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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 (SS); 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 fœtus. 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 perform; nevertheless, they all possess some common and essential structural features, and, in the younger stages of their history, some common capabilities.
FIG. 2.-DIAGRAM OF AN ANIMAL CELL.
The following are the constituent parts of a typical animal cell:
The cell body-containing
(a) The nucleus with its nucleolus ;
(b) The centrosome with the centrioles;
(c) The mitochondria.
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
tractile substance which forms the "physical basis of life." It consists of C., H., N., O., and S., combined together in different ways and in differing proportions to form various modifications of protoplasm which possess definite physical and chemical characteristics, and which receive, therefore, different names.
The cell body consists of a kind of protoplasm called cytoplasm, separable into two parts; the spongioplasm or cyto-reticulum, which forms a network or spongework; and a more fluid part, the hyaloplasm or cytolymph, which occupies the interstices of the reticulum.
The nucleus lies in the cytoplasm. It consists of a form of protoplasm, called karyoplasm, which is separable into a more fibrillar part, the karyo-reticulum, and a more fluid part, the karyo-lymph or nuclear juice. The reticulum also consists of two parts, the achromatic or non-stainable part formed of a substance called linin, and a part called chromatin, which is readily stainable.
Chromatin varies in appearance at various stages of the cell life. During the resting periods, which intervene between the periods of cell division, it is broken up into small particles which either are embedded in or are in close association with the linin network.
When cell division commences the chromatin particles are, in many cases, aggregated to form a thread-like strand, which ultimately breaks up into a number of segments called chromosomes. The chromosomes are probably of definite number in the body cells of any given species of animal. In the human subject the typical number is probably 24.
According to Winiwarter's recent observations the number of chromosomes in each oocyte I (see p. 12) is 48, and in each spermatocyte I (see p. 12) it is 47. Each mature ovum (see p. 13), therefore, has 24 chromosomes, but some spermatids (see p. 17) have 24 and others 23. If a spermatozoon (see p. 17) with 24 chromosomes unites with a mature ovum a female results, but if a spermatozoon with 23 chromosomes unites with a mature ovum a male results.
During the resting period the nucleus is bounded by a distinct nuclear membrane, which is continuous on the one hand with the karyo-reticulum, and on the other with the cyto-reticulum.
The nucleolus is a spherical vesicle which lies in the karyo-lymph during the resting periods of the cell. It disappears entirely during the periods of division. The protoplasm of which it is formed is called pyrenin. In some cases several nucleoli are present.
The nodes of the karyo-reticulum are sometimes called false nucleoli.
The centrosome is a clear spherical area of the cytoplasm which lies usually in the neighbourhood of the nucleus. Around it the granules of the cytoplasm are arranged in radial lines, and in its interior lie one or two minute, deeply staining bodies, the centrioles. The centrosome appears to play a very important part in cell multiplication; and, in the more ordinary form of cell division, it divides before the division of the cell takes place, but in certain cases it disappears before the cell divides.
The mitochondria are minute particles. They are demonstrable in the majority of cells during life; or by means of certain stains, after special methods of fixation and preservation have been used. They are believed to play an important part in the economy and life-history of the cells, and they form a very definite part of the structure of the spermatozoon or male gamete.
THE LIFE-HISTORY AND CAPABILITIES OF ANIMAL CELLS. Every animal cell is formed by the division of a pre-existing cell called the mother cell. The mother cell divides into two equal parts-the daughter cells, each of which, under ordinary conditions, possesses all the capabilities of its mother.
Reproduction of Cells.-Ordinary tissue cells increase in number by the division of the pre-existing cells into equal parts, and each part possesses similar capabilities. Every new cell has a definite life-history; it grows, performs its proper function, and ceases to exist, either by dividing into two daughter cells, or by dying and breaking up into fragments which disappear.
Whilst the multiplication rate exceeds the death-rate in any given tissue or organ, that tissue or organ grows. When the multiplication rate and the death
rate are equal, the tissue or organ is in a state of equilibrium. As soon as the death-rate exceeds the multiplication rate, decay and atrophy set in; and when the decay and atrophy have proceeded to such an extent that an important tissue or organ can no longer perform its proper functions, general death ensues.
General decay and death are, therefore, the natural results of the loss of multiplication power of the cells of the body, but life may persist after multiplication power is lost, so long as the cells last produced retain their capabilities, and death may result whilst multiplication power of the cells is retained, if the newly produced cells are incapable of performing their proper functions. Nevertheless, speaking generally, it may be said that cell multiplication is a vital necessity, and it takes place in two ways (1) by amitotic and (2) by mitotic division of pre-existing cells.
Amitotic Division.-The phenomena of amitotic division, so far as they are known, are much simpler than those of mitotic division. First the nucleus is constricted and divided; then the cell body is constricted and divided, and two similar daughter cells, each half the size of the mother cell, are produced. The part played by the centrosome during the process is not definitely known, but each daughter cell eventually possesses a centrosome. The apparently simple process of amitotic division occurs at some periods of growth, and the more complicated process of mitotic division at other periods, but the laws which govern the alternations are unknown.
Mitotic Division; Mitosis, or Karyokinesis. Mitotic or karyokinetic division is not
FIG. 3.-SCHEMA OF ANIMAL CELL IN
only the more complicated, but it appears also FIG. 4.-SCHEMA OF ANIMAL CELL IN EARLY to be the more important form of cell division. PART OF PROPHASE OF HOMOTYPE MITOSIS. It takes place in all rapidly growing tissues, especially in the embryonic and foetal stages of life, and it is the main form of cell division which occurs in the earliest embryonic periods. There are, however, two forms of mitosis, the homotype and the heterotype. Of the two, homotype is so much the more common that it Chromosomes may be looked upon as the ordinary form, for heterotype mitosis appears to be limited to one of the two cell divisions which occur during the maturation of the germ cells, and to some of the cell divisions which are associated with the production of malignant tumours.
Homotype Mitosis. The phenomena of homotype mitosis occur in four phases, (1)
FIG. 5.-SCHEMA OF ANIMAL CELL AT COM-
the prophase, (2) the metaphase, (3) the anaphase, and (4) the telophase.
The Prophase-During the prophase both the centrosome and the nucleus undergo very obvious transformations.
The centrosome and its contained centriole divide into two parts, of which one passes to one pole and the other to the opposite pole of the nucleus.
The nuclear transformations concern the nucleolus, the chromatic substance, and the nuclear membrane.
The nucleolus disappears. In some cases it passes from the nucleus into the cytoplasm, where it breaks up; in other cases the details of its disappearance are entirely unknown.