containing the nucleus and occupying the cell-cavity; its interstices are filled with fluid. In young vegetable cells such a distinction does not exist; a finely granular protoplasm occupies the whole cell-cavity (fig. 8, A).

Another striking difference is the frequent presence of a large quantity of intercellular substance in animal tissues, while in vegetables it is comparatively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. As an example of the manner in which this end is attained in animal tissues, may be mentioned the deposition of lime-salts in a matrix of intercellular substance in ossification.

Morphological Development and Division of Functions.

As we proceed upwards in the scale of life from monocellular organisms, we find that another phenomenon is exhibited in the life history of the higher forms, namely, that of Development. An amœba comes into being derived from a previous amoeba; it manifests the properties and performs functions of life which have been already enumerated; it grows, it reproduces itself, whereby several amœbæ result in place of one, and it dies, but it can scarcely be said to develop unless the formation of a nucleus can be so considered. In the higher organisms, however, it is different; they, indeed, begin as a single cell, but this cell on its division and subdivision does not form so many different organisms, but possesses the material from which, by development, the completed and perfected whole is to be derived. Thus, from the spherical ovum, or germ, which forms the starting-point of animal life, and which consists of a protoplasmic cell with a nucleus and nucleolus (see fig. 4), in a comparatively short time, by the process of segmentation which has been already mentioned, a complete membrane of cells, polyhedral in shape from mutual pressure, called the blastoderm, is formed, and this speedily divides into two and then into three layers, chiefly from the rapid proliferation of the cells of the first single layer. These layers are called the epiblast, the mesoblast, and the hypoblast.

It is found in the further development of the animal that from each of these layers is produced a very definite part of its completed body. For example, from the cells of the epiblast, are derived, among other structures, the skin and the central nervous

system; from the mesoblast is derived the flesh or muscles of the body, and from the hypoblast, the epithelium of the alimentary canal and some of the chief glands.

From the epiblast are ultimately developed the superficial skin or epidermis and its various appendages, also the central or cerebro-spinal nerve centres, the sensorial epithelium of the organs of special sense (the eye, the ear, the nose), and the epithelium of the mouth and salivary glands.

From the hypoblast is developed the epithelium of the whole digestive canal, together with that lining the ducts of all the glands which open into it; also the glandular parenchyma of the glands (e.g., liver and pancreas) connected with it, and the epithelium of the respiratory tract.

Fig. 9.-Transverse section through embryo chick (26 hrs.). a, epiblast; b, mesoblast; c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; e, primitive groove; f, dorsal ridge. (Klein.)

From the mesoblast are derived all the tissues and organs of the body intervening between these two, the whole group of the connective tissues, the muscles and the cerebro-spinal and sympathetic nerves, with the vascular and genito-urinary systems, and all the digestive canal, with its various appendages, with the exception of the lining epithelium above mentioned.

It is obvious that these tissues and organs exhibit in a varying degree the primary properties of protoplasm. The muscles, for example, derived from certain cells of the mesoblast are highly contractile and respond to stimuli readily, but they have little to do with digestion except indirectly, and again, the cells of the liver, although doubtless contractile to a certain extent, yet have secretion and digestion for their chief functions.

Thus we see development in two directions going on side by side. It speedily becomes necessary for the organism to depute to different groups of cells, or their equivalents (i.e., to the tissues or organs to which they give rise), special functions, so that the various functions which the original cell may be supposed to discharge, and the various properties it may be supposed to possess, are divided up among various groups of resulting cells.

The work of each group is specialised. As a result of this division of labour, as it is called, these functions and properties are, as might be expected, developed, and made more perfect, and the tissues and organs arising from each group of cells are developed also, with a view to the more convenient and effective exercise of their functions and employment of their properties. It would be out of place here to discuss the question as to the exact manner in which a property or function, rudimentary in a low form of animal life, is found to be highly developed as we pass up the series; neither is it our province to discuss the very complicated subject of the relationship of man to other animals, and of these to one another.

Having now briefly indicated the close connection which exists between Human physiology and Biology in general, we are better prepared to commence the study of the former as constituting a part of a great whole.

The next two chapters will be devoted to a consideration of the minute structure, or the histology (ioròs, a tissue or web) of epithelium and the connective tissues.

CHAPTER II.

THE STRUCTURE OF THE ELEMENTARY TISSUES.

THE cells of the body are described in various ways; for example, according to their shape, situation, contents, origin and functions.

(a.) Their shape varies :-Starting from the spherical or spheroidal (fig. 10, a) as the typical form assumed by a free cell, we find this altered to a polyhedral shape when the pressure on the cells in all directions is nearly the same (fig. 10, b).

Of this, the primitive segmentation-cells may afford an example. The discoid shape is seen in blood-cells (fig. 10, c), and the scalelike form in superficial epithelial cells (fig. 10, d). Some cells have a jagged outline (prickle-cells) (fig. 27).

Cylindrical, conical, or prismatic cells occur in the deeper layers

of laminated epithelium, and the simple cylindrical epithelium of the intestine and many gland ducts. Such cells may taper off at one or both ends into fine processes, in the former case being caudate, in the latter fusiform (fig. 11). They may be greatly elongated so as to become fibres. Ciliated cells (fig. 10, d) must

Fig. 10. Various forms of cells. a. Spheroidal, showing nucleus and nucleolus; b. Polyhedral; c. Discoidal (blood cells); d. Scaly or squamous (epithelial cells).

be noticed as a distinct variety: they possess, but only on their free surfaces, hair-like processes (cilia). These vary immensely in size, and may even exceed in length the cell itself. Finally, we have the branched or stellate cells, of which the large nerve-cells of the spinal cord, and the connective tissue corpuscle

a

Fig. 11. Various forms of cells. a. Cylindrical or columnar; b. Caudate; c. Fusiform; d. Ciliated (from trachea); e. Branched, stellate.

are typical examples (fig. 11, e). In these cells the primitive branches by secondary branching may give rise to an intricate network of processes.

(b.) According to their situation in the tissues cells are known. as epithelial, connective tissue cells, blood cells, glandular, and the like.

(c.) According to their contents, they are called fat cells when their protoplasm contains an excess of fat, pigment cells when it contains pigment; coloured, when their protoplasm is infiltrated with a colouring matter, as hæmoglobin.

C

(d.) According to their functions, they are called secreting, protective, sensitive, contractile, &c.

(e.) According to their origin, they are named epiblastic, mesoblastic, and hypoblastic.

Nearly all cells at some period of their existence possess nuclei. As has been incidentally suggested the origin of a nucleus in a cell is the first trace of the differentiation of protoplasm. The existence of nuclei was first pointed out in the year 1833 by Robert Brown, who observed them in vegetable cells. They are

Fig. 12.-(A). Colourless blood-corpuscle showing intra-cellular network of Heitzmann, and two nuclei with intra-nuclear network (Klein and Noble Smith).

(B.) Coloured blood-corpuscle of newt showing intra-cellular network of fibrils (Heitzmann). Also oval nucleus composed of limiting membrane and fine intra-nuclear network of fibrils. X 800. (Klein and Noble Smith.)

either small transparent vesicular bodies containing one or more smaller particles (nucleoli), or they are semi-solid masses of protoplasm always in the resting condition bounded by a well-defined envelope. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Beale is fully justified in comprising both under the term "germinal matter." They exhibit their vitality by initiating, in the majority of cases, the process of division of the cell into two or more cells (fission) by first themselves dividing. Distinct observations have been made, showing that spontaneous changes of form may occur in nuclei as also in nucleoli.

Histologists have long recognised nuclei by two important characters :

(1.) Their power of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly defined, and they are rendered more easily visible. This indicates some chemical difference between the protoplasm of the cell and nuclei, as the former is destroyed by these reagents.