No appearance of new gonads has been observed in insects. The primordial germ-cells may be sucked off from the eggs of Chry somelid beetles in early stages, as they are then protruding at the posterior end of the embryo. Even after such an early operation as this the beetles remain sterile for life. The same has been observed in crickets, castrated as young larvae, and butterflies, the young caterpillars of which have been similarly treated. In mol luscs only regeneration of the hectocotylus arm (see above) seems to have been observed, and here the germ-cells are carried into it from the gonad, which is not affected by the shedding of the arm.
Some tunicates regrow from very small pieces, becoming sexually ripe in absence of their former reproductive glands, e.g., Clavel lina. Only a short while ago the vertebrates were thought totally incapable of replacing lost gonads. Castration and spaying has been practised so widely in many of our domestic animals and birds without fertility ever having appeared that it was held quite safe to perform experiments on the influence of a foster mother on the characteristics of the young by implantation of an alien ovary. Only when results after such transplantations in hens ap peared, which, in contradiction to all other cases, seemed to prove an influence of the foster mother on the colour of her stepchildren, the suspicion arose that these chickens had, after all, been derived from eggs of her own. New tests revealed the fact that in these experiments new germ-cells had arisen from the host's tissue. But as the possibility could not be excluded that some part of the gonad had not really been totally eradicated by the operation, this alternative to the regeneration of an ovary was still left open.
A better material presented itself in the mouse, the ovary of which is enclosed with the tube of the uterus in a pouch. Here the removal of the ovary is complete, if the whole pouch is taken away. It was a surprise to most biologists that experiments of this kind yielded positive results, putting the feasibility of the regen eration of ovaries even in mammals beyond doubt.
normal bulk, freshwater worms from a piece 500 times smaller than the whole body. But the reproductive cells are in many cases even less in volume compared with the entire animal. In many of the small pieces organs for ingesting food are altogether lacking, so that the restoration of form must proceed without receiving nourishment from the exterior. When the resources of the body are too small, it will not be possible for the animal to mobilize enough substance for growing regeneration buds at all the cut surfaces. Then the old tissue is remodelled to form most of the new body, which is laid down on a reduced scale in normal propor tions. This phenomenon has been termed morphallaxis. It has been mostly studied in flatworms which afford a good opportunity for the gradual reappearance of the normal proportions and the shifting of structures from a peripheral position in the cut piece towards its normal place in the remodelled worm. The eyes and the pharynx are especially noticeable. In one kind of planarian a row of small eyes runs along the anterior margin not extending backward beyond the pharynx. When a piece of this animal, in cluding the marginal eyes, has been cut off, the posterior eyes de generate, thus ridding this part of its differentiation and making room for the pharynx and tail to develop.
Cell-size and Number.—The size of the single cells does not vary much with the size of the planarian. In regeneration and morphallaxis the size of cells is retained too. Thus the flatworm remodelled on a small scale is not composed of smaller cells in the same number as before, but of fewer cells. We see how the different organs from the old piece have to be overhauled that they may be composed of fewer cells, if the proportionate number in the various structures is to be regained. In other animals, for in stance the rotifers, the number of cells remains constant after the embryo has hatched. Here regeneration can only set in, when a part of a cell is removed. Artificially, this has been achieved by cutting away some rays of the "wheel," from which the rotifers take their name. If, on the contrary, whole cells are killed or only the nucleus of a single cell, then the structure to which these cells ought to give rise will never develop. Where now do the cells come from which in planarians, hydras, and other forms with great regenerative faculties are necessary to complete the indi vidual? Several possibilities have to be taken into account. Either differentiated cells may undergo dedifferentiation, or those not yet differentiated migrate to the places of need; lastly, special cells may be in store for the purposes of restoring the lost parts. It is probable that all three methods are practised in nature, but by far the commonest method seems to be the one mentioned sec ond of so-called interstitial or indifferent cells widely spread through the body offering a plastic material for every kind of re generation. These cells are derived from layers of tissue which, since the development of the embryo, have not undergone any special alteration or served for other purposes than connecting different regions of the body. This raw material does not con tain special rudiments for the organ it is to build up, but may be used at any point needed. Even the faculty to start the structure wanted does not reside in the undifferentiated cells ; the regenera tion bud must have its destination stamped on it by differentiated cells lying proximally to the cut surface. The proof of this fact has been given by the experiment of grafting regenerating buds to different parts of the body. When the buds are very young, taken shortly after regeneration has begun, they will differentiate not in relation to the place from which they were taken, but in relation to that where they are grafted. In a bud taken from anywhere will form a head if it has been transplanted to the head region, the regenerating bud of the hind-leg of a salamander grafted in place of an amputated fore-leg will grow into a fore leg; even the tail-bud of a newt removed to a site near the fore leg will grow out in the form of this leg. But all this is changed if buds a little older are taken as grafts. Then, be the bud attached to any place whatsoever, and it will yet give rise to the form it would have attained if left where it had originated.