MODES OF CELL-DIVISION.

13

Modes of Cell-division.-Two modes of cell-division are known—(1) direct division or budding; (2) indirect division, karyokinesis, mitosis, or karyomitosis.

Direct Division.-In the process of direct division a constriction occurs in the center of the cell which increases until the cell is split in two.

Mitosis. The process of indirect division is by far the more frequent and complicated method. The process is chiefly concerned in the nucleus, and has been divided into stages for the sake of convenience of description, although, of course, in reality they merge imperceptibly from one into the other.

Resting Stage.—In this stage the cell is, as has been described, above the chromatin filaments, being arranged in a network.

Close Skein.-The centrosome leaves the nucleus and divides into halves, which proceed to opposite poles of the cell and, together with their radiating lines and clear zone, are called attraction spheres. The nuclear network resolves itself into a fine close skein.

Loose Skein, or Wreath.-The skein becomes less tangled and more distinct. It then resolves itself into a number of convolutions arranged as a wreath. (See Fig. 3.)

Monaster. The loops of the wreath separate, leaving a series of V-shaped threads with the apex of the V toward the center of the cell, giving a radiating or starform, hence the name.

Diaster.—Each V shape in the monaster now splits horizontally into exactly similar halves whose apices begin to diverge from their respective opposite halves and point toward the nearest attractive sphere.

In this condition they present the appearance of two stars, each consisting of half the monaster, called daughter stars.

Division of Nucleus.-These daughter stars separate from each other, going toward the attraction sphere, and the sides of the nucleus begin to constrict at a zone between the daughter stars. This constriction progresses until we have the nucleus divided into two. Each half contains a daughter star and an attraction sphere.

Division of Cell-body. The cell-wall next assumes an indentation, which increases in size until the cell is

Fig. 3.-Diagram of stages of karyokinesis: a, Resting stage; b, close skein stage; c, d, loose skein or wreath stage; e, monaster stage; f, diaster stage; 9, showing constriction of cell-body and two nuclei in loose skein; h, cell divided completely, nuclei in resting stage.

divided in two, a half of the original nucleus remaining in each half cell.

Reverse Procedure.-The chromatin fibrils in the daughter stars now begin to undergo a reversed procedure.

The ends of the V-shaped fibers unite to form a wreath which tangles to become a close skein, which relapses into the nuclear network found in the resting stage. The nucleoli reappear.

The attraction sphere once more enters the nucleus,

and we have two cells, which are identical (except in size) with the parent cell.

Time Required.-These changes, while complex, take place rapidly. In the human subject half an hour may be sufficient to complete the cycle.

Development.-It has been stated above that cells are always produced by preexisting cells. This would lead us to suspect that the origin of the adult animal might be traced backward to a single cell.

Ovum. In man this origin may be traced to the ovum, a complete or typical cell generated in the ovary of the female. This cell passes down one of the Fallopian tubes to the uterus, meeting and uniting, on the way down, with a spermatozoon (the male element), provided conception has occurred.

In the uterus it multiplies by indirect division into two, these two into four, then into eight, etc. For some time this process is kept up, the cells arranging themselves to form a spheric mass, which has been termed the mulberry mass.

Multiplication of some groups of cells now proceeding faster than others, the mass begins to flatten and lengthen, and eventually arranges itself into three superimposed layers, called the primary germ layers.

Primary Germ Layers.-The upper layer is called the epiblast, or ectoderm; the middle layer is called the mesoblast, or mesoderm; the lower layer, the hypoblast, or entoderm. To these some writers have added a fourth layer, the parablast.

Specialization.-The cells of these layers in their growth now begin to assume certain characteristic peculiarities, which differentiate them more and more and cause them to resemble the various tissues they are to form. This process of specialization proceeds in a

regular and definite form, the same layer always producing the same structure in the adult.

Tissue. A tissue is an aggregation of similar cells and intercellular substance having a common function. The principal tissues originating from the primary germ layers are as follow (from Piersol):

From the epiblast, or ectoderm, are derived the epithelium of the outer surface of the body, including that of the conjunctiva and of the anterior surface of the cornea, the external auditory canal, together with the epithelial appendage of the skin, as hair, nails, sebaceous and sweat-glands.

The epithelium of the nasal tract, with its glands and communicating cavities.

The epithelium of the mouth, and salivary and other glands discharging into the oral cavity.

The enamel of the teeth.

Tissues of the nervous system.

The retina of the eye.

The epithelium of the membranous labyrinth.
The pituitary and pineal bodies.

From the mesoblast :

The connective tissues.

Areola tissue.

Tendons.

Cartilage and bone.

Dentin of teeth.

The muscular tissue (except that of sweat-glands). The tissue of the vascular and lymphatic systems, with their endothelium and circulating cells.

The sexual glands and their excretory passages as far as the termination of the ejaculatory ducts, the vagina, the kidneys, and ureter.

From the hypoblast, or entoderm :

The epithelium of the digestive tract, with that of all

THE CONNECTIVE TISSUE.

17

the glandular appendage, with the exception of those named as being of epiblastic origin. The epithelium of the respiratory tract.

The epithelium of urinary bladder and urethra.

The epithelium of thyroid and thymus bodies (in the latter, Hassel's corpuscles).

If we consider the parablast as one of the germ layers, we must omit certain structures from the above classification, as follows:

Tissue developed from parablast.

Blood-vessels.

Lymphatics and lymph-glands.
Blood-corpuscles.

Connective tissue.

All the tissues of the body may, for convenience, be divided or classified into four groups-viz.: (1) Epithelial tissue; (2) connective tissue; (3) muscular tissue; (4) nervous tissue.

The Connective Tissue. The principal connective tissues are mucous, white fibrous, yellow elastic, adipose, retiform, bone, and dentin.

Areola tissue is a tissue composed of white fibrous, yellow elastic, and adipose tissues in varying amounts. Mucous tissue is composed of large branching cells surrounded by a soft gelatinous substance (mucin). In the adult it is found in the vitreous humor of the eye. In the embryo it occurs in the umbilical cord and in the subcutaneous tissue.

White fibrous tissue is made up of very fine, delicate fibrils, which run more or less parallel, do not branch, and are united to form bundles. These bundles may unite to form larger bundles, or may cross to form a regular network. Two kinds of cells are found in this tissue-known as the wandering and the fixed cells. The fixed cells are irregularly stellate, and are direct