Book - A Text-book of Embryology 1

From Embryology

Chapter I. The Male and Female Sexual Elements (Maturation, Ovulation, Menstruation, Fertilization)

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Embryology is that department of biology which treats of the generation and development of organisms. It may refer to the development of the race or stock — Phylogeny — or to that of the individual — Ontogeny ; again, it may treat of animal or of vegetable development.

Since no observations have been made upon embryos of an age less than four or five days, and but few, indeed, upon those younger than sixteen or eighteen days, we cannot be said to possess definite knowledge of the very earliest processes of development in man. There is, however, sufficient analogy between the known facts of human development and those of corresponding stages in allied groups of animals, as well as between the various groups of animals themselves, to establish certain broad general principles of agreement in essential features. In tracing the history of human development, therefore, frequent recourse must be had to the development of animals, since in this way only is it possible at present to fill up the gaps in our knowledge of human embryology.

That a new individual may be called into existence, the union of the male element, or spermatozoon, with the female element, or ovum, is necessary. Such union is variously called fertilization, fecundation, and impregnation.

Prior to the beginning of the present century, little or nothing was definitely known concerning reproduction and development. The opinions of the biologists of early times found expression in a theory which was then called the theory of imforming or of evolution, but which more recently has \wf*u d(?Hi^natf'<l flic; prefnrmntion theory, According to this dortririe, the egg or germ contained all the parts of the adult organism in an exeeniingly minute condition, and <lcvelopment eonsisted in the simple growth or nnfolding of already formed |Mirts. v\h the throri/ of unfoltlinf/ impliiH^I the pn:formation not only of the immediate hnt of all subsc»quent offspring, its votaries were ahle to eompnt(; that the ovary of VjVi* e4>nt4iinHl 2()0,(KK) millions of hnman germs.

With the discovery of the spermatozoon in 1H77 by Hamm, a pupil of Ii<»nw<'nh(M'ek, a ermtroversy arose as to whether it was the spermatic^ fdament or the ovum that contained the germ. Those who maintained the former view were known as animahiiliHtH ; thow who h<!ld the latter, as or/Vx. According to the opinions of th<* anirnalcniists, tin* spermatozoon was the complete organism in miniature, and it required for its growth the soil or environment which the ovum alone could furnish.

The enunciation by Wolff, in I TolJ, of his (Utcfrhic ofepif/vneSM completely overturned the preformation theory. Wolif maintained that the germ was ininrf/ftnizrd mntfcr^ and that the union of male and female mat<Tial was <'ss(>ntial to repnnhiction. While Wolff's thcorv was in the main corn'ct, it remained for lat<»r in vest igji tors to show that the ovum did not consist of unorgjini/ed mntt^M*, as he thought, i>ut that it |missesswl definite^ structund characteristics. Thus, tin* g(»rminal vesicle of the hen *s egg was discovered in 1825 |)y Purkinje, and the germinal spot in lS2tl i)v Wagner. SM)n after the enunciation of the celI-<loctrine i>v S'hicidcn and S-hwann, it was seen that the ovum was in reality a typical cell, possessing all the jMirts of such a structure

It was not, however, until about th<* year IS 10 that it was shown, by Kolliker, Reiehert, and others, that the spermatozoa are the active agents in fecundation. I*n»vious|y it had Inrn held, since the refutation of the pivformation theory, that the seminal fluid pcrfomic<l this function, and that the spermatozoa were parasitic organisms.

The length of time necessary for the development of the new individual varies according to the spcnics ; in man it occupies nine calendar months or about ten lunar months — that is, from 273 to 280 days. The period of human gestation is arbitrarily divide(] hy His into three stages : (I) The stage of the ovum, comprising the first two weeks of development ; (2) The stage of the embryo, extending from the end of the second week to the fifth week, during which time the germ begins to assume definite form ; and (3) The stage of the fetus, which includes the remainder of the term of intrauterine existence.

It may he pointed out that the term ovum as employed in embryology, has three different significations : it designates the female sexual cell prior to its impregnation ; it is used in the sense note<l alxivt? to doignate the fertilized egg; and it is somewhat Ioosely applied to the product of conception during various stages of development.

The Spermatozoon

It is noteworthy that both spermatozoa and ova — that is, both sexual cells — are priMJucts of metamorphoses taking place in epithelial structures, the former heing derived from the spermat<»g<Miic cells found in the sc»miniferous tubules of the testicle, while the latt<T come from the germinal epithelium of the ovary. Till* form of the seminal fdameut varies greatly in dillrrciit spc'<*i<»s i Fig. 1), hcin;j: usually an el«»uirat<'d Hairellate cell. I'hc human spermatozoon (Fig. 2) is ahout O.tJo mm. ( . ,\„ inch) in length, consisting of a head, a middle piece, a tail or flagellum, and an end-piece.

The head is much thii^kcucd as eom|mriMl with llu' otlu'r M'giut'uts, appearing egg >ha|H'd as m-cu upon its hroader surtace, the >malh'r (»xi remit v l>riii«r iviuiccIihI with the middle pircr ; srcn in pri>Sle, it is rt»nvr\ on one >i»li' ami concave on the other. Tho middle piece is somewhat longer and much thinner than the lu'ad. whilo the tail is a slender filament ^lii^htlv mnn* than four-fifths of the entiri' Irnirih i»l' tin5^vnuato/.<H»n. I-ying in ilir i-iiitrr of the sjwrmato/iHin, and «\t«iidiii:^ thrnu$:\iou' h- eiiim- ImriK i? the slender axial fiber, wliiili i^ \»nj\uii|:»f<.l tii;riiiiy tt^-yimd ihe wil as the end-piece «m terminal filament. At the anterior end of the axial fiber is a small body, the end-knob (not shown in the figure).

Heisler 001.jpg

Fig 1. —

The power of locomotion which the spermatozoon exhibits is conferred by the vibratile movement of its tail, aec«impanied by a rotation about its long axis through an arc of 90 d^rees. The rate of progression is about 0.05 or 0.06 mm., or its own length, per second.

Fig. 2 The Spermatozoon

Spermatozoa possess remarkable vitality, remaining active in the male genital tract for several days after death. In the genital passages of the female, they may retain their activity for several weeks, and when mounted and protected from evaporation they have been known to show vibratile motion after the lapse of nine days (Piersol). Weak alkaline solutions render them more active, while acids, even quite dilute, destroy them. The spermatozoa of the bat, being deposited in the female genital passages in the autumn, retain their power of fecundating ova until the following spring.


The details of spermatozoon-formation, or spermatogenesis, vary in different animals. A cross section of a seminiferous tubnle of a mammal (Fig. 3) shows a layer of cuboidal cells called parietal cells, lying in contact with the ba.semenf membrane of the tubnle wall. This layer consists of tlio so-called Bertolli'a colnmns, or snstentacnlar cells, and of the spermatogenic cells or spermatogonia. The sustentaculnr cells are merely supporting ; the spermatogenic cells give rise to the spermatozoa.

The spermatogonia undergo re{)catcHl mitotic division with a concomitant decrease in size. The last generation of the spermatogonia, after an intervening period of growth, give rise, also hy mitotic division, to the mother-cells or primary spermatocytes, which lie nearer the lumen of the tubule than do their predecessors. The primary s|K»rmatocytes now divide to form the daughter-cells or secondary spermatocytes, and these in turn undergo division to form the spermatoblasts or spermatids. From the s}>ermatids, by rearrangement of their constituent elements and certain special modifications in form, are produced the spermatozoa.

Not all the details of the differentiation of the spermatozoon from the spermatid are as yet clear; moreover, these details vary somewhat in different spi*cies. As observed in mammals, the nucleus of the spermatid becomes somewhat flattened and elongated to become finally the head (nucleus) of the spermatozoon. The centrosonie migrates to that side of the nucleus which is toward the lumen of the tubule, becoming attached here to the nuclear membrane, while the attraction sphere (arehoplasm) goes to the opposite side. The attraction sphere produces the head-cap and lance which are present in the spermatozoa of some mammals. From the centrosome a delicate filament grows through the cytoplasm, toward the lumen of the tubule, the centrosome itself — or centrosomes, as there may be two — giving rise to at least the neck of the middle piece, that is, the part adjoining the head (Meves and Iji'uhoss^k) or, acconling to others, persisting as the end-knob. Although the axial filament seems to grow forth from the centrosome it i?s believed l)V Meves that it is differentiated from the cytoplasm of the spermatid, which latter also gives rise to the remaining part of th(» middle piece and its sheath as well as to the tail and its sheath. Meanwhile the cytoplasm in relation with the nucleus is reduced to an exceedingly thin layer, a portion of it being cut off* in some eases.

During the metamorphosis of the spermatids the Sertoli cells increase in size, elongjuing toward the lunien of the tubule. To each such S<TtoIli column a number of spermatids become attached, the Sertolli cell iH'ing apparently used up in yielding nourishment to the developing sperm-cells.

The descent of the spermatozoa from the spermatogonia is accompanied by a peculiar modification of ordinary mitosis known as the reduction of the chromosomes, or reduction-division. The spireme, or chromatin thread, of the ordinary cells of the body, known as body-cells or somatic cells in contradistinction to the reproductive or germ-cells, breaks up at the beginning of mitosis into a definite number of segments or chromosomes, which number is constant and characteristic for the species. Thus in man, the guinea-pig, and the ox, there are sixteen chromosomes ; in the mouse, the salamander, and the trout, twenty-four; in some sharks, thirty-six ; in the grasshopper, twelve. After the division of the chromatin into the characteristic number of chromosomes, in the case of a somatic cell, each chromosome splits longitudinally, so that each daughter-nucleus receives as many chromosomes as there were in the parent cell. Thus the number of chromosomes remains constant notwithstanding re])eated cell-divisons. In the division of the germ-cells, however, an important mo<lification of this process has been observed, resulting in the reduction of the number of chromosomes. This reduction-division occurs both in the development of the spermatozoon and in the " maturation of the ovum. The essence of reduction-division is that in the germ-cell the chromatin divides into half as many chromosomes as in the case of the somatic cell, but these chromosomes are tetrad and hence are equivalent to double the number of the somatic chromosomes ; and that two subsequent cell-divisions occur without intervening reconstruction of the nucleus, so that one element of each tetrad passes to each one of the four descendent cells. Thus each of the four cells descended from any one germ-cell containshalf as many chromosomes as a somatic cell, and when, during fertilizaticm, such a (male) germ-cell unites with a similar (female) germcell the normal numl)er of chromosomes is restored.

During spermatogenesis the multiplication of the spermatogonia is effected by the usual method of mitosis, as is also the formation of the primary spermatocytes from the last generation of the spermatogonia, but ^vhen the primary spermatocyte enters upon the process of mitosis its chromatin divides (in the case of the Ascaris megalocephala whose somatic chromosomes number four) into two chromosomes, each chromosome being tetrad. Without reconstruction of the nucleus the primary spermatocyte divides into two secondary spermatocytes (Fig. 4), each tetrad chromosome dividing into two dyads, one for each new nucleus. Again, without reconstruction of the nucleus, each secondary spermatiKjyte divides into two spermatids, each dyad breaking

Pritnordtal sexual cell.

Zone of proliferation. {TAe j^ene rations are tnuch larger.)

TUmt of growth.

y,perMittiHyte I ntdrr.

'.petmatiK yte, II onler. !,f'frmiitul

Zone of maturation.

Fig. - Hi:h<!ffiiit Irr <lin((ruin of HfHfrmatogenesis as It occurs In ascaris (after Ii<»virl;. (• KrK'«:tm. <1. Aimt. u. Entw.," Bd. L)

u|) into two rtiii^i<? (rhromosomes, one for each spermatid. Thiirt ciu'h ^|NTlllati(l (contains half as many chromosomes as the Hituiiiiir iiuiiilxT (•liara(!teristic for the sj>ecies, and each primary HporrnatiM-yU? is the parent of four spermatids, which riubn<M|U(;iitly become fum^tional spermatozoa.

The Ovum

The female sexual cell or ovum is remarkable among animal cells for its size, it beinj^ a rule, to which there are no known exceptions, that it is much larger than any other cell in the b<Mly of the parent. The human ovum measures, in the mature stjite, 0.2 nun. in diameter. In structure, the ovum presents the parts of a typical cell ; namely, a cell-wall, here called the vitelliae membrane, the cell-contents, or vitellna ur yolk, a nucleus or germinal vesicle, and a nucleoliis or germinal spot.

Surrounding tbe ovum is a st)niewliat loosely -fitting transl)arent, elastic envelope, the zona pellucida, and outside of this is the corona radiata. These two layers are often re

Fig. y—Eft!! Ctom a rnbbll's rolUclc nhicli was n.:> Dim, (th loflhl In diameter (•(ler W«lderer). It is mrroundtd by Ihu- Ii>no pelliirldu (t p.). ou which IhiTe rvsl It one pince rolliealur L-ells I/, i.). Ttie yolk conUiint deutoplasmjc gruiulci ('tl. In the grtinlUBllve vwklo il. b.\ the niirlMr uHtwork (l, n.) Ib eapwlallr

ferrcd to as the egg-eoTelopes ; but since they are contributed by the discus proligerus of the Graafian follicle, it must be remembered thatthey arc iif>t. properly speaking, a part of the ovum . Between the zona jielliicida and the ovum is the small perivitelline space. The radial striation of the zona is generally regarded as due to the presence of minute lauals opening into this space. The canals are thought by some to facilitate the ingres."? of spermatozoa, thus corresponding in function to the micropyle, a small aperture found in the leas easily penetrable egg-envelopes of many invertebrates and of some lis lies.

The vitelline membrane does not call for extended description. It tu:iy be regarded aw a slightly speciullzed condeDsation of the i>eriplit.Tal jwrt of the eel 1-cont eats.

The viteUiu, or cell-contents, here, as in other cells, ia essentially protoplaam or cytoplasm, to which is added material called deutoplaam, designed for the nutrition of the ovtim at the beginning of development. The pmtoplasm is also called the fonnative yolk ami the egg-plasm, while thedeutoplasm is known as the nutritiTe ya\^. In the hiimuD ovum these elements are more or less uniformly distribulLtl j there is, however, a difioreutiation into an inner, slightly less clear region, containing more yoik-granules (deiitoplaam) and a peripheral, clearer zone. The chahictoristic transparency of the human e^-eell is due to the fact that the deutoplasmic particles found in it arc not clondy as in the ova of other mammals. The following classification of ova by Balfour is based upon the arrangement of these constituents :

1. Aleclthal ova are those in which the pnitoplasm and

Fig. a.—

deiitoplasm are uniformly distributed, as in the ova of Mammalia, hiclilding man), and of aniphioxus (Fig. o).

2, Telolecithal ova are those in which the reialivelv abundant dcutoplasm is accumulated at one side of the ovum, called the vegetative pole, while the protoplasm appears as a flat germ-disk at the animal pole on the op]K)site side. Here belong the eggs of birds, reptiles, and bony fishes (see Fig. 6).

3. Centrolecithal ova are those in which the deutoplasm is centroly the protoplasm completely surrounding it, as in the eggs of arthropods (Fig. 7).

Ova are classified also according to their method of segmentation. This will be described later.

The germinal vesicle or nucleus is the most imj)oi'tant part of the cell, since, as will be seen hereafter, it is essentially by the conjugation, or more accurately by the fusion, of the nuclei of the male and female parent-cells that generation is effected. As a rule, there is but one nucleus, though there may be two. Its position is usually — if not universally — eccentric, this being more marked where there is a distinct differentiation into animal and vegetative poles, in which case it is found always near the animal pole. It is nearly spherical in shape, and like the nucleus of any other typical cell, it is composed of the nuclear network consisting of linin and cliromatin, and nuclear juice or achromatin, the former containing the latter within its meshes. Surrounding the nucleus is the well-marked nuclear membrane, while within it is the nucleolus or germinal spot. The latter may be single or multiple, according to the species, though the number is fairly constant for each species. Nagel ascril)es ameboid movement to the germinal spot.

Polarity. — The polarity of the egg has been incidentally referred to. Apparently it owes its existence to the eccentric position of the nucleus, the animal pole being that point on the surface to which the nucleus is nearest. Polarity bears a significant relation to the specific gravity of the ovum, since the nucleus reaches the surface of the latter at the animal pole and there extrudes the polar globules ; and it is also related to the segmentation of the fertilized e^g.

The Hen's Egg

As the hen's Q^g is so largely utilized for the study of development, it will be profitable to consider briefly its structure. The ovum or egg-cell is represented by the yolk or yellow of the egg, the alhuinen or white, as well as the shell ami filiell-meiiibraiie, being egg-envelopes contributed by the oviduct. As in other ova, the egg proper is a Bingle cell, having a vitelline membrane and a germinal vesicle. The enormoUH size of the cell is due to the large quantity of nutritive material or dcutoplasm present, this contriiiuting by far the greater part of the bulk, while the nmch Bmaller formative yolk or protoplasm, containing the germinal vesicle, is so eccentrically placet! that it seems to float upon tliu surface of the deutoplosra. The little whitish flpot on the surface of the yolk, known as the cicatricnla or garmlnative disk, consists of the germinal vesicle with the Niirrouniling formative yolk. It is in the germinative disk niorii- ihiit .-^fomentation takes place, and it is for this reason that i-gtfK iif iliis class are designated meroblaatlc, or partiallydMdiag «BKB'

The dnitopluMm is made up of white ami of yellow yolk

nnclens, situated under the germinative disk; of a larger mass, the latebra, more deeply placed ; and of several concentric layers separated from each other by the yellow yolk.

Such is the egg as it leaves the hen's ovary. In the beginning of the oviduct it is fertilized by the spermatozoa already there. After fertilization it passes into the longitudinally furrowed second part of the tube, where it receives a copious coating of albuminous material, the white of the %gg\ thence it goes into the villous third part of the oviduct, where it acquires a calcareous coating, the shell; finally, passing through the fourth part of the canal, it is "laid."

The layer of albumen immediately surrounding the yolk is relatively dense ; it is prolonged to either extremity of the egg, somewhat spirally twisted, as the chalazse. Enclosing the albumen is the thin tough shell-membrane. This consists of two layers, which se]>arate at the blunt pole of the egg soon after it is laid, giving rise to the air-chamber. The shell, composed largely of lime salts, is very porous and thus readily permits of the necessary gas-interchange between the contents of the egg and the external air during incubation.

Ova do not possess the remarkable vitality which is characteristic of spermatozoa. An unimpregnated ovum perishes in from seven to nine days.


The formation of ova takes place throughout the greater part of fetal life and continues for a short time (two years, according to Waldeyer, Bischoff, and others) after birth. Their number is estimated to be about seventy thousand.

The ovum, the direct derivative of the germinal epithelimn covering the free surface of the ovary, is situated in the cortical part of the latter organ, being enclosed in the Graafian follicle. As a rule, each Graafian follicle or ovisac contains but one ovum, though sometimes two, and more rarely three are present.

The Graafian follicle, in its mature condition, is a vesicle from 4 to 8 mm. in diameter, which is surrounded by a sheath, the theca foUiculi or tunica vascolosa, consisting of a condensation of the ovarian stroma. The outer, more fibrous

ru.g.-SoCIIonnr hUlPBO uvary. Includlne ^rlex : a.^ennintl epithelium of tn* (urlkrii; b, lunlu •FliiiKlnca ; r. pcTlr>heral Mrama cunMinlQg immatura Cliullali tolllcJncDi r, wcllMilvanrcd fulllclH rrum ulKwe wall latinbriuiB )-raiiuluaa liaa partially *cpaHl«l ;/, cavity of liquor folKculi: ;;. ovum sutroUDdcd by Mll-tuaa HiiiitltulliiK dlwiu |imllt^nit it>lenii)l|.

zone of tho tliota, containing large blooil-vcsaels, is distinguifthcd as the tonlca fibrosa ; the inner more cellular layer, rich in »imall vessels ami capillarien, as the tunica propria.

The fibrous wall of the follicle is lined by the membrana granulosa, which consists of many layers of epithelial e^lls ; these, at the point of contact with the ovum, project in such a manner as to surround it completely, the cellular envelope thus formed constituting the discus proligerus. The inner cells of the discus are arranged in two layers, the individual elements having their long axes radially directed. From the appearance of radial striation, conferred partly by this circumstance, the inner zone has been called the zona radiata or zona pellucida, and the outer the corona radiata. The cavity of the Graafian follicle is filled with fluid, the liquor folliculi.

The stigma, or hilum folliculi, a yellowish-white spot devoid of blood-vessels on the free surface of the Graafian follicle, indicates the point at which rupture will take place. After this event, which occurs when the ovum is " ripe, the latter passes into the Fallopian tube.

The ultimate origin of the egg^ is to be sought in that important group of cells on the surface of the ovary to which Waldeyer gave the name germinal epithelium. This first appears at about the fifth week of intra-uterine life, as a localized thickening of the cells of the structure that subsequently becomes the peritoneum. The thickened areas comprise two longitudinal elevations on the dorsal side of the future abdominal cavitv, one on each side of the median plane of the body ; these are the genital ridges. Owing to the development of connective tissue beneath the epithelium, the ridges increase in thickness, and, with the progress of other changes, finally become, in the female, the ovaries. At about the sixth or seventh week — the germinal epithelium now consisting of several layers of cells instead of being a single stratum thick, as at first — cord-like processes, the sexual cords, or primary egg-tubes, or egg-columns, grow from the surface into the underlying connective tissue, carrying with them certain of the surface-cells (see Fig. 128). Conspicuous among these are the large sexual cells, or primitive ova ; while smaller cells, likewise from the germinal epithelium, are also present. The sexual cords become divided into groups of cells, each group containing one or more primitive ova and many of the smaller cells. Gradually, the small cells of the group surround the primitive ovum, at first as a single layer of flattened cells, which are succeeded by several layers of j>olygonal cells. From these enveloping cells come the membrana granulosa and the theca of the Graafian follicle.

The primitive ova or oogonia — analogous to the spermatogonia — having undergone repeated mitotic division, cease to divide at a certain period of their history and enter upon a period of rest and growth. They thus increase in size and become fully formed ovarian egi^ or oocytes, the nucleus enlarging and the cytoplasm becoming more or less laden with deutoplasmic material or food-stuffs.

The youngest ova are found nearest the surface of the ovary, the eggs as they develop advancing toward, but never entering, the medulla of the organ. Finally, in the fullydeveloped condition of the ovum and the follicle, the size of the latter is such that its diameter equals or exceeds the thickness of the ovarian cortex, its position being usually indicated by a small prominence on the surface of the ovary.


By maturation or ripening is meant that series of changes by which the ovum is prepared for fertilization and without which the latter process is impossible. In nearly all mammals, including man, it occurs while the ovum is still in the Graafian follicle ; in some other groups it takes place after the egg has reached the oviduct.

Briefly, maturation may be said to consist in the extrusion from the cell of a j)art of its nucleus and of a small pan of its cytoplasm. The nucleus undergoes changes ])ractically identical with those of ordinan^ cell-division. First, the nuclear membrane disappears, the nucleolus disintegrates, the nuclear juice becomes mingled with the surrounding protoplasm, and the nucleus moves toward the periphery of the egg (Fig. 11). There is now formed a nuclear spindle from the achromatin substance of the nucleus. The long axis of the spindle lies parallel with one of the radii, and its direction is ilctermineil hy the position of the pole-corpuscles. Each pole-corpiisole is surrounded by a radiatiou, the attraction-sphere or polar striation. These bodies exercise a contmlling intlnence u|>on t)ie nuclear spindle, so that it assumes

Fig. U.— Pnrtliini of the ova or Atleriai glariatft.nho^iaK rliiiiigi't Hni'i.-l Inutile

t, germtnal Bpot, cnrnpoei-i] nI aiiclvln niid lArauuclEtn (r; : <t, niirtear uplnille In praceii of RirmMlon.

such a position that each of its apices points towanl a polecorpu.scle.

The outer extremity of the nuclear spindle, being made to protrude by the continued onward movement of the nneleiis, becomes detached (Fi^. 12); this separated piece, with the

Fro. 12.— Fonnallcn of the poUr bodlea In the dy« of Aileriai gUicinUi iHeitwtg): pt, polar spindle : pb', flnt polar body : pb", ■econil polar hod;r. "i nucleiu returning Id condition of reat.

small 8urrt>unding constric ted-off toasa of protoplasm, constitutes the first polar body. Krom the remnant of the first

niic\vav !^piiit]li>, a sccun<] ono is funiiGd, \vhicli in the same nianuer extrudes the second polar body. M'liat reiniiuis of the nucleus dow moves toward the center of the cell and U kii'>wn as the female pronncleos. The [u^ition of the female proniicleiis is nearlv or absolutely central. The egg U now ready for fertilization.

For some timo after their extrusion, and pending their final disaiipeurance and disint(>gration, the jxilar l>odies are to be seen lying in the perivitelline syaix. The formation of polar globulcM is prolrably almost universal throughout the animal world. It is of interest to note thatin some partbenogenetic eggs — that is, e^s capable of developing into a new individual without contact with the male element, as, for example, the summer e^;s of plant lice and of some other arthropiHls— only one polar globule is said to be formed, and it has recently l>een shown (Sobotta) that in the maturation of the ovnm of the mouse only one polar body was tormed in the miijoritv of cases.

The maturation of tiie ovum is essentially ii rediielion of the chromosomes precisely analogous to the re<^l net ion-di vision seen in the descent of the s[>ermatozoon from tiie primary spermatocyte. The List generation of oogoniu having increase<l in size after iheir stage of rest, uud liavinj; thus become the ovarian eggs or primary oocytes, now niiderj^ mitosis, but in a uianner differint; from that of their pndecessors. The chromatin lhrea<l of the nucleus, instead of dividing into the number of chromosomes characteristic for the somatic cells, divides into half that number. These chromosomes are tetrads, that is, each one consists of four more or less loosely associated elements conceived to result from a primary longitudinal splitting of the chromosome, followed possibly by a transverse division of the two halves. In the division of the primary oocyte (Fig. 14) to form two secondary oocytes (one of which is the first polar body) each tetrad is halved so that the same number of dyads goes to

Primordiai egg-cell.


Germinal zone.

Zone of mitotic division. ( The nutnber of generations is much larger than here represented.)

' Zone of growth.

Oocyte I. order

Odcyte II. order Matured cvntn.

o I. It-fiL I Zone of maturation.

11. P. B.

Fig. 14— Scheme of the development and maturation of an ascaris ovum (after Boveri) : P. /?., Polar bodies. (From '* Ergebn. d. Anat. u. Entw.," Bd. I.)

each new nucleus. The secondary oocytes now undergo mitosis, but without reconstruction of the nucleus, each dyad chromosome giving one of its elements to each of the new cells, that is, to the now mature ovum and the second |K)lar body. It will be apparent, therefore, that the casting off of the polar bcxlies is a cell-division, but one which results in the production of cells of very unequal size. It is noteworthy, as pointed out by E. B. Wilson, that the chromatin of the nucleus is exactly halved at each division, notwithstanding the disproportion in the division of the cytoplasm.

In comiiaring the phenomena of maturation with those of s[>ern]ut<^encisis it i^ to he nuted thatiu the latter case all four pr<^etiy of the primary spermatocyte become funotiooal si>erniatozou, while in the furiiier case three of the progeny cunie to naught, only one of the number, the mature ovum, being functioually imjKjrtant. E. B. Wilson points out that the nfluction of tiie chromosomes in the germ-cells is for the piir[M»sc of maintaining the constancy of the number of chnimoisomes which is peculiar to the specieij, since, if reduction did not occur, the number would be doubled at each generation ; lie further points out, however, that " the real jirobleni is why the number of chromosomes should be held constant."


Extrusion of the ovum from the Graafian foUicle, or ovt!lation, occurs upon the completi<«n of the process of maturation. As the time for this event approachetr, the wall of the follicle at the site of the stigma iKxximes much thinned and finally ruptures, ati<l the ovum passes into the Fallopian tube (Fig. l^). If, instead of ])assing into the tube, the

ovum maintains its connection with the ovary and is fertilized there, it may undergo partial development in xitu; such a condition constitutes one variety of eztra-nteiine pregnancy or ectopic Kestation.'

Ova are extruded from the ovary, one or niorc at a time, ' Other varictiw of orlc.pic j^lntion are ahdominal «,nA tubal, ibe naniea of whicb are miffii-iontly iluHTiptiro.

ali-, from imhorty to the at regular, generally nmnthly, climacteric.

After tbe eseajie nf the ovum, hemorrliage into tlit- empty follicle occurs, the resulting clot being the corpus hemoirhagicnm. According to Leopold, if niptiir« occurs during the iDtermenstrual period instead of at the time of menstruation, hemorrhage will be amall or entirely wanting, the resulting corpus lut«um being called then atyplcid, to distinguirih it from the ii/plca/ body formed in tbe ordinary manner.

The hl(md-c!ot is soon permeated by cells originating in tbe Willi of the follicle, some of wiiicb are fusiform connective-tissue cells, while others are large cells containing the yellow pigment, lutein. Meanwhile, the follicular wall thickens and becomes plicated. I^ater, upon tbe replacement of the mass of clot and cells by fibrous tiwsne and the development of capillaries within it, the body assumes a yellowish cicatricial appearance and is known as the corpna Intenin. (Fig. 16). The color of the corpus varies considerably in

- Fig.,— Oviifli different species of animals, the yellow color being characteristic for the human subject.

If the ovum is not fertilized, the («rpU8 luteum attains its maximum development in less than a week and begins to shrink at about the twelfth day, becoming completely absorbed in a few weeks. If fertilization occurs, it continues to grow for two or three months and accjuires a size onefourth or one-third that of the entire ovary ; persisting till toward the end of gestation, it finally shrinks to a small white scar, which may not totally disappear until a month or more after labor.

It has been customary to designate the larger, better developed yellow body, the true corpus lateuxn, or the corpus Inteuxn of pregnancy, in contradistinction to the so-called fedse corpus Intemn of menstruation, and to regard the presence of the former as absolute proof of previous impregnation. This view is no longer tenable, since bodies identical in appearance with true corpora lutea have been found in virgin ovaries (Hirst).

The relation of ovuhition to the menstrual function has been much discussed. While the two processes usually occur at the same time, they are not to be reganled as dependent one upon the other. It has been shown by Coste, whose observations have been confirmed by Leoi)old, that as a rule Graafian follicles burst during menstruation, tliough they may rupture before or after this event. It has also been shown that in the rabbit sexual intercourse hastens the rupture of the follicle.


Menstruation, or the eatamenial flow, is considered here because of its natural association with the function of ovulation.

Menstruation may be defined as a periodical discharge of bl(HHl and disintegrated epithelium and other structural elements of the mucous membrane of the bodv of the uterus, mixwl with mucus from the uterine glands and the vagina, occurrinii: normallv about everv twentv-eii^ht davs, an<l ass(MMat(Hl with more or less disturbance of the entire sexual system. The inauguration of the function marks the age of puberty, the beginning of the sexual life of woman ; its cessation, known as the climacteric, or menopause, indicates the termination of the child-1 Hearing j)eriml.

In temi)erate climates, the menses are established between the thirteenth and seventeenth years and cease between the ages of forty and fifty. In the tropics, they appear somewhat earlier; in cold climates, somewhat later. The function is suspended during pregnancy and, usually, during lactation.

The quantity of the discharge, though subject to considerable variation, is usually from 4 to 6 fluidounces. The blood is venous in character, and, owing to admixture of alkaline mucus, does not coagulate unless present in excessive amount.

The menstrual cycle of twenty-eight days may be divided into four periods : the constructive stage, comprising from five to seven days ; the destructive stage, lasting about five days ; the stage of repair, covering a period of three or four days ; and the stage of quiescence, including the remaining twelve to fourteen days.

In the constructive stage, which occupies the six to seven days preceding the discharge, the mucous membrane of the uterus becomes markedly swollen, the normal thickness of from 1 to 2 millimeters being more than doubled. The uterine glands become wider and longer and also more branched. The blood-vessels, especially the capillaries and veins, undergo great increase in size, and the connective-tissue cells are increased in number. The thickened mucous membrane resulting from these alterations is the decidua menstrualis. The term "constructive" is applied to this series of changes for the reason that their apparent purpose is the preparation of the womb for the reception of a fertilized ovum.

The destructive stage, corresponding to menstruation proper, lasts from three to five days. It consists essentially in the partial destruction of the hypertrophied mucous membrane, the menstrual decidua, accompanied by hemorrhage. The initial step is the infiltration of blood into the subepithelial tissue ; according to Overlach, this takes place, not by rupture of capillaries, but by diapedesis. In a day or two the superficial layers of the mucous membrane disintegrate and are cast off, those portions of the enlarged uterine glands included within this stratum sharing the same fate. By the loss of the epithelium and the subjacent strata, the blood vessels are exposed. Subsequently these rupture, giving rise to the characteristic hemorrhage. Fatty degeneration accompanies the death of the cast-off tissue, and was thought by Kundrat and Engelman to be the direct cause of the hemorrhage ; it is probable, however, that fatty degeneration is not present until after the flow of blood has begun.*

The stage of repair, comprising the three or four days following the period of the discharge, witnesses the return of the uterine mucosa to its usual condition. With the gradual subsidence of the swelling, the superficial layers, which were lost, are replaced by the growth of new tissue from the deeper layers, which persisted. The formation of the new epithelium begins at the mouths of the uterine glands.

The stage of quiescence extends from the close of the preceding stage to the end of the cycle, or, in other words, to the beginning of the next constructive stage.

Other parts of the sexual apparatus, including the ovaries, the Fallopian tubes, and the mammary glands, show more or less sympathy with the uterus during menstruation, the changes in them consisting chiefly in swelling, hyperemia, and tenderness.

The Relation of Menstruation to Ovulation and Conception

The function of menstruation and the extrusion of ova from the Graafian follicles, though closely associated, are not depeiKl(?nt upon each other. Ovulation occurs perhaps most commonly during the time of the menstrual discharge, but it may take phic(» before or after this event. While it is now general ly accepted that the two functions are not mutually interdependent in the sense that one is a necessary part of the other, yet, since the turgescence incident to sexual intercourse has been shown to hastim the rupture of the follicles, it seems reasonable to sup(K)se that the ovarian hyperemia attendant upon the menstrual epoch woyild exert a like influence.

Since the function of menstruation is normally susj)ended during pregnancy, the relation Ixitween menstruation and

MarHhair8 "Vertebrate Embryology;" Miiiot's '^ Human Embryology

ovulation, and of these to conception, are of practical interest in determining the date of labor. The duration of pregnancy is from 270 to 280 days, nine calendar, or ten lunar, months, and it dates from the moment of conception. But since the ovum retains its vitality for about a week after its extrusion from the Graafian follicle, and since the activity of the spermatozoa may continue for several weeks after their entrance into the female genital tract, it is impossible to fix accurately the date of conception even in those cases in which there has been hut one coitus. It is now believeil by most embryologists that the ovum is fertilizable only while it is in the Fallopian tube, a period probably of about seven days ; if this be true, it follows that conception must occur within a week after ovulation, although it may be effected as late as two weeks after coitus. Since the ovum is usually discharged from the ovary during the menstrual j)eriod, it is evident that the time most favorable for conception is the week following menstruation ; and inasmuch as the latter function is suspended during pregnancy, it is obvious that the most reliable basis for calculating the probable date of conception is the last menstruation. The method usually employed is to count nine months and seven days from the first day of the last menstruation. After what has been said it is perhaps needless to remind the reader that this can furnish only approximately the date of labor. In a case where conception occurred a few days prior to the first omitted period, there would be a discrepancy of several weeks between the actually and the calculated, termination of pregnancy.


Fertilization is that peculiar union of spermatozoon and ^g-cell which initiates the phenomena resulting in the formation of a new individual. As implied in a preceding section, impregnation is possible in the higher organisms only after the completion of maturation, while in others, as for example the maw-worm of the horse, spermatozoa enter the ovum before the extrusion of the polar bodies, and thus one process overlaps the other.

'XW* iiMifit primitive methrxl of fertilization is that effected witli/>ut ^5ri|>iilation of the parent organismi^y or extemil fertUizattoi; thin (K^rturH in 088eous fishes, in some amphibiarm, and in many invertebrates. In these groups, both ova and Mcmen are discharged into the water and there \%%iH*i, In frogK, however, there is a quasi-copulation, the nml<? <?mbnuiing the female during the breeding season and i\i*\Hm\!\\\^ mwM'W u|K)n the eggs as they are evacuated. In all liiirh'T animals, internal fertilization occurs, this being i*tViutU*il by Mf'Xiial congress.

In man, f(f*rtili/Jition normally occurs in the outer third of iUit Fallopian tnlH». The semen having been deposited in iUtt vagina, or the utcnis, or even upon the vulva, the speruuiiny4rti umU(* tlir»ir wav into the oviduct bv the vibratile motion of tl^'ir tails. Meeting the ovum, they swarm around it, and mtttio, of i\u'U\ pass through the zona pellucida into tli« |Mfrivit<*lline HjiarM*. It is believed by many investigators that lUii ranaln of i\w. zona (ronHtitiite the avenues of entrance for iUt* n\u*nuaUy/Ani, In the rather firm egg-en veIoj>es of Umu*iii and ^ime finheH, there is a small aperture, the microVfU, through wliirli the spermatozoa gain entrance.

Whihi many Hpcrmatozoa may pass through the zona, only mi' — thai onn wliow? liciMJ first impinges against the vitelline m^mlimnn nnf<*rH \\m\ ovinn. Why others do not or cannot MUlnl' l» unknown; ponhiMy ht^i^ause the egg's power of attraction U Mnnilll^d (Minolj. Polyspermia, or the i)enetration of mivim'mI ttpMrniMlo/oii, may o<*(Mir, however, if the ovum is linlutMllliV I Mud In Home Iowit types it is sjiid to be normal.

A(i (liM lapMriMMloKoon Ih ahout (o strike the vitelline merabnmni lIlM pnilnplilnm MWelU up at tlH> iM)int of contact into (hit li>oiiMMVi> MnMMlHotioi* ( l**i^;. 17). Through this the spermHh»#»«»n ImiH'o IIo nvmv, !{« tail heing absorbed by the cyto\\\\\^\\\ \^^ Mu^ MVnm or tiring \v\\ outside in the ease of the h^M^ Uh^hhr V\\\' middli' pife*', t»r at least a j>art of it, h\\^b^vl^»y4i \W v\\^\ lxh»*h» \\\\\v\\ luHrr rcpix'smts the eentro*>vw>^ \^\ v\\N <^vUHHU^Ii \^\\W\^ iIm' oNUUi with the head (uu\^W^^ \^* ^hv i^s VUS4^\^'»\^N^l\ V\\\n uueieus or head now en^MV>^ ^S\v^*^^'^dL ^^^^y^ ^^^v^ "*^*'^ ^*<MuM«eA'M/*. The male and female pronuclei approach eat'h other and finally meet in the center of the ovum, the two bodies apparently fnsing to form the single segriientotioit-Hiivleas or vlmvage-mideus. It must not be mulerstood, however, that an actual single membranatc nucleus is formc<I, As the proniiclot approach each other the centrosome of the spermatozoon lies between them surrounded by its attraction-sphere, and gives rise to a unclear spindle after the manner of onlinary mitosis, the chromatin threads of the two pronuclei lying in relation with its equator, but on opposite sides from each other. In other words, the chromatin contributed respectively by the two pronuclei of AtterUtt glacial!*, ahowlng the approach > le ovum (Hertwlg) : a. fcrtillstnK n.ftle eleme >/, b", aUgts u( fUalon or the head of Ihe si retains in each case its identity, the " segmentation-nucleus " entering iii^n the processes of mitotic division without previous intermingling of the chromatin of the ovum with that of the spermatozoon. As will be shown in Chapter II., one-half of each chrontatiii thread goes to one pole of the spindle, while the other hidf of each goes to the ()p{>osite pole, to give rise to the two daughter-nuclei resulting from this first segmentation of the segmentation-nucleus. It will be evident that the segmentation -mid ens consists of chromatin substance derived from each {Mirent. As this fact has been thought to explain, anatomically, the otTspring's inheritance of both paternal and maternal characteristics, it has been made tho basis of a theory of heredity formulated by Hertwig and independently advanced by Strasburger.

L, fcrtlKntovumnrerlilDus iHertwlg): I ) nre apprOBcblnR: In B Iher hnre almait ion of fenlltiallon (Hertvrlg) : 1.71., irgmei e ratio {a) aod the female

Artiflclal fertilization, or the bringing about of the development of the ovum by urtifioial (chemieal) means, without the partioijMtion of the male element, has been rcpently experimentally effected with the eggs of the sea-urchin by Ijoeb, of Chicago, These eggs, when first immersed for about two hours in a mixture of sea-water and a weak Boliition of magnesium chlorid, and then transferred to normal seawater, were found to undent complete and normal development, producing perfect larvse. This artificially induced development different from that of the ordinary method only in being slower.

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Glossary Links

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols

Cite this page: Hill, M.A. 2017 Embryology Book - A Text-book of Embryology 1. Retrieved October 22, 2017, from

What Links Here?
© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G