Part 3

When gromia oviformis reproduces by mitosis it gives off a bud (small cell), which finally separates from the parent form and constitutes a distinct individual. This process may be repeated many times, so that a great number of separate individuals may be formed, all of which lead detached and independent lives.

Most of the protozoa, which are produced by fission (cell division by mitosis), separate entirely from each other, as in Gromia; but in many of these unicellular animals, the new creatures produced by fission do not separate from one another, but remain more or less closely connected, and thus form _colonies_ of Protozoans. These colonies are of the greatest interest, for they represent a lower stage of the _cell colonies_ of the Metazoa (multicellular animals). They reproduce, in many cases, in a way which is strongly suggestive of reproduction in the Metazoa.

=Microgromia socialis= is a little unicellular animal, having a thin, nearly globular, calcareous shell that it secretes upon its surface. It multiplies by fission, and forms a number of distinct individuals which have the curious habit of fusing their pseudopodia and uniting into a more or less closely associated colony. The individuals sometimes remain at a distance from one another, but sometimes associate themselves together into a compact colony. These individuals are all alike, performing the same functions. There is no division of labor among the units, but they live practically an independent life. If the individual animals were detached from one another, they would live and build new colonies.

=Codosiga umbellata= is another unicellular animal—a flagellate Protozoan. It has a collar-like extension of its ectoplasm from the anterior extremity of its body, forming a sort of funnel from the bottom of which the thread-like structure (flagellum) arises. The vibrations of this flagellum cause a current of the surrounding fluid to set into the funnel so that particles of food reach the soft protoplasmic substance which serves as a mouth. The nucleus is seen near the base of the collar. Near the posterior extremity of the body two contractile vesicles are to be observed. This posterior extremity of the animal has a cylindrical extension of its ectoplasm by which it attaches itself to an object. This protozoan multiplies by longitudinal fission. In some species the animals separate completely from one another and lead entirely independent lives. But in _Codosiga umbellata_ the fission does not extend through the cylindrical extension, so that a group of animals are associated in a colony.

=Rotalia or Globigerina.= The shelled amœba (_Lagena_) gives off a bud (unseparated cell) which grows to the full size, secretes its calcareous shell and remains connected with the parent form. This process may be repeated a number of times until a colony of shelled amœbæ, of varying pattern in different species, may be formed and permanently associated together (Fig. 4). The individual amœbæ are all alike and perform the same functions. There is no division of labor, no specialization, among them. If the individual animals could be separated from one another they would live and build new colonies.

=Pandorina morum= forms a small colony of sixteen cells (solid sphere) of mulberry-like shape and enclosed in a common gelatinous envelope. Each cell in the mulberry mass bears two flagella on its peripheral end. These project out beyond the surface of the gelatinous envelope, and are agents for locomotion of the colony. The cells in the colony are all alike. There is no division of labor among them. They all act alike. The cells (flagellate protozoans) of the colony may reproduce in two ways. Each animal in the colony may subdivide into sixteen smaller units, each of which by growth and multiplication may form a new mulberry mass, a new colony, each unit of which acquires two flagella. Or two of the small units may amalgamate (conjugate), and then develop (by fission) into a new colony. The conjugating units are nearly of the same size and look very much alike.

=Volvox globator= is a spheroidal shaped colony (hollow sphere) of unicellular flagellate animals, about one-half a millimeter in size. It was formerly supposed to be a fresh-water _Alga_. It is now known to be a colony of _Protozoans_. All the animals in this colony are not alike. There is a division of labor among the cells, for some are merely vegetative, serving purposes of nutrition, and having no reproductive powers; while other members of the colony are purely reproductive animals. Furthermore, there is quite a marked specialization of the reproductive cells. Those reproductive cells that may be spoken of as the female cells are large and non-motile encysted cells. The male cells are small and actively motile, in that they have two flagella developed on them. A small flagellate male cell penetrates the large encysted female cell, and as the result of this conjugation, fission takes place repeatedly, and a new colony of flagellate protozoans (volvox) is formed. Volvox approaches very suggestively towards the type of animals known as Metazoan.

In order to comprehend, in some measure, the transition from _Colonial Protozoa_ to _Metazoa_, it will be well for the reader to study a typical sponge. For a long time the Porifera (Sponges) were looked upon as compound Protozoa (colonial Protozoa), but while they are nearer the Protozoa than any of the other types of Metazoa, their position in the animal series is unquestionably among the Metazoa. The Sponge, like the rest of the Metazoa, develops from a fertilized egg by a process of cell multiplication, differentiation, gastrulation, etc.

CELL REPRODUCTION BY MITOSIS.

The multiplication of cells plays a part of such fundamental importance in Evolution, and therefore in Embryology and studies in Heredity, that it is necessary to study the subject somewhat in detail. It is a wonderful process, and is worthy of very careful attention.

The process by which one cell (a mother cell) divides into two cells (daughter cells) is called _mitosis_, and is inaugurated by the _centrosome_ (Fig. 8, A, _at_). The centrosome divides into _two_ centrosomes, which at first remain close together (Fig. 8, B, _sp_), and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the _attraction sphere_; also, at the same time, a spindle-shaped bundle of achromatin fibers, called the _spindle_ (Fig. 8, B, sp), extends between the centrosomes. In the meantime, important changes have been taking place in the chromosomes (hereditary threads) of the nucleus. The chromosomes, which at first are arranged in an apparently inextricable tangle or network, frequently assume U-shaped or V-shaped forms (Fig. 8, C, _chr_), and the nuclear membrane disappears. Sooner or later each chromosome splits longitudinally into two daughter chromosomes, with which the achromatin fibers of the spindle become connected (Fig. 8, D). In this phase of mitosis the split V-shaped chromosomes form a single group called the _equatorial plate_ (chr), and extend across the axis of the spindle. It is to be observed from the diagrams in the figure, that one of the centrosomes has traveled to the opposite pole of the nucleus, thus causing the achromatin fibers of the spindle to extend across the original site of the nucleus. The equatorial plate of split V-shaped mother chromosomes (hereditary threads) thus divides the fibers of the spindle into two parts, one half extending from one centrosome to one group of daughter chromosomes, while the remaining half extend from the other centrosome to the other group of daughter chromosomes. Soon the achromatin fibers of the spindle contract, and in this way separate the two groups of daughter chromosomes, so that one group is drawn towards one centrosome, and the other group to the other centrosome (Fig. 8, E, F, G, H). After the two groups of daughter chromosomes have been drawn to their respective centrosomes, each group assumes the tangle or network phase like the nucleus of the mother cell, and an investing nuclear membrane reappears for each (Fig. 8, I). Thus from the mother nucleus of the mother cell we get two daughter nuclei (I). In a further phase of the mitotic process, a furrow appears on the surface of the protoplasm and surrounds it in the form of a ring. This furrow is in a plane at right angles to the long axis of the spindle, and gradually deepens until the protoplasm is divided into two parts, each segment of protoplasm containing its own nucleus and centrosome; in short, the mother cell has divided into two daughter cells (I).

It will thus be observed that the centrosomes and their achromatin fibers are a beautiful mechanism by which the heredity threads (chromosomes) are exactly divided into two equivalent halves.

There are some cells (_Amœba proteus_, for instance,) which divide in a much simpler manner than by mitosis; in these there is no complicated rearrangement of the chromosomes and no disappearance of the nuclear membrane, the nucleus simply becoming separated into two parts (_Amitosis_).

=The Human Ovum.= The human ovum is a typical cell about one-fifth of a millimeter in size and spherical in shape. It is a nucleated piece of protoplasm possessing an enveloping cell-wall (metaplasm). The protoplasm contains nutrient material or yolk (metaplasm). The _maturation_ of the ovum essentially consists in throwing out half of its chromosomes. In doing this the nucleus (mother) approaches the surface of the protoplasm (Fig. 9, A), and divides, by mitosis, into two daughter nuclei; then the unripe ovum divides into two cells, but of very unequal size. This process is repeated a second time. Thus two small cells are formed which are known as the _polar bodies_ (Fig. 9, A, B, pol. b). The large cell remaining after the formation of the two polar bodies is the _mature ovum_ (B). Its nucleus, which recedes towards the center of the protoplasm, is called the female _pronucleus_ (Fig. 9, B, f. pr.). The pronuclei of mature ova differ from the nuclei of all the other cells of the body in that they only contain half as many chromosomes (hereditary threads).

The ripe _spermatozoid_ (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess.

=Fertilization.= The male sexual cell (_spermatozoid_) is vastly smaller than the female sexual cell (ovum). Having a _flagellum_ (Fig. 9, B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at (Fig. 9, B, s). The tail or _flagellum_ of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its _pronucleus_ (C, m. pr.), accompanied by its _centrosome_ (C, m. c.), approaches the _female pronucleus_ (f. pr.) of the ovum (Fig. 9, C, D). Finally the male and female pronuclei coalesce to form a single nucleus (Fig. 9, E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through _mitosis_, takes place.

The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (_oösperm_), and contains now the normal number of _hereditary threads_ (chromosomes); for those of the male have been added to those of the female.

=Segmentation of the Oösperm.= The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed (Fig. 10). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or _morula_ (Fig. 10, 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by _Pandorina_. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the _blastula_ (Fig. 10, 5). This phase of man’s existence is quite suggestive of _Volvox_.

Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called _gastrulation_ technically) is the formation of a sort of cup. This is the _gastrula_ (Fig. 10, 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp).

The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called _epiblast_ or ectoderm, and the inner one _hypoblast_ or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the _mesoblast_. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The _epiblast_ develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The _hypoblast_ develops into the cellular parts of the digestive canal, the liver, lungs, etc. The _mesoblast_ develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic _oösperm_ is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.

A careful study of the life-histories of the few unicellular animals and plants mentioned in the preceding pages will help to let one realize the wonderful powers of nucleated pieces of protoplasm (cells). If isolated pieces of protoplasm can accomplish so much, one will not be astonished to learn that many diverse cells associated together in intimate correlations, as occurs in the higher animals, may accomplish results that are profoundly interesting and marvelous. As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together.

All of the wonderful results of Embryology are accomplished through cell-multiplications, cell-differentiations, cell-associations, invagination and evaginations of cell-groups (tissues or organs), and unequal growth of parts (cells or groups of cells).

In concluding this brief but, we hope, useful study of a few selected cells, we may say that an eminent English physiologist has made the statement that a student who has not looked through the microscope and observed the circulation of the blood in the web of a frog’s foot is not fit to study medicine. However beautiful, fascinating and instructive the sight of this circulation may be, we are tempted to make the assertion that the student who has not looked through the microscope at some of the superficial ooze from the bottom of any slow-running stream, in summer, and observed the structure and actions of that wonderful little unicellular animal, the Amœba proteus, is still less prepared to study medicine. In this little speck of protoplasm the problems of life are reduced to their simplest forms, for all higher plants and animals may be regarded as groups of more or less modified amœbæ peculiarly associated together. In our studies of the amœba we will be forcibly reminded of a very clever trick which is practiced in India and is called the mango-trick. In this trick a seed is put into the ground and covered up, and after divers incantations a full-blown mango-bush appears within five minutes. We have never met any one who knew how this thing was done, nor have we ever seen a person who believed it to be anything else than a conjuring trick. So it is with the _amœba_, a beautiful and fascinating trickster of nature. We understand some of its activities, interesting and exceedingly instructive, but there are many others beyond our ken. We see the commencement and ending of many of its chemical activities, but there are numerous other intermediary processes that take place in the hidden recesses of the protoplasm, and concerning which we know nothing. It may fall to the lot of some reader of this little book, as a patient and keen observer, to unravel some of these mango-like tricks of the amœba or other unicellular creature.

SECTION II.

HEREDITY WITH VARIATION.

HEREDITY WITH VARIATION.

That an offspring always inherits from its parents many of their characteristics is well known; that it always varies, more or less, from them is also equally well known. Heredity and variation are twin forces that play upon every creature, holding it rigidly true to the parental type or compelling more or less divergence therefrom, according to the strength of the one or other power; so that every creature is the resultant of the activities of these two great parallel forces. Variation is coextensive with heredity, and every living creature gives evidence of the existence of variations.

=Examples of Variations.= No two leaves on a plant are exactly alike; no two children of the same parents give a perfect resemblance; no two individuals of the same species are molded in precisely the same pattern; of the thousands and thousands of faces that we observe in a city in the course of a year, each has some distinctive peculiarity.

The trained eye of the gardener recognizes each hyacinth among hundreds of bulbs; of the shepherd, each sheep in his flock; of the Laplander, each reindeer crowded in his herd like ants on the anthill. In a flock of 1,000 sheep each mother can even recognize a variation in the _voice_ of her own lamb, all alike to _us_.

Every part of an animal is subject to variations, not only in bodily structure, but also in habits and instincts, and these variations are large in amount, numerous and diverse in character. Many observations, experiments and measurements that have been made at various times attest the truth of this assertion. Not only do variations take place in animals and plants under domestication, but also in the wild state.

=Illustrations of Heredity.= _Mental heredity_ can be illustrated by studying the genealogies of such persons as Aristotle, Goethe, Darwin, Coleridge, Milton, etc. Probably the Bach family, of Germany, supply one of the best illustrations of the inheritance of intellectual character that we know of. The record of this family begins in 1550, lasting through eight generations to 1800. For about two centuries it gave to the world musicians and singers of high rank. The founder was Weit Bach, a baker of Presburg, who sought recreation from his routine work in song and music. For nearly two hundred years his descendants, who were very numerous in Franconia, Thuringia, and Saxony, retained a musical talent, being all church singers and organists.

When the members of the family had become very numerous and widely separated from one another, they decided to meet at a stated place once a year. Often more than a hundred persons—men, women and children—bearing the name of Bach were thus brought together. This family reunion continued until nearly the middle of the eighteenth century. In this family of musicians twenty-nine became eminent.

_Inheritance of moral character_ is well established. Heredity, in its relation to crime and pauperism, has been thoroughly investigated by Mr. Dugdale in his most instructive little work entitled _The Jukes_. In this work the descendants of _one_ vicious and neglected girl are traced through a large number of generations. It reveals that a large proportion of the descendants of this woman became licentious, for, in the course of six generations, fifty-two per cent. of the females became harlots and twenty-three per cent. of the children were illegitimate. It shows also that there were seven times more paupers among the women than among the average women of the State, and nine times more paupers among the male descendants than among the average men of the State.

The inheritance of _physical peculiarities_ is so obvious as to need no illustration. Among the ancients the Romans stereotyped its truth by the use of such expressions as the _labiones_, or thick-lipped; the _nasones_, or big-nosed; the _capitones_, or big-headed; and the _buccones_, or swollen-cheeked, etc. In more recent times we read of the Austrian lip and the Bourbon nose.