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perfect, as for instance the brain and spinal medulla, it requires only a closer study to reveal many points of difference between the right and left halves.
The line on the front of the body along which the median plane reaches the surface is termed the anterior median line; whilst the corresponding line behind is called the posterior median line.
It is convenient to employ other terms to indicate other imaginary planes of section through the body. The term sagittal, therefore, is used to denote any plane which cuts through the body along a path which is parallel to the median plane (S S'); and the term coronal or frontal is given to any vertical plane which passes through the body in a path which cuts the median plane at right angles (C C'). The term horizontal, as applied to a plane of section, requires no explanation.
Any structure which lies nearer to the median plane than another is said to be medial to it; and any structure placed further from the median plane than another is said to lie lateral to it. Thus in Fig. 1, A is lateral to B; whilst B is medial to A.
The terms internal and external are applied to the walls of hollow cavities or organs; thus, the ribs possess external surfaces, that is, surfaces away from the cavity of the thorax, and internal surfaces adjacent to the cavity.
The terms anterior and ventral are synonymous, and are used to indicate a structure (D) which lies nearer to the front or ventral surface of the body than another structure (E) which is placed nearer to the back or dorsal surface of the body, and which is thus said to be posterior or dorsal. In some respects it would be well to discard the terms "anterior" and "posterior" in favour of “ventral” and "dorsal," seeing that the former are only applicable to man in the erect attitude, and cannot be applied to an animal in the prone or quadrupedal position. They are, however, so deeply ingrained into the descriptive language of the human anatomist that they cannot be entirely discarded. A similar objection may be raised to the terms superior and inferior, which are employed to indicate the relative levels at which two structures lie with reference to the upper and lower ends of the body. The equivalent terms of cephalic or cranial and preaxial are, therefore, sometimes used in place of "superior," and caudal and postaxial in place of "inferior."
The terms proximal and distal should be applied only in the description of the limbs. They denote relative nearness to or distance from the root of the limb. Thus, the hand is distal to the forearm, whilst the arm or brachium is proximal to the forearm.
By A. H. YOUNG and ARTHUR ROBINSON.
Rewritten by ARTHUR ROBINSON.
THE ontogenetic or developmental history of every human individual is separable into two main periods, pre-natal and post-natal.
It is to the knowledge of the phenomena of the earlier or pre-natal period that the term human embryology is applied, and as pre-natal development takes place in an organ called the uterus, it is frequently spoken of as intra-uterine development. The period of pre-natal development extends through nine lunar months, and may be divided into three sub-periods: (1) the pre-embryonic period, during which the zygote, from which the embryo is formed, shows no definite separation into embryonic and non-embryonic portions. This period lasts about fourteen days; (2) the period of the embryo, in which the zygote is definitely separated into embryonic and non-embryonic portions, but the embryonic part has not yet assumed a clearly human form. This period terminates at the end of the second month; (3) the foetal period, which commences at the end of the second month, when the embryo assumes a definitely human form and is called, thenceforth, a foetus. The foetal period ends at birth, when the foetus becomes a child and postnatal development commences.
Only the general phenomena of the pre-natal period of development are considered in this section; the details of the pre- and post-natal development of the various organs and systems will be dealt with in the sections devoted to the descriptions of their adult conditions.
THE STRUCTURE OF ANIMAL CELLS.
The human body is formed by the multiplication and differentiation of animal cells, therefore it is essential that the student should possess a knowledge of the main features and capabilities of
such cells before he commences the study of the details of human embryology.
Animal cells differ from each other in minor points of structure, in association with the positions they occupy and the functions they membrane perform; nevertheless, they all possess some common and essential structural features, and, in the younger stages of their history,
some common capabilities.
The following are the constituent parts of a typical animal cell:
All the essential parts of the cell consist of a substance called protoplasm.
In its simplest form protoplasm is the semifluid, viscous, irritable, and con
The chromatic substance is aggregated to form first a fine and afterwards a thicker thread or spirem. At the same time, a spindle of achromatic fibrils appears between the two daughter centrosomes, and the nuclear membrane disappears. As soon as the achromatic spindle is definitely established the chromatic thread breaks up into a number of segments, the chromosomes, which arrange themselves around the equator of the achromatic spindle.
The chromosomes may be V-shaped, rod-like, cuboidal or spheroidal, and each
may be a single structure, or it may consist of two or four parts which are closely bound together. There is evidence which tends to support the belief that, whether the chromosome appears to consist of one, two, or more segments, its constituent particles are derived partly from the maternal and partly from the paternal ancestor of the cell; and it is believed that the maternal and paternal portions undergo similar division during the last three phases of mitosis. In any case, whether the chromosomes are single or compound structures, each becomes
FIG. 6.--SCHEMA OF ANIMAL CELL IN META- attached to, or very closely associated with, one
PHASE OF HOMOTYPE MITOSIS.
pole of spindle
of the fibrils of the achromatic spindle.
At the end of the prophase the nucleus as such, and the nucleolus, have entirely disappeared, and the cell body contains, in their place, two centrosomes, an achromatic spindle, and the chromosomes. The centrosomes lie at the opposite poles of the achromatic spindle with the granules of the protoplasm grouped radially around them, and the chromosomes are grouped round the equator of the achromatic spindle.
The Metaphase. During the metaphase each chromosome divides into two equal parts,
Fig. 7.—Schema OF ANIMAL CELL AT END the rods or loops dividing longitudinally; and
OF ANAPHASE OF HOMOTYPE MITOSIS.
FIG. 8.-SCHEMA OF ANIMAL CELL AT END OF
the division, in all cases, commences at the point where the chromosome is in relation with the fibrils of the achromatic spindle.
The Anaphase. In the anaphase the halves of the chromosomes, i.e. daughter chromosomes, move towards the opposite poles of the achromatic spindle, and when they reach the vicinity of the daughter centrosomes the anaphase ends and the telophase begins.
The Telophase. At the end of the anaphase, or the commencement of the telophase, a constriction appears around the periphery of the cell, at the level of the equator of the achroTELOPHASE OF HOMOTYPE MITOSIS. The matic spindle. After its appearance the concell has divided into two daughter cells. striction gradually deepens until the cell is Red and blue indicate the original paternal completely divided into two halves, the daughter cells, each of which contains the typical number of chromosomes, and a portion of the achromatic spindle.
and maternal derivatives.
The Resting Stage. During the resting stage, which lasts for a variable period, a nucleus is formed in each daughter cell by the appearance of a nuclear membrane around the chromosomes, as they repass first to the thread-like and then to the granular form of chromatic substance, and by the reappearance of a nucleolus. The cell increases in size also.
The Period of Cell Life.-The period of cell life varies, but in all cases it mately ends in death; for a time comes when cells no longer transmit to their
descendants the power of division, or the capability of growth and function. If it were not so, growth and function, or at least maintenance and function, would continue uninterruptedly, and in the absence of accident or disease individual life would continue for ever, and " old age" would be unknown.
It appears, therefore, that the ancestors of certain tissue cells are capable of producing only a certain number of descendants, which grow to the normal size and perform their proper functions for a more or less fixed period, whilst in other cases the power of division appears to be transmitted continuously, but the more remote descendants become less and less capable of performing their proper functions. The result in both cases is the same; gradual decay, terminating in death.
Heterotype Mitosis. In ordinary or homotype mitosis the chromosomes are divided into equal parts, and, when the process of cell division is completed, each daughter cell possesses the same number and same kind of chromosomes as the mother cell from which it was derived (Figs. 3-8). In heterotype mitosis, the number of chromosomes is reduced during the cell division, and each daughter cell possesses only half the number of chromosomes that was present in the mother cell. The details of the division of the chromosomes during heterotype mitosis differ in different groups of animals, but the end is the same in all cells in which the process occurs, and is the reduction of the number of the chromosomes in the daughter cells to half the number typical for the ordinary cells of the animal.
The most typical form of heterotype mitosis is seen during the first maturation division of many germ cells, in which, during the spirem or thread-like stage of the chromatic substance, careful examination of the thread shows that it consists of a number of alternate segments attached end to end, the number of segments corresponding with the number of the chromosomes typical for the ordinary cells. of the animal. Towards the end of the prophase, the segments of the thread become attached together in pairs which form a number of twin chromosomes. These arrange themselves around the equator of the achromatic spindle, and it is obvious that the number of twin chromosomes is only half the number of the chromosomes originally present in the cell (Figs. 12 and 13). (See note 1,
p. 79.) The total number of chromatic segments is still the same, for each twin chromosome consists of two ordinary chromosomes attached side to side.
The process of reduction takes place during the metaphase, when the two segments of each twin chromosome become separated from one another. During the anaphase the separated segments pass to the opposite poles of the achromatic spindle, and when the telophase is completed the number of chromosomes in each daughter cell is half that which was present in the mother cell (Figs. 12-19).
In some cases, at the commencement of heterotype mitosis, the chromosomes are not arranged in pairs as twins, but in groups of four, called tetrads, each tetrad consisting of a pair of dyads. In those cases the two dyads of each tetrad are separated from one another during the metaphase, and when the telophase is completed each daughter cell possesses only half the number of chromatic particles which were present in the mother cell.
It is known that a cell which contains only half the typical number of chromosomes can divide once, therefore from each original cell which underwent heterotype mitosis four grand-daughter cells may be produced. It is still uncertain, however, whether or not cells which contain only half the typical number of chromosomes can further subdivide, or whether they can continue to live and function. So far as the observations made can be relied upon, it appears that such cells either die or they unite with another cell containing half the typical number of chromosomes to produce a new cell which contains the typical number of chromosomes and which possesses also the capability of reproducing itself by division.
The Gametes. The gametes are the germ elements by whose union, in pairs, new individuals are produced.
They are of two kinds, female gametes or ova and male gametes or spermatozoa. Both female and male gametes are modified cells, by means of which hereditary characteristics are transmitted from generation to generation, and they are derived from cells called primitive germ cells, whose origin will be considered in association with the development of the germinal layers.