has the rudiments of all the functions which we possess.

A little projection of the outer, clearer layer of protoplasm, a pseudopodium, appears; into this the whole animal may flow and thus advance a step, or the projection may be withdrawn.  And this power of change of form is a lower grade of the contractility of our muscular cells.  Prick it with a needle and it contracts.  It recognizes its food even at a microscopic distance; it appears therefore to feel and perceive.  Perhaps we might say that it has a mind and will of its own.  It is safer to say that it is irritable, that is, it reacts to stimuli too feeble to be regarded as the cause of its reaction.  It engulfs microscopic plants, and digests them in the internal protoplasm by the aid of an acid secretion.  It breathes oxygen, and excretes carbonic acid and urea, through its whole body surface.  Its mode of gaining the energy which it manifests is therefore apparently like our own, by combustion of food material.

  [Illustration:  1.  Amoeba proteus.  Hertwig, from Leidy.
  ek, ectosarc; en, endosarc; N, food particles;
  n, nucleus; cv, contractile vesicle.]

It grows and reaches a certain size, then constricts itself in the middle and divides into two.  The old amoeba has divided into two young ones, and there is no parent left to die, and death, except by violence, does not occur.  But this absence of death in other rather distant relatives of the amoeba, and probably in the amoeba itself, holds true only provided that, after a series of self-divisions, reproduction takes place after another mode.  Two rather small and weak individuals fuse together in one animal of renewed vigor, which soon divides into two larger and stronger descendants.  We have here evidently a process corresponding to the fertilization of the egg in higher animals; yet there is no egg, spermatozoon, or sex.