Perhaps the most interesting numerical relation of this kind are those recently discovered in the division of teloblasts, where the number of divisions is directly correlated with the number of segments or somites. It is well known that this is the case in certain plants (Characece), where the alternating nodes and internodes of the stem are derived from corresponding single cells successively segmented off from the apical cell. Vejdovsky's observations on the annelid Dendrobana give strong ground to believe that the number of metamerically repeated parts of this animal, and probably of other annelids, corresponds in like manner with that of the number of cells segmented off from the teloblasts. The most remarkable and accurately determined case of this kind is that of the isopod crustacea, where the number of somites is limited and perfectly constant. In the embryos of these animals there are two groups of teloblasts near the hinder end of the embryo, viz. an inner group of mesoblasts, from which arise the mesoblast-bands, and an outer group of ectoblasts, from which arise the neural plates and the ventral ectoblast. McMurrich ('95) has recently demonstrated that the mesoblasts always divide exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times, before relinquishing their teloblastic mode of division and breaking up into smaller cells. Now the sixteen groups of cells thus formed give rise to the sixteen respective somites of the post-naupliar region of the embryo (i.e. from the second maxilla backward). In other words, each single division of the mesoblasts and each double division of the ectoblasts splits off the material for a single somite ! The number of these divisions, and hence of the corresponding somites, is a fixed inheritance of the species.
The causes that determine the rhythm of division, and thus finally establish the adult equilibrium, are but vaguely comprehended. The ultimate causes must of course lie in the inherited constitution of the organism, and are referable in the last analysis to the structure of the germ-cells. Every division must, however, be the response of the cell to a particular set of conditions or stimuli ; and it is through the investigation of these stimuli that we may hope to penetrate further into the nature of development. It must be confessed that the specific causes that incite or inhibit cell-division are scarcely known. The egg-cell is in most cases stimulated to divide by the entrance of the spermatozoon, but in parthenogenesis exactly the same result is produced by a different cause. In the adult, cells may be stimulated to divide by the utmost variety of agencies — by chemical stimulus, as in the formation of galls, or in hyperplasia induced by the injection of foreign substances into the blood; by mechanical pressure, as in the formation of calluses ; by injury, as in the healing of wounds and in the regeneration of lost parts ; and by a multitude of more complex physiological and pathological conditions, — by any agency, in short, that disturbs the normal equilibrium of the body. In all these cases, however, it is difficult to determine the immediate stimulus to division ; for a long chain of causes and effects may intervene between the primary disturbance and the ultimate reaction of the dividing cells. Thus there is reason to believe that the for
mation of a callus is not directly caused by pressure or friction, but through the determination of an increased blood-supply to the part affected and a heightened nutrition of the cells. Cell-division is here probably incited by local chemical changes; and the opinion is gaining ground that the immediate causes of division, whatever their antecedents, are to be sought in this direction. The most promising field for their investigation seems to lie in the direction of cellular pathology through the study of tumours and other abnormal growths. The work of Ziegler and Obolonsky indicates that the cells of the liver and kidney may be directly incited to divide through the action of arsenic and phosphorus; and several others have reached analogous results in the case of other tissues and other poisons. The formation of galls seems to leave no doubt that extremely complex and characteristic abnormal growths may result from specific chemical stimuli, and some pathologists have held a similar view in regard to the origin of abnormal growths in the animal body.
Suggestive as these results arc, they scarcely touch the ultimate problem. The unknown factor is that which determines and maintains the normal equilibrium. A very interesting suggestion is the resistance theory of Thiersch and Boll, according to which each tissue continues to grow up to the limit afforded by the resistance of neighbouring tissues or organs. The removal or lessening of this resistance through injury or disease causes a resumption of growth and division, leading either to the regeneration of the lost parts or to the formation of abnormal growths. Thus the removal of a salamander's limb would seem to remove a barrier to the proliferation and growth of the remaining cells. These processes are therefore resumed, and continue until the normal barrier is re-established by the regeneration. To speak of such a " barrier " or " resistance " is, however, to use a highly figurative phrase which is not to be construed in a rude mechanical sense. There is no doubt that hypertrophy, atrophy, or displacement of particular parts often leads to cornpensatory changes in the neighbouring parts ; but it is equally certain that such changes are not a direct mechanical effect of the disturbance, but a highly complex physiological response to it. How complex the problem is, is shown by the fact that even closely related animals may differ widely in this respect. Thus Fraisse has shown that the salamander may completely regenerate an amputated limb, while the frog only heals the wound without further regeneration.' Again, in the case of coelenterates, Loeb and Bickford have shown that the tubularian hydroids are able to regenerate the tentacles at both ends of a segment of the stem, while the polyp Cerianthus can regenerate them only at the distal end of a section (Fig. 142). In the latter case, therefore, the body possesses an inherent polarity which cannot be overturned by external conditions.