The simplest case is that of a separation of the first two blasto meres; if this be done each will develop independently of the other. In some cases a complete embryo of half the normal size will develop from each, in others a complete half-sized embryo from one, and one incomplete by the absence of certain parts from the other, whilst, in a few cases, half an embryo arises from each, the individual blastomeres segmenting as if the other were actually present.
Thus it seems certain that the cytoplasm of a fertilized egg may actually possess a structure which determines the course of its further development. But this predetermination of the fate of a cell by its structure can only be partial; it can be tested by experiments on regeneration (q.v.).
If, for example, the leg of a newt be amputated, the cut end of the stump rapidly heads over, and a small white conical accumu lation of cells appears at its extremity; this grows in such a way that it eventually attains exactly the size and structure of the part of the leg which was originally cut off. In such a case it is clear that cells which, in an unmutilated animal, would never have formed part of a limb, have not only been stimulated to grow out, either by the effect of the wound or by the removal of the inhibiting influence of a complete leg, but have been com pelled by a control exercised by some structure or structures in the rest of the animal, to do so in such a way as to acquire a definite structure.
Regeneration experiments, suitably designed, may throw much light on the whole problem of the control of development; but allied to them are others in which, instead of allowing normal regeneration to take place, part of the animal is transplanted to a new situation, where it is exposed to the control of a new environ ment, and should, if capable of modification, take on a structure appropriate to its new position.
Of these experiments the most important are those which have been carried out on the embryos and larvae of Amphibia. It has been discovered that fragments of one amphibian embryo im planted in another will grow even if the two belong to different species or even genera. This fact enables implants of a larva whose cells appear black to be grafted on to larvae whose cells are white. In this way it is possible to follow with great cer tainty the fate of the implanted fragment. These experiments have shown that the greater part of the surface of a newt's gas trula is indifferent, a set of cells which, if left in their natural position, would become part of the skin covering an external gill, may, if transplanted, become part of the brain or form the retina of an eye. Continued exploration by this method has
shown that only one small region, that of the dorsal lip of the blastopore, has its fate determined ; it is an organizer, which, if introduced into any Urodele blastula into which it becomes in corporated, will determine the establishment of an embryo whose parts stand in a definite relation to it. (See EXPERIMENTAL EMBRYOLOGY.) But this organizer can be seen to arise from a particular part of the cytoplasm of the fertilized egg, the grey crescent, a struc ture which appears at fertilization at a point immediately opposite to that at which the spermatozoan enters.
It has then to be considered whether it is the entrance of the spermatozoan which establishes a structure in the fertilized egg or whether such a localization exists in the unfertilized but ripe ovum. Evidence on this point may be sought by two methods: An attempt may be made to destroy any structure the unfertilized ovum possesses by making use of the difference in specific gravity which exists between the cytoplasm and its inclusions, either by merely inverting the egg or by the more powerful forces which are available in a centrifuge. The eggs with their contents so rearranged may be fertilized immediately, and an attempt made to correlate any abnormalities presented by the embryos resulting from such fertilizations with the displacements which may be observed by cytological methods in eggs similarly treated. As a control, eggs which have been fertilized must be subjected to exactly the same treatment. An easier, though much more indi rect mode of investigation is through genetics.
No two animals, even belonging to the same brood, are alike; whilst exhibiting a close similarity to their parents and to one another, each will differ from all others either to a small extent or even very considerably in one or many respects. The investigation of the nature and causes of these variations, and of the extent to which they are passed on from one generation to the next, forms a most important part of the science of zoology. It is clear, even on a cursory investigation, that variations may be either of such a kind that they form a continuous series con necting one extreme of structure within the species with the other, or may represent definite steps between which no inter mediates can be found. This distinction between continuous and discontinuous variation is one of the greatest importance for a clear understanding of the problems at issue.