Eugenio Rignano Upon the Inheritance of Acquired Characters (1911) - Paperback

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER THREE PHENOMENA WHICH POINT TO THE EXISTENCE OF A CENTRAL ZONE OF DEVELOPMENT HYPOTHESIS OF THE STRUCTURE OF THE GERM1NAL SUBSTANCE. /. Phenomena Which Point to the Existence of a Central Zone of Development The only group of organisms in which one can say that the existence of a central zone of development is directly demonstrated is that of the unicellular organisms, in which this zone is constituted by the nucleus. In pluricellular organisms it is only indirectly that we are able to arrive at the conclusion that the central zone exists. Experiment has shown that the necessary and sufficient condition for the ontogenetic development of the Infusoria is the presence of a nucleus. This latter constitutes therefore for them an effective central zone of development and consequently ontogenesis consists in them in a true and proper centroepigenesis. If one divides an amoeba or a rhizopod or an infuso- rian already completely developed, into many pieces, that one of these fragments which remains provided with its nucleus though it be the smallest of all, is yet capable of reproducing by new formation all the missing organs and of developing again into a normal individual; whereas theother fragments, without nuclei, are incapable of it even though they may be much larger. Especially we recall the researches upon artificial division of the Infusoria made by Nussbaum and Gruber. If, for example, one cuts a stentor into three pieces, of which each contains one portion of its moniliform nucleus, in the space of twenty-four hours each piece regenerates the missing part. The anterior extremity regenerates the posterior and vice versa; the middle piece reforms the two extremities, that is to say, both the rather complex peri- stomal region with its mouth, its p...