Talk:Book - A Laboratory Manual and Text-book of Embryology 1

CHAPTER I THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION

The Germ Cells

The highly dififerentiated human organism, like all other vertebrates and most invertebrates, develops from the union of two germ cells, the ovum and spermatozoon.

The Ovum

The female germ cell, or ovum, is a typical animal cell produced in the ovary. It is nearly spherical in form and possesses a nucleus with nucleolus^ chromatin network, chromatin knots, and nuclear membrane (Figs. 1 and 2). The cytoplasm of the ovum is distinctly granular, containing more or less numerous yolk granules and rarely a minute centrosome. The nucleus is essential to the life, growth, and reproduction of the cell. The function of the nucleolus is unknown; the chromatin probably bears the hereditary qualities of the cell. The yolk granules, containing a fatty substance termed lecithin, furnish nutrition for the early development of the embryo. A relatively small amount of yolk is found in the ova of the higher mammals, since the embryo develops within, and is nourished by, the uterine wall of the mother. A much larger amount occurs in the ova of fishes, amphibia, reptiles, birds, and the primitive mammalia, the eggs of which are laid and develop outside of the body. The so-called yolk of the hen's egg (Fig. 3) is the ovum proper and its yellow color is due to the large amount of lecithin which it contains.

Ova become surrounded by protective membranes, or envelopes. The vitelline membrane, secreted by the egg itself, is a primary membrane (Fig. 2). The follicle cells about the ovum usually furnish other secondary membranes, e. g., the zona pellucida. Tertiary membranes may be added as the egg passes through the oviduct and uterus — the albumen, shell membrane, and shell of the hen's egg are of this type (Fig. 3).

The human ovum is of small size, measuring from 0.22 to 0.25 mm. in diameter (Fig. 1). The cytoplasm is surrounded by a relatively thick radially striated membrane, the zona pellucida. The striated appearance of the zona pellucida is said to be due to fine canals which penetrate it and through which nutriment is carried to the ovum by smaller follicle cells during its growth within the ovary.

'I hf- tff't0rt awl pf mth «A the <wufn within the ovary (oogenesis) are described on f^>. 2)4- 217, We may <ttate here that each growing ovum is at first surrounded the Graafian follicle, within which the ovum is eccentrically located (Figs. 4 and 230). The cells of the Graafian follicle immediately smrounding the ovum form the corona radiata (Fig. 1) when the ovum is set free.

I'll), I . ttiiOMn t»'tiin r<(Bniinnt fmh In the liqunr folUcuU OValdeyer). X 415. The eooa, pellurliln 1( ovn Hfi n lliUk. ilmr iiinllr niirriHinilnl by the cells of the coroiu ndiata. Tltencariy mature run ilwll ihiiw* n i-i'iilinl itrntiulnr ileuliipUHmic arrn and a peripheral clear layer, and endoses the niiclrui In which in nrvn Ilu> iiuclei>lti«. At ihr rinhl b a !i)irrmatoioOn cortcspoDdingly enlarged.

liv s«ia1) mun'iiw vvlK Viiowii a^ 'fhii.-^f -rJvs. ITx-sc incifAse in munbca' during A^i^VAn'ns Ivtwwn tlios*' .Vu-- Kww-.-^ ftjVsi «iih fl^i.^ Aiiii ihas ioncs » sac.

Fig. 3. — Diagrammatic longitudinal section of an unincubated ben's egg (Allen Thomson in Heisler): b.l. germinal disc; iF.y, white yolk, whicb consists of a central tkskshaped mass, and a number of concentric layers surrounding the yellow yolk (y,y.); ^-'^ vitelline membrane; i, a somenliat Quid albuminous layer which immediately envelops the yolk; w, albumen, composed of alternating layers of more and less fluid portions; ckJ, chalazx; a.ch, air chamber ■t the blunt end of the egg — simply a space between the two la>-eT5 of the shell membranei t.s.m, inner, i.m, outer layer of the sheU membrane; i, shell.

n 1 d ng rtex g rm nal p th 1 m f free ur( b tun ca albu (^ ea penph ral t ma ntain mg mma re Graaban foUi Its {(f) w U d n d f 11 1 from whose waU membntna granulosa has partially separated; /, cavity of liquor folliculi; I, ovum surrounded by cell mass constituting cumulus oophonis (Piersol).

Fig. 5. — Section of well -developed Graafian fdlicle from human embryo (von HerS); the enclosed ovum

Fig. 6.— Uterine tube and ovary with mature Graafian follicle about ready lo burst (Ribemont-Dessaignes) .

Ovulation and Menstruation

When the ovum is ripe, the Graafian follicle ix large and contains fluid, probably under pressure. The ripe follicles form bud-like projections at the surface of the ovary (Fig. 6), and at these points the ovarian wall has become very thin. It is probable that normally the bursting of the (iraafian follicle and the discharge of the ovum are periodic and associated with the phenomena of menstruation, as maintained by Fraenkel and Villeimn. 'I'hut ovulation or discharge of the ovum from the ovary may occur independently of the menstrual periods has been proven by the observations of Leopold and it( Kiivano, Also in young girls ovulation may precede the inception of menHlrualion and it may occur in women some time after the menopause.

Fig. 7.— T. ImnMlun (hIIU-If i-tini«inin]i six uv«. Kront Ihf o\*ar)- of a >-oung monkey. X 430.

At birth, or shortly itfter. nil of the ova arc fi>rmed in the ovary of the female thiUI. Hcniti-i) i-stinmtoii llwl ii nurmul human fomale may develop in each ovary M) riiv ovii. Most of tln> yo\mK o\;i. wliich may numbor 50.000. degenerate and lU'Vt'i vi\ti'h mKtuntv. At ovul«li»n but »>no ovum is normally ripened and disihitrjrrd fhim iW oviiry, SfviTid ma. howvvcr. may be produced in a single i\\\ih\\' ilk rai't' *'iis»>N. Stub HUiltiplc (ulliiU-s ha\o Kx-n olt«T\-wl in human o\'aries Aud itif ol frrt)Ui'i)t iwviinvmv in tlu- ovary oi the monkey vFig. 7),

The Spermatozoa

Thf S(Wrnt«tointiit. V\\v male a-ll or s)XTm;it\^iKin of man is a minute cell (1 lV\* mm Kin^, sinviali^-il loi ai livo mo\ rmcm Inx^iiuso of their acti\-e movements, spermatozoa were, when first discovered, regarded as parasites living in the seminal fluid. The sperm cell is composed of a flattened heady short neck^ and thread-like tail (Fig. 8).

The head is about 0.005 mm. in length. It appears oval in side view, pear-shaped in profile. When stained, the anterior two-thirds of the head may be seen to form a cap^ and the sharp border of this cap is the perfaraiorium by means of which the spermatozoon penetrates the ovum. The head contains the nuclear elements of the sperm cell. The disc-shaped neck contains the arUerior centrosomal body. The tail begins with the posterior centrosomal body and is divided into a short connecting piece y a chief piece ox jlageUum^ which forms about four-fifths of the length of the sperm cell, and a short end piece, or terminal filament. The connecting piece is marked off from the chief piece by the annulus. The connecting piece is traversed by the axial filament (filum principale), and is surrounded (1) by the sheath common to it and to the flagellum; (2) by a sheath containing a spiral filament; and (3) by a mitochondrial sheath. The chief piece is composed of the axial filament surrounded by a cytoplasmic sheath, while the end piece comprises the naked continuation of the axial filament.

The spermatozoa are motile, being propelled by the movements of the tail. They swim always against a current at the rate of about 2.5 mm. a minute. This is important, as the outwardly directed currents induced by the ciliary action of the uterine tubes and uterus direct the spermatozoa by the shortest route to the infundibulum. Keibel has found spermatozoa alive three days after the execution of the criminal from whom they were obtained. They have been found motile in the uterine tube three and one-half weeks after coitus. They have been kept alive eight days outside the body by artificial means. It is not known for how long a period they may be capable of fertilizing ova, but, according to Keibel, this [K-riod would certainly be more than a week. Lode estimates that 200 million spermatozoa are liberated at an average ejaculation.

Fig. 8. — Diagram of a human spermatozo5n, highly magnified, in side view (Meves, Bonnet).

Mitosis and Amitosis

Before the discharged ovum can be fertilized by the male germ cell, it must undergo a process of cell division and reduction of chromosomes known as matu++++ration. As the student may not be familiar with the processes of cell division, a brief description is appended. (For details of mitosis see textbooks of histology and E. B. Wilson's "The CeU.")

Amitosis. — Cells may divide directly by the simple fission of their nuclei and cytoplasm. This rather infrequent process is called amitosis. Amitosis is said by many to occur only in moribund cells. It is the type of cell division demonstrable in the epithelium of the bladder.

Mitosis. — In the reproduction of normally active cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (.thread) in distinction to liviiiotl for convenience into four phases

Prophase. — 1. The centrosome divides and the two minute bodies resulting from the division move apart, ultimately occupjdng positions at opposite poles of the nucleus (I-III).

2. Astral rays appear in the cytoplasm about each centriole. They radiate from it and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (II).

3. The nuclear membrane and nucleolus disappear, the nucleoplasm and cytoplasm becoming continuous.

4. During the above changes the chromatic network of the resting nucleus resolves itself into a skein or spireme^ which soon shortens and breaks up into distinct, heavily-staining bodies, the chromosomes (II, III). A definite number of chromosomes is always found in the cells of a given species. The chromosomes may be block-shaped, rod-shaped, or bent in the form of a (J++++5. The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U-shaped, the base of each U is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure and at the end of the prophase this is called a monaster,

Metaphase. — The longitudinal splitting of the chromosomes into exactly similar halves constitutes the metaphase (IV, V). The aim of mitosis is thus accomplished, an accurate division of the chromatin between the nuclei of the daughter cells.

Anaphase. — At this stage the two groups of daughter chromosomes separate and move up along the central spindle fibers, each toward one of the two asters. Hence this is called the diaster stage (V, VI). At this stage, the centrioles may each divide in preparation for the next division of the daughter cells.

Telophase. — 1. The daughter chromosomes resolve themselves into a reticulum and daughter nuclei are formed (VII, VIII).

2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic spindle (VIII). Two complete daughter cells have thus arisen from the mother

cell.

The complicated processes of mitosis, by which cell division is brought about normally, seem to serve the purpose of accurately dividing the chromatic substance of the nucleus in such a way that the chromatin of each daughter cell may be the same qualitatively and quantitatively.

This is important if we assume that the chromatic particles of the chromosomes bear the hereditary qualities of the cell. The number of chromosomes is constant in the sexual cells of a given species. The smallest number of chromosomes, two, occurs in Ascaris megalo ifphalft unlvtUem, a round worm parasitic in the intestine of the hoisc. The largest number known \% found in the brine shrimp, ArUmia, where 168 have been counted.

The number for the human cell is in doubt. Guyer (1910) and Montgomery (1912) found 22 in the upermatogonia of negroes, and Guyer (1913) reported considerably larger nutnbrrM (count not given) for white spermatogonia. According to Winiwarter's recent work on whites (Arch, de Biol., T. 27, 1912), the number of chromosomes in each immature ovum or o^kytc in 48, in each spermatogone 47. Wieman (1913) found the most frequent tiuniltrr in varioun white Mmatic cells to be 34, but recently (Amer. Jour. Anat., vol. 21, 1917) hr UAMTtH thnt the numl>er in both negro and white spermatogonia is 24, thereby agreeing with DurMbrrg (19()6).

W(* hdve Hcen that reproduction in mammals is dependent upon the union of male and female germ cells. The union of two germinal nuclei (pronuclei) would neceHsarily double the number of chromosomes in the fertilized ovum and alw) thr number of hereditary qualities which their particles are supposed to bear. I'hJH mulllplication of hereditary qualities is prevented by the processes of maturation which take place in both the ovum and spermatozoon.

Maturation

Maturation may be defined as a process of cell division during which the number of chromosomes in the germ cells is reduced to one-half the number 1 haracterlstlc for the species.

The sptrmittotoa take their origin in the germinal epithelium of the testis. Their <lr\Tlopment» or s pcrmalogettesis , may be studied in the testis of man or of the rati their nuituration stages in the tubular testis of Ascaris, Two types of cells may he recognized in the germinal epithelium of the seminiferous tubules, the sHxttHiiivuhir vtliK (of Sertoli), and the male germ cells or spermatogonia (Fig. \{)). The s|HTmatogt)nia divide, one daughter cell forming what is known as a primiiry .v/^rwii/<>f v/r. The other daughter cell persists as a spermatogone, and, hy auitinvuHi <llvislon durlt^g the sexual life of the indiWduaK gives rise to other primary s|H*rn\atiK\vtes. The primary sjx^rmatocytes correspond to the ova hel\Mv luaturatUm. Kach a>ntains the numlxT of chromosomes typical for the n\ale wi the sihhIos. The pnHX\^< of maturation ajnsists in two cell divisions of the prln\ary s|H^rmatiHytes» each pnnlucing first, two secondary spermatocytes ^ {\\\\\ thcHO \\\ turn four ivils known as spermatids. During these cell divisions the uvuuIhm ot \ h<>>u\\w\^n\es Is ^hIuvxhI to half the original number, the spermatids jHVH^v^Huing \\\^\ half an u^aiu chi>>n\i^siunos as the svxTmatogi>nia. Each spermatid uv^w IvwMuo^ traUHfonuinl iut\^ a n\atur\" sjHTmatojtivn vFig. U\ The nucleus t\Mu\^ 0\c Ku^vr |Mrt of the IunuI, the vvnttwivMuo divivU^, the resulting moieties |V^>i>iU\ii t\^ the e\tu^u\itiov v\t the uivk. The jyviterior vYntrvvfk^me is prolonged to form the axial tUament, and the cytoplasm forms the sheaths of the neck and tail. The spiral filament of the connecting piece is derived from the cytoplasmic mitochondria.

Fig. 10. — Stages in the spermatogenesis of man arranged in a composite to represent a portion of a seminiCeroua tubule sectioned transversely. X 900.

Fig, 11. — Diagrams of the development of spermatozoa (after Meves in Lewis and Sffihr). a^., .\nterior centrosomc; a.f., axial filament; c.p., connecting piece; eh.p., chief piece; g.c., cap; n., nucleus; nt., neck; p., protoplasm; p.(., posterior centrosome.

The way in which the number of chromosomes is reduced may be seen in the spermat<^enesis of Ascaris (Fig. 12). Four chromosomes are typical for Ascaris megalocepkala bivalens and each spermatogone contains this number. In the early prophase of the primary spermatocyte there appears a spireme thread consisting of four parallel rows of granules (B). This thread breaks in two and forms two quadruple structures known as tetrads {D-F) ; each is equivalent to two original chromosomes split lengthwise to make a bundle of four. At the metaphasc (G) the two tetrads split each into two chromosomes which already show evidence of longitudinal fission and are termed dyads. One pair of dyads goes to each of the daughter cells, or secondary spermatocytes (G-I). Without the formation of a nuclear membrane, the second maturation spindle appears at once, the two dyads split into four monadsy and each daughter spermatid receives two single chromosomes, or one-half the number characteristic for the species. The tetrad, therefore, represents a precocious division of the chromosomes in preparation for two rapidly succeeding cell divisions which occur without the intervention of the customary resting periods. The easily understood tetrads are not formed in most animals, although the outcome of maturation is identical in either case. A diagram of maturation is shown in Fig. 13. The first maturation division in Ascaris is probably reductional, each daughter nucleus receiving two complete chromosomes of the original four, whereas in the second maturation division,. as in ordinary mitosis, each daughter nucleus receives a half of each of the two chromosomes, these being split lengthwise. In the latter case the division is equational, each daughter nucleus reciving chromosomes bearing similar hereditary qualities.

Fig. 12. — Reduction of chromosomes in the spermatogenesis of Ascaris mcgalocephala tnvalens

(Brauer, Wilson). X about 1100. .4 -C, successive stages in the division of the primary apennatocyte. The original reticulum undergoes a very early division of the chromatin granules which then form a doubly split spireme (B, in profile). This becomes shorter (C, in profile) and then breaks in two to form two tetrads (D, in profile), (S, in end). F, C, II, first division to form two secondary spermatocytes, each reccivinK two dyads. /, secondary spermatocyte. J, K, Che same dividing. L, two resulting spermatids, each containing two single chromosomes.

Fig. 13. — Diagrams of maturation, spermatogenesis and oogenesis (Boveri).

In some animals the sequence of events is reversed, reduction occurring at the second maturation division. In many insects and some vertebrates it has been shown that the number of chromosomes in the oogonia is even, the number in the spermatogonia odd, and that all the mature ova and half the spermatids contain an extra or accessory chromosome (see p. 32).

During oogenesis, the ova undergo a similar process of maturation. Two

iH If IK cEku cells: mitosis, maturatiox axd fertilization

(rll flivihionH take place but with this difference, that the deavage is unequal, liiul, inHf(r;ul of four cells of equal size resulting, there are formed one large ripe ovum or odryle and three rudimentary or abortive ova known as polar bodies or Iwlotytrs. The numlxrr of chromosomes is reduced in the same manner as in \\\v Hpi'rmatocyte, si> that the ripe ovum and each polar cell contain one-half I he nurnbrr of chromosomes found in the immature o\'um or primary oocyte. The* frrnali* ^crm cells, from which new ova are produced by cell division, are (iillfd oiii^onia and their daughter cells after a period of growth within the ovary an* the primary oocytes, comparable to the primary spermatocytes of the male (I'i^. 12). During maturation the ovum and first polocyte are termed secondar> (WW yt<*H (comparable to secondary spermatocytes), the mature ovum and second pnltMytc, with the (laughter cells of the first polocyte, are comparable to the Np«Tmiili(ls. Kach spermatid, however, may form a mature spermatozoon, but only i»nr (»f the four daughter cells of the primary oocyte becomes a mature ovum. The* ovum develops at the expense of the three polocytes which are abortive and (h*genernte eventually, though it has been shown that in the ova of some insects the polar cell may be fertilized and segment several times like a normal ovum, in most animals, the actual division of the first polocyte into two daughter cells Ih Huppressed. The maturation of human ova has not been observed, but such a pi'oeeHH undoubtedly takes place. The reduction of the chromosomes may be beHt observed in the germ cells of Ascaris and of insects. The mouse offers a favorable opportunity for studying the maturation of a mammalian egg as the ova are easily obtained. Their maturation stages have recently been studied by Long and Mark (Carnegie Inst. Publ. No. 142).

Maturation of the Mouse Ovum

The nucleus of the ox'um after maturation U k nown a» t \\\\lfmiik f*ropimlnis. When the sfXTmatozoon penetrates the mature ov\nn it loHeH its taiK and its head becomes the male pronucleus. The aim and end ol /rr/i7i(«i/ii»»i consists in the loiiopi of the chromatic elements contained in the w«»/e «»♦*«/ lem^ile profimln apul the initiation of cell division. In the mouse, the ln«tt |H«lovvte i>( lornuHJ while the ovum is still in the Graafian follicle. In the hMmati\M) ol the nvaturation spindle no astral rays and no t\'pical centrosomes have lHHi\ \»bHe\vnl The eh^^^nu^somes art* V-sha|x\l. The first polar cell is \\M\Ht\t\ \\^\ \\\\\\\ the \^v\uu and lie^ In^uMth the A>na jx^Uucida as a spherical mass i\U\^\\\ V^ \\\\\\{\ in \lian\etei vKijkj. IIV l^\^th vwum and jx^lar cell (secondary \^\^\teH^ w^ntain H^ vlu\»n\\»MMnes, \m hall the numlvr normal for the mouse. \\\\^ ^n^»^ n\.U\n.Uu^n \liNiMvM\ iv the nxluvtivMul one ami the chromosomes take

After ovulation has taken place, the ovum lies in the ampulla of the uterine tube. If fertilization occurs, a second polocyte is cut off, the nucleus of the ovtmi forming no membrane between the production of the first and second polar bodies (Fig. 14 A-D). The second maturation spindle and second polar cell are

++++Fig. 14.— Maturation and fertilization of the avum of the mouse (after Sobolta). A , C-J. X 500; B X 750. A-D, entrance of the spermatozoon and formation of the polar cells. D-E, dci-clopmenl of the pronuclei. F-J, successive stages in the first division of the fertilized ovum.

smaller than the ^t. Immediately after the formation of the second polar cell, the chromosomes resolve themselves into a reticulum and the female pronucleus is formed (Fig. 14 D).

Fertilization

Fertilization of the Mouse Ovum

Normally, a single spermatozoon enters the ovum six to ten hours after coitus. While the second polar cell is forming, the spermatozoon penetrates the ovum and loses its tail. Its head is converted into the male pronucleus (Fig. 14 D). The pronuclei, male and female, approach each other and resolve themselves first into a spireme stage, then into two group>s of 20 chromosomes. A centrosome, possibly that of the male cell, appears between them, divides into two, and soon the first segmentation spindle is formed (F-H). The 20 male and 20 female chromosomes arrange themselves in the equatorial plane of the spindle, thus making the original number of 40 (/). Fertilization is now complete and the ovum divides in the ordinary way, the daughter cells each receiving equal numbers of maternal and paternal chromosomes. The fundamental results of the process of fertilization are : (1) the union of the male and female chromosomes to form the cleavage nucleus of the fertilized ovum, (2) the initiation of cell division or cleavage of the ovum.

These two factors are separate and independent phenomena. It has been shown by Boveri and others that fragments of sea urchin's ova containing no part of the nucleus may be fertilized by spermatozoa, segment, and develop into larvae. The female chromosomes are thus not essential to the process of segmentation. Loeb, on the other hand, has shown that the ova of invertebrates may be made to develop by chemical and mechanical means without the cooperation of the spermatozoon {artificial parthenogenesis). Even adult frogs have been reared from mechanically stimulated eggs. It is well known that the ova of certain invertebrates develop normally without fertilization, that is, parthenogenetically. These facts show that the union of the male and female pronuclei is not the means of initiating the development of the ova. In all vertebrates it is, nevertheless, the end and aim of fertilization.

Lillie (Science, vols. 36 and 38; 1912, 1913) has recently shown that the cortex of sea urchin's ova produces a substance which he terms fertilizin. This substance he regards as an amboceptor essential to fertilization, with one side chain which agglutinates and attracts the spermatozoa, and another side chain which activates the cytoplasm and initiates the cleavage of the ovum. According to Loeb, the spermatozoon activates the ovum to develop by increasing its oxidations and by rendering it immune to the toxic effects of oxidation.

Spermatozoa may enter the mammalian ovum at any point. If fertilization is delayed and too long a period elapses after ovulation, the ovum may be weakened and allow the entrance of several spermatozoa. This is known as polyspcrmy. In such cases, however, only one spermatozoon unites with the female pronucleus.

Fertilization of the Human Ovum

This has not been observed, but probably takes place in the uterine tube some hours after coitus. Ova may be fertilized and start developing before they enter the uterine tube. If they attach themselves to the peritoneum of the abdominal cavity, they give rise to abdominal pregnancies. If the ova develop within the uterine tube tubal pregnancies result. Ovarian pregnancies are known also. Normally, the embryo begins its development in the uterine tube, thence passes into the uterus and becomes embedded in the uterine mucosa. The time required for the passage of the ovum from the uterine tube to the uterus is unknown. It probably varies in different cases and may occupy a week or more. The ovum may in some cases be fertilized within the uterus. Fertilization is favored by the fact that the spermatozoa swim always against a current. As the cilia of the uterus and uterine tube beat downward and outward the sperms are directed upward and inward. They may reach the ovarian ends of the uterine tubes within two hours of a normal coitus.

Twin Development

Usually but one human ovum is produced and fertilized at coitus. The development of two or more embryos within the uterus is commonly due to the ripening, expulsion, and subsequent fertilization of an equal number of ova. In such cases ordinary or fraternal twins, triplets, and so on, of the same or opposite sex, result. Identical twinSy that is, those always of the same sex and strikingly similar in form and feature, are regarded as arising from the daughter cells of a fertilized ovum, these having separated and each having developed like a normal ovum. Separate development of the cleavage cells can be produced experimentally in many of the lower animals. The ofifspring of the armadillo are normally produced in this manner (Patterson).

The Significance of Mitosis, Maturation and Fertilization

It is assumed by students of heredity that the chromatic particles of the nucleus bear the hereditary qualities of the cell. During the course of development these particles are probably distributed to the various cells in a definite way by the process of mitosis. The process of fertilization would double the number of hereditary qualities and they would be multiplied indefinitely were it not for maturation. At maturation not only is the number of chromosomes halved, but it is assumed also that the number of hereditary qualities is reduced by half. In the case of the ovum, maturation takes place at the expense of three potential ova, the polocytes, which degenerate, but to the advantage of the single mature ovum which retains more than its share of cytoplasm and nutritive yolk.

Mendel's Law of Heredity

Experiments show that most hereditary characters fall into two opposing groups, the contrasted pairs of which are termed allelomorphs. As an example, we may take the hereditary tendencies for black and blue eyes. It is supposed that there are paired chromatic particles which are responsible for these hereditary tendencies, and that paired spermatogonial chromosomes bear one each of these particles. Each chromosome pair in separate germ cells may possess similar particles, both bearing black-eyed tendencies or both blue-eyed tendencies, or opposing particles, bearing the one black, the other blue-eyed tendencies. It is assumed that at maturation these paired particles are separated along with the chromosomes, and that one only of each pair is retained in each germ cell, in order that new and favorable combinations may be formed at fertilization. In our example, either a blue-eyed or a black-eyed tendency bearing particle would be retained. At fertilization the segregated tendency-bearing particles of one sex may enter into new combinations with their allelomorphs from the other sex, combinations which may be favorable to the offspring.

Three combinations are possible. If the color of the eyes be taken as the hereditary character, (1) two "black*' germ cells may unite; (2) two "blue" germ cells may unite; (3) a "black** germ cell may unite with a "blue" germ cell. The offspring in (1) will all have black eyes, and, if interbred, their progeny will likewise inherit black eyes exclusively. Similarly, the offspring in (2), and if these are interbred their progeny as well, will include Douhiz^ bui blue-eyed individuals. The first senentioii from the crass in (3) will have black e>^YS ac4thr. tor black in the present example is d<nmin*imt, as it is termed. Such Mack-eyed ir>ii>S±ujils. Devenheles&, possess blue-eyed bearing chromatic panides in thdr germ cells; ii :^ psvf^ray reselling fiom the interbreeding of this dass the original condition is repeated — puinr b^cks. impuie blacks vhich hold Uue recessive, and pure blues vill be formed in the ra'.x' ^xf 1 : 5 : 1 respectively. It is thus seen that Uue-eyed children may be bom of blackex^ ;uLresiiL vhereas hlue<e>Td parents can never have black-eyed offspring. Many such iiLdrmurfttati pairs ol uni: characters are known.

Determination of Sex

Tbe JL553£mpuoci that the chromosomes an? the carriers of hereditary tenoto."»v i> boc^e ocit by the obs^rv-atioDs ol c^tologisls on the gcnn odk of inver*tfOri:rs, es;%e\'ii2\- iieiects. and i^" some \TMnebraies. AccordSiig to Winiwaiter Ar.-^ ie Bxv. T. 27, 1912 the nucki vX huzoan sp«»mito^iQBa coDtain 47 chios.^*c:ifs. wt£ie zix^^if of ibe vV^xdi cvxitain 4$. When matoratiaii and reduction .Tii :^ .■ir.iCk^jocDcs tike pLiar in the niiie ofik. ooe unjuinsi chromoeome fails •, ivii^ u^i ru.si$t:s irtacc to ooe v-**- the ocher diurfiter ceSs: hence half of the ^pirma.dii' :*.tmiz .14 c^^rvHoot^ocaeSv the oibe^- rjuf s?CLy 25. AH ti«^ oocytes and i%.u.W5ssv .Tt lif CvXimrw . c*.xi:jl£:i 24. Tb^rnr i> th-,:s voe e.Ttn chrooosome in

Sr^riasse ,-c >ftr; m-iSjs cc >L2tr .r <ikt?e, ex:: S: x.:<?fid5t-c e*isJ> ni rsirry ammak, i.iii. i> ttrrmr'i ^ Aiweyr i .ifywwsicMfcr. MvV!:::^;^ wj;:^ tic irsc tJ xssmie that "lie iir'.vi^Ty :3r'.-cQ,'^5Cia>tf ^ a >*:x >>fv^fr?ri7^t.T.:. I: biL> >ao* be«!i shovn by TMst-tL Davis iju: xi«s rMt tie JLvXt^?5^c^v .'?;:r.*r*o?ccie rj-nris the tinoale srriiil .'Oitrii-'ti'rrs WS:!r. i:r t^ .us^r u^is^c v.vn>;ct^rj:i:n. -i sfvranitoziwm with I-r .iir.aiiw^ciinr> :n:r^iiL:<rj^ iT ,'vi:r. tic ?ir^tL":|i .'?r>r>v ii^ a iinije. tcs scicatic

ixn\» 2? ,im-mi.^ints •*!ti^;u•: tht io.v:j^Ty cax',»rn,*^*frrv.' *,*rA;uo;s ^ zmiJie vtth

.TiiW»:v»:r tiic jti is ruasmittt^i by th<; hou-^j .•7r-,*irrv*^irtc< e^5«j*?«idaJy m the Hiuifltr itrscrilx-i. whicrt i;jr»:xs '*iuh tit eu:?il> jO:<T^t,\i ,*av.«iv*tri\.?ia in insects.