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are formed by two successive mitotic divisions, of which the first is heterotypical and produces reduction of the chromosomes, and the second is homotypical.
The two divisions differ from the corresponding divisions of the oocytes in three important respects: (1) centrosomes are present; (2) the four grand-daughter cells produced are of equal size and presumably of equal value, so far as capability of uniting with a mature ovum to form a zygote is concerned; (3) each of the four grand-daughter cells possesses two centrosomes.
In the prophase of the first or heterotype division the nucleus and nucleolus disappear in the ordinary way. The centrosome divides, and an achromatic spindle appears, which has the daughter centrosomes at its poles and half the typical number of chromosomes at its equator. The chromosomes are twin chromosomes. During the metaphase the two segments of each twin chromosome separate from each other. In the anaphase they travel to the opposite poles of the achromatic spindle, and consequently, when the cell divides in the telophase, each daughter cell or spermatocyte II contains a centrosome and half the typical number of chromosomes.
The second maturation division, which takes place without the intervention of a resting stage, is of the homotype form. The centrosome divides, a new achromatic spindle appears, and the daughter chromosomes gather at its equator. In the metaphase the chromosomes divide into equal parts, which travel to the opposite poles of the spindle during the anaphase, and when the telophase is completed the grand-daughter cells, which are called spermatids, possess a centrosome and half the typical number of chromosomes. In the resting stage which follows, the chromatic particles become enclosed in a new-formed nucleus, and the centrosome, if it has not already divided, separates into two parts, one which lies nearer the nucleus and is called the anterior centrosome, and another, farther from the nucleus, termed the posterior centrosome (Fig. 22). Numerous mitochondria are present, and an indefinite structure, called the accessory body, is also found in the cell protoplasm. A spermatid, therefore, differs from a typical animal cell not only because it possesses the chromatic substance of only half the typical number of chromosomes, but also because it possesses an accessory body and two centrosomes.
From Spermatid to Spermatozoon.-The reader will have noted that the female gametes become mature and ready for conjugation with male gametes directly after the second maturation division is completed. In the case of the male germ cells, however, the spermatids which result from the second maturation division have still to undergo a complicated process of transformation before they become converted to spermatozoa or mature male gametes. The process of transformation takes place in association with the nurse cells in which the developing spermatozoa become embedded.
The details of the process of transformation are difficult to follow, and the knowledge regarding them is still to some extent indefinite. Certain points, however, are well established; but before they are considered it is necessary that the reader should be acquainted with the anatomy of an adult spermatozoon.
A spermatozoon is a minute organism consisting of a head, a neck, a body, a tail, and an end-piece. Its total length is about 50 μ, that is, its length is about the same as the diameter of the nucleus of the ovum.
The head has the form of a laterally compressed ovoid. It is separable into anterior and posterior portions, and the anterior portion is more or less completely covered by a head-cap, which culminates in a sharp ridge. The length of the head is about 4.5 μ.
The neck is an extremely short constricted region which intervenes between the head and the body. At its anterior end, where it joins the head, there is a deeply staining anterior centrosome, and at its posterior end a similarly deepstaining posterior centrosome, from which a deep-staining axial filament extends posteriorly through the body and tail into the end-piece (Fig. 21).
The Body.—The body is a little longer than the head, and its constituent parts
are: (1) a portion of the axial
The axial sheath is a thin layer of protoplasm immediately surrounding the axial filament.
The spiral sheath consists of a spiral fibril embedded in indifferent protoplasm, and the mitochondrial sheath, which sur-. rounds the spiral sheath, is formed by protoplasm containing numerous mitochondria.
Nucleus -Anterior centrosome Posterior centrosome
The end-ring closes the posterior ends of the spiral and mitochondrial sheaths, and it is perforated by the axial filament and its sheath as they pass from the body into the tail.
FIG. 20.-HUMAN SPERMATOZOA
The Transformation of the Spermatid into the Spermatozoon. As the transformation progresses the nucleus of the spermatid becomes the head of the spermatozoon. The axial filament grows out from the posterior centrosome of the spermatid, which divides into two parts, one of which becomes the posterior End piece centrosome of the neck of the spermatozoon, whilst the other becomes the end-ring of the body of the sperma- FIG. 21.-STRUCTURE OF A HUMAN
SPERMATOZOON (after Meeves).
the anterior centrosome of the neck of the spermatozoon. The cytoplasm of the spermatid forms the axial sheath, the indifferent protoplasm of the spiral sheath and mitochondrial sheath. The origin of the spiral filament and the origin of the head-cap are uncertain, but it is stated that, in some animals, the head-cap is formed from the accessory body, which is not shown in Fig. 22.
FIG. 22. SCHEMA OF TRANSFORMATION OF SPERMATID
The Object of the Reduction of the Chromosomes.-The most striking phenomenon of the process of the maturation of the gametes is the reduction of the chromosomes. The constancy of the reduction tends to emphasise its importance, but, as we have no definite knowledge of the functions of the chromatic substance, the object of the reduction
can only be surmised. The evidence which has been accumulated tends to the
Sheath of axial filament
conclusion that the particles of the chromatic substance are the bearers of hereditary tendencies and capabilities. If this is the case, then they are the means by which ancestral possessions, in the morphological sense, are transmitted from generation to generation. There is evidence also, first ascertained by Mendel and substantiated and increased in recent years by his followers, which lends probability to the belief that the tendency carriers form two main groups: (1) those which carry certain tendencies; (2) those which carry opposite tendencies. The bearers of tendencies and the bearers of their opposites are allelomorphic or alternative to each other, and are called allelomorphs. Thus the particles which bear tallness and dwarfness respectively are allelomorphs, that is, they are alternative to each other.
Further, the facts which are known suggest the idea that in the primitive germ cells, and their descendants which contain the typical number of chromosomes, the character-bearing particles are arranged in pairs of which both elements may bear the same tendencies, or one may bear one tendency and the other the opposite.
For example, if red and blue be supposed to be opposite tendencies carried by different particles or allelomorphs, then the germ cells of any given animal, male or female, may contain either a pair of red-bearing particles, a pair of blue-bearing particles, or a red and a blue bearing particle associated together as a pair.
The reduction of the chromosomes during the maturation divisions of the germ cells is an admitted fact, and it is believed that the reduction is a necessary preliminary to the union of a male and a female gamete to form a zygote from which a new individual may arise. It is assumed that the purpose of the reduction is the segregation of the different tendency bearers from each other in order that they may enter into new combinations. If this assumption is correct, then every mature germ element or gamete contains only one element of any given pair of tendency bearers, in the supposititious case under consideration, either the red or the blue bearer, but not both; and the object of the reducing division is the segregation of the allelomorphs in order that they may enter into new and possibly into different combinations, producing new and possibly varied results.
If, in the case of any given group of animals, the mature germ cells of some of both sexes contain the blue-bearing particles and others the red-bearing particles, it necessarily follows that three possible results may ensue when impregnation occurs, that is when two mature germ cells unite to form a zygote.
(1) A female gamete bearing red tendency particles may fuse with a male gamete bearing red tendency particles; (2) a female gamete bearing blue tendency particles may meet and fuse with a male gamete bearing blue tendency particles; (3) a female gamete bearing red tendency particles may meet and fuse with a male gamete bearing blue tendency particles. The constitution of the zygotes formed may be stated as follows:
and the character of the individual developed from the zygote will vary according to the combination. If two red tendency bearing gametes meet, the individual will be red; if two blue tendency bearing gametes meet, the individual produced will be blue; but when a gamete bearing red tendency particles unites with a gamete bearing blue tendency particles the individual will be either red or blue. or a combination of the two, the result depending upon the relative potency or dominance of the two tendencies.
Further exposition of this interesting subject would be out of place in a textbook of anatomy, but it is of such great importance in association with the transmission of hereditary characteristics and hereditary diseases that every medical student should make himself familiar with its possibilities by consulting the works of Bateson, Punnet, and other writers and observers who are attempting to solve the complicated problems which it presents.
1 It must be understood that this function, if it exists, does not prevent the chromatic particles possessing other functions, and that there is no evidence that the potency of a tendency depends upon amount of chromatic substance.
Fertilisation is the term applied to the union of the male with the female gamete to form a zygote which contains the typical number of chromosomes (Fig. 23). The meeting of the gametes and their union take place, normally, in the upper or middle part of the uterine tube.
The details of the process are unknown in the case of the human subject, but in many animals it has been noted that as the spermatozoon approaches the ovum the latter shows signs of excitement, and a small prominence, called the cone of attraction, appears on its surface. At the same time its pronucleus undergoes changes of form. As the two gametes meet the spermatozoon pierces the oolemma which surrounds the ovum and passes through the cone of attraction into the body of the ovum.
In some cases apparently only the head, neck, and body of the spermatozoon effect an entrance, but in others the whole spermatozoon enters the body of the ovum. After the entrance occurs and before the second polar body is formed, the parts of
FIG. 23. SCHEMA OF THE FERTILISATION OF THE MATURE OVUM AND THE FORMATION OF THE ZYGOTE.
the spermatozoon which have entered remain quiescent. After the second polar body is formed they disappear and are replaced by a nucleus which contains half the typical number of chromosomes and which is accompanied by two centrosomes. At this period the impregnated ovum contains two pronuclei, both of which contain half the typical number of chromosomes; but the female pronucleus has no accompanying centrosomes.
Shortly after the appearance of the male pronucleus the two pronuclei unite and then the zygote, formed by the union of the male and female gametes, consists of a cell body enclosing a nucleus called the first segmentation nucleus; and two centrosomes.
The first segmentation nucleus is the product of the union of the male and the female pronuclei. It contains the typical number of chromosomes, half being derived from the male and half from the female gamete; and it is accompanied by two centrosomes, both of which appear to be derived from the male gamete, though their exact origin has not yet been definitely established. The zygote and the polar bodies which are still present are enclosed within the oolemma.
Immediately after its formation the zygote is separated, by a series of consecutive mitotic divisions, into a large number of cells which are grouped together in the form of a solid spherical mass, called a morula on account of the mulberrylike appearance of its surface. This period of division is called the period of segmentation (Figs. 24-27).
The segmentation divisions are of the homotype form, and there is evidence which tends to the conclusion that the earliest divisions, by which the zygote is divided first into two and then into four parts, are quantitatively and qualitatively equal. After a time, however, the divisions result in the formation of cells of different sizes and different capabilities, definite and circumscribed functions being allocated to certain groups of cells and their descendants. It is probable FIG. 24.-SEGMENTATION OF ZYGOTE that at this time cells are set apart which are the progenitors of the germ cells of the next generation, and which therefore retain all the capabilities of their ancestors. These cells are the means by which the species is reproduced and the hereditary tendencies are transmitted from generation to generation. At the same time other cells are set apart for the production of the tissues and organs of the individual which will be produced from the zygote, and in which the germ cells and their descendants will be lodged and protected till they attain their maturity.
After the morula is established one of the first FIG. 25.-SEGMENTATION OF ZYGOTE definite changes which occurs in its constitution is the differentiation of its cells into an outer layer and an inner mass (Fig. 26).
In the human subject, as in many other mammals, Outer layer the cells of the outer layer constitute the trophoblast or trophoblastic ectoderm, which plays a most important part in the nutrition of the embryo and foetus. It enters into the formation of the chorion, or outermost envelope of the growing zygote, which is subsequently differentiated into a placental and a nonplacental portion and which serves, in the first instance, both as a protective and a nutritive covering.
In many mammals the cells of the inner mass soon separate into two main groups, the ecto-mesoderm and the entoderm; but it appears probable that, in the human subject, they differentiate into three groups, ecto-mesoderm, primary mesoderm, and entoderm.
FIG. 26.-SEGMENTATION OF ZYGOTE.
In the majority of mammals, immediately before or as the differentiation of the inner mass occurs, a cavity appears in the zygote. As soon as the cavity appears. the morula is converted into a blastula and the cavity enlarges until it separates the inner mass from the outer layer, except at one pole of the zygote, where the inner mass and the outer layer remain in contact. The cavity is called the segmentation cavity. It would appear, however, from the evidence at present available, that this primitive cavity never exists in the human subject, for as the main part of the inner mass separates from the outer layer the cells of the primary mesoderm segment of the inner mass proliferate rapidly and form a jelly-like tissue which completely fills the space which would otherwise become the segmentation cavity. At the same time the ecto-mesodermal and entodermal segments of