HUMAN EMBRYOLOGY.

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.

Nucleolus

Animal cells differ from each Nucleus other in minor points of structure,

in association with the positions Nuclear
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.

Spongioplasm
(cyto-reticulum)

Hyaloplasm

-Attraction sphere

Centrosome

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

Centrosome

with centrioles

Nucleus

Nucleus with
chromatic
substance in
skein form

Daughter centrosome

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)

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.

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

Daughter centrosome

Chromosomes

dividing into.

equal parts

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

Daughter centrosome

Chromosomes at

pole of spindle

Achromatic

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.

Centrosome

Nucleus

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 ultimately ends in death; for a time comes when cells no longer transmit to their