VI.

THE PHYSICAL BASIS OF HEREDITY.

BY FRANK MACE McFarland.

ALL living organisms, animals as well as plants, are built up of certain elementary parts or units termed cells. No matter how widely divergent

The cell theory. in external appearance or habitat they may be, the elephant and the lily, the sponge and the palm, are each aggregations of structural units, fundamentally alike, and no form of animal or plant life is known to exist which does not conform to this general law. To the studies of Schleiden upon plants and of Schwann upon animals (1838-'39) we owe the foundation of the "cell theory," more precisely formulated by Max Schultze in 1861. Since the time of these pioneer studies upon the cell, investigation has been carried on by a constantly increasing number of students with methods and instruments steadily improved in their efficiency, and the accumulated results already throw a wealth of light upon some of the most abstruse problems of biology. Yet the most enthusiastic and sanguine of these workers will not assert that we have advanced further than the threshold of this domain in which are concealed the answers to the questions as to the ultimate structure of living matter and even to the very nature of life itself.

the term "cell."

The accumulating results of patient study have totally changed the earlier conceptions of the cell. Two and a quarter centuries ago, by the aid of the newly invented microscope, minute cavities were discovered in certain plant tissues, and from their resemblance to a honeycomb were termed "cells." This study of such substances as ordinary cork, in which the cells are dead and empty, easily led to the idea that the cell wall was the all-important feature, and it has not been until within the past forty years that this error has been set aside. The name "cell" itself is someThe meaning of what misleading in that it implies, in the ordinary usage, a cavity with definite walls of considerable firmness, which is by no means always true. Great groups of cells have no solid walls whatever, but are soft and changeable in form, and the majority of cells have no cavities, but are masses of semifluid consistence. The appearance of empty cavities, or clear fluid-filled spaces, is a condition which comes about in plant cells late in life, and scarcely ever in animal ones. The unwearied study of biologists, aided by constantly improved instruments and methods. of research, have shown that it is the contents of the cell which form the essential living substance. But, although the cell wall has lost the significance which it formerly was held to possess, the term cell has become firmly fixed by usage, and such terms as "Energide," as proposed by Sachs, though much more happily chosen, are very slow of adoption.

The simplest forms of life of which we know anything are minute microscopic organisms found in both fresh and salt water and under the most varied conditions. Each one of these is composed of a single cell, and each one carries out in a general way the varied functions of movement, respiration, growth and multi

Unicellular and multicellular organisms.

plication, assimilation, secretion, excretion, irritability, etc., functions which, in multicellular organisms, are divided up among a vast number of the constituent cells. Thus while the onecelled amoeba has its muscular, nervous, and digestive systems united within the limits of a single microscopic mass of protoplasm, the higher animals have their various functions divided up among definite groups of thousands and millions of cells, each group carrying out some particular function. response to this physiological division of labour among the cells has come about a corresponding modification in their structure, so that we find certain forms and types characteristic of the particular function which the respective cells carry out. The muscle cell, for example, is one whose special work is that of contraction. Within its substance has been developed a system of highly contractile fibrils, and the whole cell has assumed an elongated shape. For this one function of contractility have been sacrificed more or less completely the other properties of protoplasm, and thus it has become dependent upon its fellows which have assumed various other functions. The bone cell, the gland cell, the epithelial cell-all have equally complicated specializations of structure in other directions and, all united together into an organic community, are co-ordinated and directed in their various activities by the nerve cells.

The essential parts of the cell.

However diverse the form and function of the adult tissues may be, they all have the same fundamental structure, and they all have a common origin and descent from the fertilized egg cell. The essential parts of a cell consist of the cell body and the cell nucleus, which together make up the living substance. The body of the cell is made up principally of a granular, viscid, semi

fluid substance termed protoplasm or cytoplasm. By this term is understood not a homogeneous, definite chemical substance, but rather an organThe protoplasm. ized association of extremely complex organic compounds, belonging mainly to the group of proteids. The majority of these substances are but little understood as yet, nor is this at all surprising when we reflect that the living cell is the theater of constant changes, both synthetic and analytic, and that the dead protoplasm subjected to the chemist's analyses is no longer protoplasm, but has suffered profound transformations in passing from the living to the lifeless condition.

In its simplest form the cell is an approximately spherical, viscid, granular structure in which oftentimes there may be made out, in the living state, more solid substances in the form of threads or networks of delicate filaments. This threadwork is probably made up of rows of granules, and varies in arrangement in different cells and in different parts of the same cell. Within the meshes of this reticulum is inclosed a clearer fluidlike portion-the cytolymph or hyaloplasm.

The nucleus.

Inclosed in the cytoplasm lies a spherical or ovoidal body-the nucleus-set off from the rest of the cell by a more or less distinct boundary membrane. This structure is of constant occurrence in all cells, and plays an extremely important part in their life history, forming apparently the controlling centre of the constructive processes in growth and multiplication. Its significance will be better understood further on in connection with the problems of cell development and heredity.

In chemical and physical properties the nucleus differs markedly from the rest of the protoplasm. When studied in detail by the aid of suitable reagents and

magnification, it is found to consist essentially of a delicate network or system of threads, the "linin network," bearing granules of a substance rich in phosphorus, which, from its affinity for certain staining fluids, has been termed "chromatin." One or more rounded structures, behaving in certain definite ways. toward reagents, may also be found in the nucleus. These, the "nucleoli," are probably not always of the same nature in different cells, and their significance is at present much less clearly understood than is the case with the other nuclear structures. Filling the meshes of the nuclear network is found a clear semi-fluid material, the "karyolymph," and a more or less clearly defined wall, the nuclear membrane, incloses the nuclear substances and separates them from the cytoplasm.

In 1876 Van Beneden announced the discovery of a minute rounded body at the poles of the spindle in the dividing eggs of Dicyemids, which has since been found in nearly all kinds of animal cells, both in division and in the "resting condition," and may probably be regarded as of universal occurrence, so far at least as animal cells are concerned. In plants, however, it has thus far been identified with certainty in but few forms. To this structure Boveri, in 1888, gave the name of "centrosome," and showed it to be a cell organ of probably constant occurrence and of the greatest significance in cell multiplication. It often lies in a more or less specialized area of the cytoplasm, the "attraction sphere," or "archoplasm," near the nucleus, but in some forms it is doubtless without any such surrounding structure.

The centrosome.

The foregoing paragraphs must be taken merely as the briefest outline of cell structure. It would far exceed the limits of this article to attempt to discuss the finer detail of the subject, or to enter upon the many