Book - Human Embryology (1945) 2

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Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.

   Human Embryology (1945): 1 Introductory Concepts | 2 Formation Maturation and Structure of Germ Cells | 3 Cyclic Changes in Female Genital Tract | 4 Fertilization Cleavage and Formation of Germ Layers | 5 Implantation of Blastocyst and Development of Foetal Membranes Placenta and Decidua | 6 Fate of Germ Lavers and Formation of Essential (Primary) Tissues including Blood | 7 Growth of Embryo Development of External Form Estimation of Embryonic and Foetal Age | 8 Determination Differentiation Organizer Mechanism Abnormal Development and Twinning | 9 Cardio Vascular System | 10 Alimentary and Respiratorv Systems Pleural and Peritoneal Cavities | 11 Urogenital System | 12 Nervous System | 13 Skeletal System | 14 Muscle and Fascia | 15 Integumentary System | 16 Comparative Vertebrate Development | Figures
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Chapter II Formation, Maturation and Structure of the Germ Cells

Amongst the mdi%iduals of most metazoan species there are certain contrasung chanctenstics which may be classified as belonging to either the jntilr oryrww/r sex -It is not possible however, to give a rigid definition of se\ In a broad sense sex ma^ be defined as the power of the individual to produce germ cells (gametes) — spermatozoa or ova The glands le gonads (testis or ovar^) which produce these germ celK are the pnmary sex organs In all vertebrates the tebtes or ovaries develop in the posterior wall of the abdominal cavit) The germ cells produced by the testes are called spermatozoa (sperms) and b) the ovary ova (eggs) A male may be defined therefore as an individual who normally is orwillbe or has been capable of producing spermatozoa and a female one who normally is, orwillbe or has been capable of producing ova Iri sexual reproduction the fusion of one of each these two sets of cells constitutes the starting point m development of a new individual.

Functional germ cells are produced throughout the reproductive life of the human subject In man the process of sperm production spermatogenesis by the testis is normally continuous fiom just after puberty until old age This is also the condition in many primates and domesticated mammals e g , rabbit, pig. In some domesticated mammals eg ferret and m many wild mammals eg squirrel and deer spermatogenesis is not a continuous process but seasonal (consult Marshall 1936 Allen 1939 and Asdeli 1946 for details) In the males of those species m which the breeding season is continuous the sex changes are less distinct than in the female while in those species m which there are restricted breeding seasons the marked cyclic sex changes occur m both sexes In the human male there are no cyclic changes, other than phasic activity m the difTcrcnt semtm/erous tubules

The production of ova oogenesis by the ovanes is a cyclic process It is characterized by the formation groitlh and maturation (ripening) of the ovum (or ova) in a vesicle the ovanan folltele hen mature the ovum (or ova) is shed by the rupture of the follicle into thejientoneal cav ity or a recess of this cavity the process is known asoiulation After ovulation the collapsed" follicle undergoes a rapid modification which results in the formation of a temporary endocrine gland called the corpus luteum In the human female the changes just described are regularly repeated in a cyclic manner throughout mature sexual life Thi« periodicity of the female sex function is established gradually at puberty and is continuous until the menopause unless modified by pregnancy or disease

The changes occurring in the testes and ovanes are under the control oC hormones produced by certain of the ductless glands cspeaally the anterior lobe of the pituitary (page 34)

The Testis and its Ducts

As has becn stat^ the testes in vertebrates develop on the posterior aWominal wall and m most \ ertebrates including some mammals (c g the elephant and the whale), remain in this position throughout life In most mammals however the testes descend (see page 254 and \\islocki ^933) from this intra abdominal posiuon and come to he cither permanently (carnivores ungulates primates) or temporarily (moles shrews rodents and bats) m a pouch located in the perineal region or the lower abdominal wall . In man the descent is permanent and the testes are located in a scrotal_sac. They are covered by peritoneum, the tunica vaginalis (page 255), ha\^ their vascular connexions directly with the abdominal aorta and inferior' vena cava, and receive a nerve supply from the abdominal sympathetic trunks. Each testicular duct [vas deferens) passes back through the abdominal wall into the abdomen on its way_to the uiethra The scrotum is probably a device for maintaining the testes at a lower temperature than that of the abdominal cavity, for in most mammals the spermatogcnic tissue of the testes will degenerate at the higher temperature found in this cavity.

There are two elements in the testis of primary importance m reproduction, (i) the seminiferous or convoluted tubules, the cellular walls of which give rise to the spermatozoa, and scattered in the stroma between the tubules the iniersiilial gland tissue for the elaboration of the male sex hormone, testosterone, which maintains the function of the accessory sex organs and the characteristic male behaviour.



A schematic representation of the testis and Its ducts, it shows the course taken by the sperms from the testis to the exterior


The spermatozoa produced in the seminiferous tubules of the testes reach the exterior by a series of genital ducts A schematic representation of the testis and its ducts is given in Fig I. JThe testis is surrounded by a fibrous capsule, the tunica albuginea, whicfTis thickehed'posteriorly to foim the mediasiinum^efisjxora which fibrous septa pass into the interior of the ofgan'and divide it into lobules. Each lobule has "bile or more seminiferous tubules; at intervals, between the tubules, there are groups of interstitial cells. The seminiferous tubules, which are very numerous, are about the diameter of a horse hair and, if uncoiled, are several inches long. The tubules are the essential structural units of the testis and each forms a loop continuous at each end with a straight tubule The straight tubules from all the seminiferous loops open into a network of channels lying in the mediastinum testis and named the rete testis, which is continueiLii^ the effe rent ductules of the epididymis; these in tufn~open into the canal of the epididymis which is continuous with the vas deferens, a thick walled duct which passes from the epididymis through the anterior abdominal wall and then behind the bladder towards the


prostate gland. The vas def erens is joined n^t — itsjCTnunatmn by the duct of the seminal vesicle and the resulting common ejaculatory duct opens into the prostatic part of the urethra.


The Seminiferous Tubule

Microscopically the wall of each tubule (Fig 2) shows a basement membrane, lined by a number of layers of cells with spermatozoa either free in the lumen or attached to the inner surface of the epithelium This epithelium is made up of two cell types; tall pyramidal cells (the Sertoli or nurse cells), extending from the basement membrane to the lumen, and rounded cells of several sizes, the male germ cells The latter range from small spermatogonia next the basement membrane through the large primary spermatocytes, and the somewhat small secondary spermatocytes, to the still smaller spermatids more or less embedded in the central pseudopodium-hke ends of the Sertoli cells These spermatids show all stages of metamorphosis into typical spermatozoa (Fig 3).


Spermatogenesis

A number of important changes are rfTectetl dunng ibc process of fonrtalion and maturation of the spermatozoa First, spermatogonia mullipl> b> mitotic diMsion thus assuring a con tinued suppl> of germ cells Second some of these spermatogonia grow increasing setcral times in bulk and are pushed towards the lumen of the tubule to form primary spermatocytes uhich apparently contain enough nutriment to carr\ them through the succeeding stages of maturation at some distance from the basement membrane and the source of nutrition from the capillaries between the tubules Third, the primary spermatocytes prepare for division into secondary spermatoevtes in such a wav that the chromosome number m the latter becomes one half that of the parent primary spermatocyte Flus is called the first maturation or heterotypical division (meiosis) It assures that the sperm cells will contain only the haploid number


Fig 3 — A t^ans^r^sc srcuonora scmmircrous tultuleand intmtitial tissue of the t«us The maturing spcmu are attached bj iheir heads to the Sertoli cells whilst their tails project into the lumen of the tubule ( \rter Sliese in von MollendorfT 1930) x c 330


of chromosomes so that when fertilization occurs the haploid numbei of egg chromosomes will be restored to the diploid number and the new individual will inherit equally from each parent (biparental inbentance)_ Fourth the secondary spermatocy tes quickly divide to form sperma tids thus further m'ulfiplylng the possible number of male germ cells and reducing their size m anticipation of engulfment by the Sertoh cell cytoplasm Fifth engulfment in the Sertoli cells takes place and immediately profound metamorphtc changes begin m the spermatids These changes consist mainly m the loss of almost all the spermatid cytoplasm the concentration of its nuclear material into a dense sperm head and a formation of the sperm neck middle piece and tail m short, the metamorphosis of a simple cell into a specialized motile organism, the functional spermatozoon capable of swimming to an egg and fertilizing This meta morphic stage of spermatogenesis is called spcrmiogcnesis grows out through the surface of the cell The centnolcs no\% pass towards the side of the nucleus opposite to that where the acrosome is attached __As-the-centnoles move t owards _ the nucleus the a^l filament is drawn into the cytoplasm of the spermatid A posterior n uclear cap developed from a posterior nuclear bod^ grows around the postenoLparl-ot-ttw— nucieus'ahd’comes intoTdhtacrwith the acfSomic cap' ~Thc antenqr^r pr oximal centnole "TiScomes cldsel>" related to the gramile which lie5~bchind the posterior nuclear ^apjn flielncck repon of the spermatid “ The postenor or distal centrToIe'enlargcs becomes ring shaped and moves away from the'pro'timal centnole along the axial filament it comes to rest at a position which will mark the posterior end of the middle piece of the sperm Most of the mitochondrial granules which are scattered in the cyto plasm of the spermatogonia and spermatocytes pass to Ibrm a sheath for that part of the axial filament between the proximal and distal centnoles of the spermatid as it is transformed into the spermatozoon The remainder of the cytoplasm mitochondria and Golgi apparatus are separated from the ripe sperm and he free in the lumen of the tubule where they degenerate.

Sperm Structure

The ripe human spermatozoon possesses a head neck middle piece and flagellum The head consists of the nucleus covered by two caps an anterior cap or acrosome (derived from the Golgi appara tus of the spermatocyte) and a post nuclear cap The two caps meet approximately at the equator In the centre of the head there is a relativ ely large cav ity or v acuole Behind the head is the neck granule which attat-hes it to the middle piece (Gatenby and Beams, 1935)

This middle piece extends from the anterior centnole to the posterior centnole and has an axial filament around which the mitochondrial granules are arranged The axial filament is prolonged through the ring of the distal centnole as the flagellum or tail Recent studies with the electron microscope (Culp and Best 1949) have added some further details of the structure of the sperm (Fig 4) Physiological intcrpreta tion of the morphological details is largely lacking Pollister and Mirsky (1946) using trout sperm demonstrated the actuality of the long postulated chromosome integrity in the sperm head with special staining techniques which revealed the individual chromosomes

Maturation of the Sperms in the Male Genital Tract

The exact length of time necessary for cither spermiogenesis or the whole maturation process is unknown, but it is probable that the diagrammatic

development of a sperm from a pnmary spennatocyte takes only a few matur"no™al hum‘an days hen fully formed the sperms become free in the tubule lumen ipcrmatozoon as seen and are pissed on to the straight tubules rcte testis ductuli cfferentia micr^

the epididymal portion of the vas deferens and finally to the isccndmg amflest and pelvic portions of the vas where they eventually reach the prostatic urethra and are ejaculated w ith the secretions of the accessory glands as semen ^\ hile the sperms are to all appearances fully formed when they leave the Sertoli cells pliysiologically they appear to be matured further as they pass through the epididyTnis for it is known that their capacity for fertilizing eggs is greater if taken from the tail of the epididymis than when taken from Its head or the testis itself M the sperms mature in the epididymis and vas deferens thev increase in vigour and fertilizing power but if not ejaculated they soon degenerate and are absorbed within the tubules of the cpididyims (Simeone and Young 1931).


The Ovary and Oogenesis

The human ovaiy possesses an outer cortical and an inner medullary portion (Fig. 5 ) The medulla consists of richly vascularized connective tissue During the child-bearing period the cortex constitutes more than one-half of the thickness of the ovary. It consists of a stroma of connective tissue in which are embedded the ovarian follicles contaimng within them the developing ova The cortex is covered by a modified part of the peritoneal mesothelium called the germinal epithelium During mature sexual life the ovary is the seat of complex cyclic changes involving principally the ovarian follicles and the germinal epithelium Consequently



Fig 5 — Schematic representation of the sequence of events occurring m the mammalian ovary during a complete ovarian cycle The development, growth and rupture of the ovarian follicles and the formation and retrogression of the corpus luteum are shoivn




Fig 6 A schematic dra^vmg of the relation of an oogonium to the germinal epithelium T ^ cuboidal epithelium covers the ovarian stroma

In B a solid downgrowth of germinal epithelium passes into the ovarian stroma In C the downgrowth consists of follicular cells surrounding a large central cell, the ogonium The ovarian simma c)in>.rc concentric arrangement preparatory to the oogonium The ovarian stroma shows a formation of the ovarian theca the histological picture presented by the cortex is always difficult to interpret and sometimes confusing. (The details of the development of the ovary are considered in Chapter XI)

Formerly it was believed that the ova present m the adult ovary were all developed during foetal life. It was thought, in the human, that during childhood many of these ova degenerated but that some lay dormant until puberty when one matured each month and was shed from the o%ar> There is now good endcnce thato\aintheo\ar> have a short life (E\ans and Swezy 1931) and that the o\a which develop during foetal life degenerate after birth and arc replaced b> oogoma formed b> new proliferation from the germinal epithelium on the surface of the ovary (Latta and Pederson 1944) or from the immediately subjacent stroma W'hcther these postnatal oogoma arise from the germinal epithelium itself or from primor dial ova which have migrated to the neigh bourhood of this epithelium during the earlier development of the gonad has not been definitelv established

The classical description of the pro liferation of surface germinal epithelium into the ov ary is that it occurs in the form of narrow tubular ingrowths {Pj^ug/r s tubes) during the foetal period These arc bchev cd to break up into groups of cells an inner cell of each becoming a primary oo^le while the outer cells arrange themselves around this central oocyte to form a laver offollicu lar epithelium (Fig 6) In the light of our present knowledge three important quah hcations must be made to this classical concept First as already stated there is much e\ idence to indicate that proliferation of germinal epithelium occurs periodically throughout active female reproductive life Second while typical Pflugers tubes are found in certain species (dog seal bear) more commonly solid cords of cells are found In many species irregular masses of cells or even single cells bud off and migrate into the cortev to become oocvtes Third m only a few species do the cells of the epithelial ingroviths clearly take part in the formation of follicular epithelium In most cases indeed the first sign of this epithelium is a layer of verv thin squamous cells directly applied to the oocyte and apparently derived from the surrounding stroma celb These prtmor dial follicles (Fig 7) become primary Jolltcles (Fig 8) when their epithelium becomes cuboidal There is no reason to assume on theoretical or any other grounds that follicular epithelium may not be formed from either germinal epithelium or stroma.


Fig 7 — A schematic drawing of a growing primary oocyic of approx diameter 3311 The nucleus con tains a disimct nucleolui I he fat globules are coloured yelloi mitochondria blue and GoIpi apparatus black these structures form the couehe Mtellogene and are situated at one side of the nucleus The follicular cells have become Hattened (This Tig and also Figs 8 and g are based on the work of Aykroyd 1938 ) x c 1 60


Fig 8 — A schematic drawing of a primary oocyte shosmg the dispersal of the miiochondna and Golgi apparatus around the nucleus The fat giobjles (coloured grey) are less numerous The foil cular cells have become columnar and have developed a basement membrane x c 800


The cells destined to become ova, whether derived from the germinal epithelium or not ^ j I ' spond to the spermatogonia and are called oogonia. As these enlarge they become primary compaiable with primary spermatocytes, but they continue to grow to a relatively huge / c as they aie enveloped by the follicular structures, and are commonly spoken of as “ova” aithovK^h 'I.e\ are technically oocytes (primary or secondary) until after the second polar bodv L.viri off As the oocyte grows it accumulates an increasing amount of cytoplasm in nh'ch t*i( tc are a Golgi apparatus near the nucleus, mitochondria forming a crescent-shaped ma-s I a "couche vitellogene” at one side of the nucleus, and, amongst the mitochondria, some fat c'o.)ul.-s (Fig 7) At this stage the oocyte measures about 35 microns m diameter.


rv> Pi-N ZONA

PE^LUCJDA


FOLLICULAR BASEMENT

LAYER MEMBRANE


schematic drawing of a primary oocyt

showing the further dispersal into the peripher of the mitochondria and Golg apparatus and the remains of the fat globule which are now less numerous Irregular block

S Pellucfda) develoi bcUNcen the surface of the oocyte and the columna follicular cells The ovarian stroma is con "‘""‘"Sed around the follicular cells


Fig I o — The oocyte has grown to its full size and measures now approx 120/1 in diameter The mitochondria and Golgi apparatus are dispersed throughout the cytoplasm, the zona pellucida is now complete, the follicular cells have multiplied to form a multicellular layer around the oocyte and amongst these cells at one pole of the oocyte a follicular cavity has appeared X c 580


nnrl oocytc increases in size the Golgi apparatus is dispersed through the cytoplasm

firct ^ periphery of the oocyte The mitochondria become scattered

flicnp ^ later throughout the cytoplasm (Fig. 9) The fat globules are also

cells b many disappear until, in the mature ovum, only a few are found. The follicular

them and a homogeneous membrane, the zona pellucida, develops between

unite tn f ^ At first, the zona pellucida appears as irregular blocks which later

nenvitell ^ ^ rf • applied closely to the vitelline membrane of the oocyte, no

lavers space eing piesent (Fig 9) The follicular cells proliferate until there are many

vrannlnsa ^ona granulosa A cavity then appears between the cells of the membrana

or Graa/iari 'foUirlT^ surface of the ovary (Strassman, 1923), thus forming an ovarian,

’ • ne oocyte with the follicular cells, cumulus oophoncus, which surround it,


is at this stage attached b> the discus proltgenis to the deep aspect of the follicle (Fig 1 1) The oocyte now has a diameter of about 120 microns and liquor folltculi accumulates in the cavity of the follicle, It IS presumably secreted by the cells of the membrana granulosa (Shais 1927) As the follicle grows and ripens the stroma cells in contact with the membrana granulosa are differentiated to form the tArm of the follicle (Fig 10) Later (Fig n) these cells of the theca interna constitute a well defined thecal gland (Mossman, 1937 Stafford and others 1942) Between the theca interna and the granulosa cells there is a delicate basement membrane {membrana propria) A fibrous theca externa can also be distinguished m the human ovary The follicular fluid and the granulosa cells contain the oestrogenic hormone oestradiol, which is absorbed from the follicle by the blood vessels of the theca mterna There are no blood vessels m the membrana granulosa, it apparently derives its nutntion from the capillaries of the theca interna.


Fig 11 — The mature ovarian (Graafian) foJLcle The follicular cavit) has become di tended with fluid and now measures approT 6 mm in diameter The oocyte surrounded by some of the follicular cells (cumulus oophoncusi is attached to (he inner aspect of the follicle on lU deep sde The enlarging follicle nor produces a ss elling on ihe surface of the ovary Immedi3tel> surrounding the follicle (he ovarian stroma forms the theca interna or thecal gland X c i



The follicles are situated m various positions in the cortex of the ovary Before ovulation can occur the follicle must reach the surface of the ovary As the follicle approaches the surface the cumulus cells become loosened from one another by the accumulation of intercellular fluid so that they separate readily from the follicular wall and, together with the enclosed ovum are shed with the follicular fluid when the latter suddenly escapes at the time of rupture This IS the process of ovulation the cumulus cells round the shed ovum being now called the corona radiala (Fig 26) The actual cause of ovulaUon is still unknown Previous to the rupture of the follicle there is a sudden increase in the amount of fluid, this is the secondary liquor folhcuh and it may play an important role m the mechanism of rupture and expulsion of the oocyte. During the pie-ovulation period m many mammals, including the human female, the theca interna undergoes an eccentric proliferation on the superflcial aspect of the follicle to form a thecal cone The stroma lound the cone becomes oedematous prior to the rime of ovulation thus facilitating rupture of the follicle Follicular rupture has sometimes been attributed to muscular contraction but the investigations of Claesson (1947) have shown that there is no such involuntary muscle in the wall of the follicle The final pre-ovulation stages have not been observed m man, but m mammals the site of impending rupture of the ovarian surface becomes thin and translucent Walton and Hammond (1928), Markee and Hmsey 936)5 -md others have described this process.

Ovulation is normally limited to the child-bearing period of life, and only rarely occurs before puberty or after the menopause In most mammals ovulation occurs during the period of oestrus when, amongst other things, the female sex urge manifests itself In primates and man, however, ovulation occurs at a period which probably corresponds to oestrus m everyway except that the sex desire of the female may not be appreciably stronger. After ovulation the follicle collapses and becomes converted into a corpus luteum (page 24).



ZONA PEULUClOA CORONA RAOIATA


primary oocyte showing the ist maturation spindle of division This oocyte was found free in a large, unruptured follicle (drawn after Stieve) x c 230


At a stage vvhen a cavity has appeared in the ovarian follicle, and the primary oocyte has grown to its full size, important changes occur in its nucleus; these changes are comparable to the changes that occur during spermatogenesis and are knowm as maturation of the oocyte. The formation of the first polar spindle (Fig 12) initiates the first maturation, or 1 eduction, division, the oocyte dividing into a larger cell, the secondary oocyte, and a much smaller cell, the first polar body. In this division the number of chromosomes in each cell is reduced to half (page 19). A second polar spindle is now formed and at this stage ovulation occurs Human ova at this stage of development have been recovered from the uterine tube


by Allen et al. (1930) It was formerly believed (Dixon, 1927) that both polar bodies were formed before ovulation It would appeal, however, that the human oocyte is like that of the majority of mammals in that the first polar body and the formation of the second polar spindle occur before ovmlation and that the second polar body is not separated until after fertilization (Hamilton, 1944) (Fig 27)


Chromosome Reduction

The number of chromosomes in human somatic cells is 48 (24 pairs) (Painter, 1923) Evans and Swezy, 1931) This is the diploid number or set of chromosomes The genes controlling sex are carried in a single pair of these chromosomes (the sex chromosomes) and the remaining 23 pairs are the autosomes. In the female the two sex chromosomes, knowm as the X-chromosomes, are similar to one another and differ only slightly from the autosomes in appearance though they possess, among others, the genes w'hich determine sex. In the male, however the sex chromosomes constitute an unlike pair of which one is an \ chromosome received from the mother, the other the \ chromosome of paternal origin is a small atypical chromosome carrying ver^ few genes of which apparemlv none are actively involved in sex determination The oogoma and spermatogonia of the gonads are iden tical in chromosomal constitution with the somatic cells of the corresponding sex

During the division of the sperma togonia and oogoma to form primary spermatoevtes or ooc>tes respecincl> the chromosomes split longitudinally forty eight going to each daughter cell (normal mitotic div ision) The chromosomes m the primary spermato cy'tes and oocytes arrange themselves m pairs (bivalents) (Fig 13) In the male each cell has 23 paired autosomes and one pair of sex chromosomes (X\) while in the female each cell has 23 paired autosomes and one pair of sex chromosomes (\X}

During the reduction or mciotic division the bivalent chromosomes separate longitudinall> along the line of the previous pairing so that m the male 23 + X chromosomes go to one secondary spcrmatoc>'te and 23 + \ chromosomes go to the other In the division* of the secondary spermato cytes to form spermatids each chromo some of the reduced number spins longitudinally m the usual way so that two spermatids are produced with 23 d X chromosomes and two with 23 + \ chromosomes Each there fore contains only half the number or haploid number of chromosomes found m the somatic cell

In the primary oocyte a similar splitting occurs during the reduction division, 23 + X chromosomes going to the secondary oocyte and 23 d- X

Although this appears to be a mitotic division with the reduce number of chromosomes It does not owing to cros mg over (chiasma formation) of the chromosomes result in an identical distribution of the genes to the daughter cells Hence some segretation of the genes occurs at this stage and as Men delian segregation is a result of meiotic di\ iston the division of secondarv spermatoevtes (or ooc tesj must he regarded as a second meiosis not a mitosis chiomosomes going to the first polar body. At the second maturation division, as in the male, a longitudinal splitting of the half number of chromosomes occurs, giving rise to 23 + X chromosomes in the ovum and 23 + X in the second polar body. The first polar body may also divide. All ova are alike in having 23 X chromosomes. A primary oocyte, therefore, gives rise to three or four cells, one of which, the ovum proper, retains most of the cytoplasm, the others, polar bodies, each contain a very small amount of cytoplasm surrounding a nucleus which has the same chromosomal value as the ovum.


Fig 13 A sell me to show the maturation of the sperma toevte tnd oocylc with the reduction division In the spcr^locyte there are 23 pairs of autosomes and an X+Y pair of chromosomes If the ovum A is fertiliied by a sperm containing an \ chromosome the resultme individual IS a female B if the ovum is fertilized bv I spCTin obtaining a \ chromosome the resulting individual ir a male C The polar bodies each contain "3 autosomes and an A chromosome



(Fig- 13) At fertilization the sperm, with half of the somatic number of chromosomes, enters the ovum, also containing half the somatic number of chromosomes. When the two nuclear masses unite the full number of chromosomes is restored It will be seen, therefore, that meiotic division is the antithesis of fertilization; in the former the diploid number of chromosomes is halved while in the lattei the diploid numbei is lestoicd by the summation of the haploid sets from the sperm and ovum.

If a sperm with 23 + X chromosomes unites with an ovum with 23 + X chromosomes the total number will be 46 + X + X, giving rise to a female zygote. On the other hand, if a sperm ^vlth 23 + Y chromosomes units with an ovum with 23 -f X chromosomes the total number will be 46 + X + Y and a male will result. As there is an equal number of sperms with 23 + X and 23 + Y chromosomes and all ova have 23 + X chromosomes the chances of a male or female zygote resulting from fertilization should be equal. In fact, however, the number of males at birth is slightly greater than the number of females in the proportion of 105 to 100.

Genetically the sperm and ovum are both capable of transmitting the characteristics of the race to future generations. Except for the slight chromosomal differences described above, the male and female germ cells must be legarded as genetically equivalent. The zygote contains genes in duplicate derived from the mother on one side and the father on the other (Chapter I).


References

Allen, E. (1939) Sex and Internal Secretions 2nd ed Bailliere, Tindal & Co\, London.

Pratt, J P , Newell, Q. U , and Bland, L J (1930) Human tubal ova, related early corpora lutea and

uterine tubes Contrib Embryol , Carnegie Inst Wash , 22 , 45-76 Asdell, S A (1946) Patterns of Mammalian Reproduction. Comstock, Ithaca, j >

Aykroyd, O E (1938) The cytoplasmic inclusions in the oogenesis of man ^eits f " rntKr - na ,

Claesson, L (1947), there any smooth musculature in the wall of the graafian follicle’ Ada Anal, 3 ,

Corner, G W (1938) The sites of formation of oestrogenic substances in the animal body. Physiol Rei ,

Gulp, o’ S^and^Best, J W (1949) Morphology of human spermatozoa observations with the electron microscope J Urology, 61 , 446-456 ^ j u ,

Dempsey, E. W , and Bassett, D L (1943) Observations on the fluorescence, birefringence and is oc

of the rat ovary during the reproduction cycle Endocrinology, 33 , 384-401 ♦ „ cpr-nnd

Dixon, A F. (1927). Normal oocyte showing first polar body and metaphase stage m forma 10

polar body Irish J Med Sci , 149-151 , , , TImv

Evans, H M , and Swezy, O (1931) Ovogenesis and the normal follicular cvcle in adult mam Calif Mem, 9 , 119-224 , r

Gatenby, J B , and Beams, H W (1935) The cytoplasmic inclusions in the spermatogenesis o

Jour Mtcr Set, 78 , 1-30 r j ’ 7 SI

Hamilton, W J (1944) Phases of maturation and fertilization in human ova J Anat, on ’ J Hisaw, F L {1925) The influence of the ovary on the resorption of the pubic bones of P

La..., of ova and fo.hc.e ce.ls f„» ■*.= “if

the ovary of the albino rat as demonstrated by selective intravital staining w

Markee, J E , and Hinsey, J C (1936) Observations on ovulation in the rabbit ^”p! ,^Yran^’ Roy ,

Marshall, F H A (1936) Sexual periodicity and the causes which determine

Mossman, H W. (1937) The thecal gland and its relation to *^'1*80-^20. ^

changes in the ovary of the pocket gopher, Geomys bursarius (Shaw) Am j > 3 Quart

Odor D L and Blandau R J (*949) Obser\ations of fertilization phenomena in rat o\a inat Rec 103 580 (ahst )

Painter T b (19 3) Studies m mammahan spermatogenesis II The spermatogenesis of man J Exp Zool 37 291-335

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   Human Embryology (1945): 1 Introductory Concepts | 2 Formation Maturation and Structure of Germ Cells | 3 Cyclic Changes in Female Genital Tract | 4 Fertilization Cleavage and Formation of Germ Layers | 5 Implantation of Blastocyst and Development of Foetal Membranes Placenta and Decidua | 6 Fate of Germ Lavers and Formation of Essential (Primary) Tissues including Blood | 7 Growth of Embryo Development of External Form Estimation of Embryonic and Foetal Age | 8 Determination Differentiation Organizer Mechanism Abnormal Development and Twinning | 9 Cardio Vascular System | 10 Alimentary and Respiratorv Systems Pleural and Peritoneal Cavities | 11 Urogenital System | 12 Nervous System | 13 Skeletal System | 14 Muscle and Fascia | 15 Integumentary System | 16 Comparative Vertebrate Development | Figures
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.


Cite this page: Hill, M.A. (2020, November 26) Embryology Book - Human Embryology (1945) 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_(1945)_2

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