The important point is that in both the experiments, in the case of the frog and of the triton, the cell-material, separated at the first cleavage, was turned to a use quite different to its use in the formation of a normal embryo.
We may conclude with a very convincing proof. In the above-mentioned abnormal development of the frog's egg it happened that one edge of the blastopore, on account of its weight, was very much bent outwards. In consequence of this the cleft of the blastopore lay between the normal blastopore-lip and the everted border of the other lip. When the notochord and the nerve-plate appeared, as a result of this abnormal condition, they grew from a cell-material that was quite different to that which gives them origin in normal cases.[15]
In these cases Weismann cannot apply his accessory conception, the existence of supplementary idioplasm, only to the nuclei arising from the first division; he must extend it to the thousands of embryonic cells that arise by division up to the time for the appearance of the nerve-tube and notochord. The behaviour of these cells under fortuitously changed conditions shows them all to be endowed with the capacity of development in different directions.
FIFTH GROUP OF FACTS.—PHENOMENA OF VEGETATIVE AFFINITY.[16]
Many considerations, taken from the region of general physiology, support the view that all the cells of an individual, of any species, are alike, and are to be distinguished from one another only by the special development of one character.
Formerly, indeed, many biologists, relying upon the optical appearances presented in microscopical investigation, have been inclined to the view that the visible qualities of a tissue, as revealed by the microscope, were the only, or the chief, distinctive characters. For instance, by microscopical investigation one cannot distinguish the tendons, nerves, bones, and cartilages of a dog from the corresponding tissues in a horse. So far as their special use in the organism goes, one might interchange the corresponding parts in these two mammals. A tendon from the dog, if large enough, might be attached to the muscle of a horse, and would transmit the pull of the muscle on the bone just as well, and would completely satisfy the mechanical duties of the horse's tendon. The same might happen in the case of a bone, of a cartilage, or of a nerve-fibre.
As a matter of fact, the idea that parts of the tissues of different animals may serve to replace one another has been employed repeatedly in science, especially in the science of medicine. But I believe that our ideas are not yet clear upon the matter. The erroneous impression to which I have alluded has arisen because we do not bear in mind that each tissue, each part of an organ, each cell, possesses, in addition to its obvious characters, very many characters that are invisible to us. Such characters are inherent in the tissue-cells because these are parts of a definite organism. In consequence of their specific tissue characters, which are visible to us, we assign cells their place in histological classification; in contrast, we may denote the other characters as constitutional, or species, characters.
No doubt tissue cells are in the same case as genital cells. So far as microscopical characters go, egg cells and spermatozoa are wonderfully alike in all the mammalia; in many cases we could not distinguish between those of different animals. But, because they bear the specific characters, we cannot doubt but that they are as distinct as are the species, although invisibly to us.