Book - A Text-book of Embryology 13
CHAPTER XIII. THE DEVELOPMENT OF THE GENITO-URINARY SYSTEM
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
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.
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Owing to the intimate anatomical and functional association of genital organs with the urinaiy apparatus, it to iliseuHH the development of these two systems
Development of the Kidney and Ureter
M^iu ol* v ki(hu'y and ureter of the higher vcrt U rtwn^oiat^^d with the development of certain fetal and the mesonephros, which represent Ss' kidiiry t»f hirval amphibians and the perm »r llfthrw. In man and other allied types, the U^ui^ 4^^^^^^o i« of litthM)r no importance functionally, \\luK ihv' Iwwv t\iiu»lioimteH (hiring a i)art of fetal life as the '«»<.. ^^ \a uuu.M\ *'\oi'vti(»n, prior to the development of the |i\Muu^ui lvidiu\\.
»r ht»A(l kidney constitutes the most primiVvw ^I'U^UiAW I.VM^ pr)unc(*hiinism for the excretion of urine, iln iui«(\iir mii',mi*Uvi iu the foUowing manner: When Ok pu.ivtil iiu .mUiiu, Nshioh Hnbsc<jncntly divides into the
MMU, \ .\\iM\\\ (o riAjuMuto Ironi the lateral plate of mesoU i»u \Ux i\\,i |tvit t lU'o oouneoted for a time by an interven(M. )m.( I .>( K air, ibc middU plate or intermediate cell-mass \U. u». I hi ihii'kndiig of this intermediate cell-mass y\ ' l»i . 1 1*. Wu^UiiiU iidgt». whioh projects into the ctelom or I- ■ U M'iv ri»*' uu'^iuU'iiual i»r mesenchymal eh»ments of I'l \\ i'Uli in udm* brroiiu^ i^i'ouprd into cords of cells which it ui iiu. I liMU .u \'i'ii;iiu points with the mesothelial cells
I \U ■ s liMii I hr iU'iv'iu of (hcs(« et»ll-cords of the Wolffian \.A^,. \i\ hiiij^; biiu u Uk;Uici' ot' dispute, some authorities
maiiituiniiig that they come from the mesothelium of the
body-wivity, while others believe that they are of ectotlerraic
origin. Further cimiiges bring about the hollowing out of
the cell-cords so that there results a long tube, the pronephtla
or Besmental duct, which has several siiort tranaverre tnbtilen
— ill some vertebrates, sis ; in man, two — ojwiiing into it ami
communicating by their opposite oi>cn extremities, the nephridial fonnels or nephrostomata, with the cojlom (Fig. 117).
In the human embryo the pronephric tubules have been
found with oi»cn nephridial funnels, but without connection
with the pronephric duct. The mesothelium in imnifxliato
proximity to the open end of each short tubule is invaginated
Axia/trnr. , IflanU canml.
Lttttal fUuti/sr
by a tuft of capillary blood-vessels from the adjacent primitive aortro to constitute a glomemlna (Fig. 117, hh). The
pronephric duct passes tailward and opens into the cloaca,- a
receptacle which receives, in common, the terminal orifice of
the primitive bladder and that of the primitive intestine. It is
apparent, therefore, that the pronephros or head-kidney is anatomically adapted to the function of removing certain substances from the blood by virtue of the action of the cells surrounding the glomeruli or tufta of capillary blood-vessels, and
that these substances may be conveyed away through the duct
into the cloaca and thence evacuated from the body. This
organ is functionally active, however, only in certain lower classes of vertebrates, as in the Amphibia during the larval
stage and in bony fishes. In mammals it is exceedingly
rudimentary and very soon gives place to a more important
organ, the mesonepliros.
The MesonepliroB or Wolffian Body. — As in the case of the pronephros, the origin of the mesonephros is to be found in the Wolffian ridge. Reference has been made, in treating of the primitive segments, page 77, to the middle plate (Fig. 116) as a tract of mesodermic tissue connecting the paraxial tract with the parietal plate. When the paraxial mesoderm
Fig. 117 - Diagnim of pronephros (F) and proncphric duct (Prf): Al, allantols; G, gut; a, cloaca; W>, glomeruli.
Fig. 118.— Diagram of Wolffian body and duct : AU allantols ; (?. gut ; (jy, cloaca ; K. kidney evagination.
segments to form the somites the middle plate likewise undergoes segmetation, each segment being designated a nephrotome. Each nephrotomo, in the lower vertebrates, contains a cavity which communicates with the general bodycavity and which is, therefore, in effect, an evagination of the mesotheliuni of this space. In mammals, however, as well as in reptiles and binls, the nephrotome is a solid cord of cells. I5y the hollowing out of these cell-cords or nephrotomes a scries of transversely directed tubules is formed, each nephrotome, in fact, becoming converted into a short
Fig, 119.— TmiiJivcrso •cction of ■evenWen-day sheep ombryo iBonnetl; n«,
■nnlnn: on, amniotic atic-. n. nearal canal; >. ■omlte dUTerenllatecl Into miucleplate: H'd, Walfflau dut^l: R'b, Wulffliiii body: pm. psHvCal meioderm; vm. visceral lueaadenn; a, a, tUiIng primlllre aortas ; I, InlcsMne.
canal. These tubes acquire oonnection by their deeper ends with the previously formed proncphric duct (Fig. ] 1 7), which
Fig. lai.-DtepnaUlo
6.6 cm (12 In.l long (To
, Wolffian body :
^ m-ary : a. InBUlnal llgamtnl:
- , diBphrBpn^llo IlKBine
nl:5,«OTi
ach ; 0. tnwstinp
7, bU.lder: s. unibilitiil Httery.
is known hereafter, therefore, as the mesonephric or Wolffian duct (Fig. 118). The latter duct and the short transveraa
liibulow which open into it constitute the Wolffian body or
meaoaephros (Figs. 119aiid 120, 1). the tissue of the intermediate eell-nias!) from whicii the W'ollflan tiihiil<'s develop
is designated, by Sedgewick. the Wolffian blastema, and by
Rabl and by Schreiner, the nephrogenic tissue. At tliii* stage
of its development the Wolffian bmiy consists of a tube or
duct lying behind the parietal layer of the mesoderm, parallel
with, and lateral to, the primitive vertebrid column, and
opening at the caudal end of the embrj-o into the cloaca ; and
of a series of transverse Wolffian tubules opening into the
duct and abutting by their opposite ends upon the bodycavity. At the bead-end of the Wolffian duct the now atrophic pronephric tubes are still in connection with it.
As a farther step in the development of an organ adapted to the function of the secretion of urine, each Wolffian tubule becomes somewhat saccular midway between its two extremities, and this dilated ptirt of the tubule is invaginated by the capillary branches of an artery from the aorta. In this manner the cells that line the tubules are brought into relation with the bltK)d of the fetns and acquire at the same time the charaotors of secreting epithelinra. Such an invaginating tuft of capillaries, known as a glomernlns, with its enveloping capsule of Bowman, which latter is the invaginated saccular part of the tubule, constitutes a primitive Malpighian cor* puBcle, a ritructure analogous to the Malpigiiian corpuscle of ihe permanent kidney. Thi.t simple form of the mesonephros is seen as a permanent structure only in some of the lowest vertebrates. In all higher vertebrates it attains to a more complex degree of development, reaching its maximum in man in the seventh week of fetal lii'c. Its complexity ia increased by the dcvelopmont of secondary tubules and Malpighian corpuscles conneoled with those first forme<l. White at first the number of tubules corresponds with the number of nephnitomes, tlus corre^^pondenee is soon lost by the appearance of the secondary tubules.
The horizontal or transverse tnbnleB of (be Wolffian body arc divisible into an anterior or iipf)or scries, diatiiiguislied us the sexual aegmeut, anil a lowir set of atrophic tubules — atrophic for reasons that will appear hereafter. In certain vertebrates that are of higher type than those in which the pronephros functionates, such as adult amphibians and fishes, the Wolffian body persists throughout life as an organ of urinary secretion. In binls and mammals, however, its functional activity is but temporary, since it is supplanted, before the end of fetal life, by the permanent kidney. In man it disappears relatively early, retrogression beginning in the eighth week and the Malpighian bodies having almost disappeared by the fifth month. The presence of the mesonephros as a temporarily functionating organ in birds and mammals, while it is a permanent structure in certain lower members of the vertebrate series, exemplifies the embryological principle elsewhere referred to, that the higher types pass through stages during their development that are permanent in some of the forms below them in the scale of evolution.
The Metanephros or Permanent Kidney
While the Wolffian body is temporarily functionating as a kidney, a structure is developing from the lower, caudal end of the AVolffian duct which is to form the permanent organ. It has been stated that the Wolffian duct opens into the cloaca. From the dorsal asjiect of this duct, near its cloacal end, a small diverticulum, the kidney evagination (Fig. 1 1 8 and Plate VII., 1), grows forth and soon lengthens into a tube which grows head ward, dorsomesial to the Wolffian duct, penetrating into the nephrogenic tissue or mesonephric blastema (p. 236).
The cephalic end of the tube dilates somewhat to form the
primary renal pelvis, the anlage of the adult pelvis of the
kidney, while the duct itself becomes in time the ureter.
From the primary renal pelvis several small divertictila pouch out (Fig. 121), while the surrounding blastema becomes condensed and vascular.
Fig. 121.— Diagram to show extension and branching of kidney evagination and separation of its stalk
from the Wolffian duct: u,primitive ureter; ;>, pelvis of ureter; WD,
Wolffian duct ; Bl^ bladder ; u*, urogenital sinus ; CI, cloaca ; G, gut.
The development of the kidney from this stage onward has been for some years a disputed question. According to the older conception, still maintained by Golgi and by Minot, the small tubes which branch from the primary renal pelvis become the collecting tubules of the kidney and themselves give off branches which, increasing in length and acquiring tortuosity, become the secreting tubules (proximal and distal convoluted tubules, loops of Henle, etc.). The blind end of each convoluted tubule, becoming dilated and saccular, is invaginated by a tuft of capillary blood-vessels, thus being converted into a capsule of Bowman. The invaginating mass of blood-vessels constitutes a glomerulus, and
Mesodermic tissue.
^KiiMs'MiUu ausi s^i|wulo of Bowman together make up a llH^VA^t^UH sSMl^UiK^Wx Thus the entire system of tubules i^y K I hi I w \\\\ \\w i^lvin aiul the ureter have a common origin lU'iu \\u^ ^A\\^^\\ \'\\\\ \»t* thv Woltlian duct, while the blood\v V I \\\x\ Ks^wwwMW' {\^<\\K\ as well as the capsule, originate u villi ()i. uiuMtiubu^ Huvvnvhymo.
\. V I'lvliu^' lo \\\\^ K^\\wy viow — S»mi>er, Sedgewick, Balfour, »u.l ill. I \\\\\\\ iasUHiiiuhI bv the n»searches of Schreiner, wU- \y nil. li»\*' tnvh v^mtlmuHl for the most jwirt by UnJ». I. »li. « ^uixmIiu^hI uibuKvH and the capsule of Bowman ..ULUint ux.i \ *\U II .u^h.'t v»t* ilivertieula from the primitive \K\\.\\ |ul\i . tiui iiuU^ik^ikU^uIv tViMU the nephrogenic tissue,
and in the folluwiag inanoer : The nephrogenic tissue into
which the kidney evagination peaetrates shows a differentiation into two zones — an mTtefOfif,immediatelysurrounding the
primitive renal jielvisij consisting of epithelioid cells, and an
oiUer zone of less differentiated mesenchyme. Soon after the
appearance of the small diverticula which evaginate from the
primitive renal pelvis and which arc designated the primary
collecting tubules (which lattercorresjxtnd with the " primitive
renal vesicles" of Haycraft), the nephrogenic tissue breaks up
into smaller cell-masses, each snch mass surrounding a primary
collecting tubule (Fig. 123, mk). This part of the nephro
Fig. 1Z3.— Section throuRh the kldn«]r tiT human TeIus <if Eeven monlhs (from Felii. sfterSchreinert: Sr, collt-cllnu tubules of wlilrh three nre ehuwn. each with itfl cap of metauepbTogenlc tiHue, mt; in relation with each is un early UTiniferoua tubule, the three latler. n, b, c, cuch nbovlnif a dlHerent itBgc nf development— a, showing beglnnltiK of eipanslon; b, evaclnatlon at At; r, S-sbapeil aiage, M Indicating development of Bouniana capsule.
genie tissue Schreiner calls the metanephrogenic tiasoe by way of distinction from the remaining part of this mesenchymal aggregation which, from its relation to the development of the mesonephros, is called the meBonephiogenic tissue. Each primary collecting tubule, after becoming bulbous at its end, divides into two tubules, each one of whicli in ttirn divides into two, this process of division Iwing repeated several times. These tubules become the adult straight collectinK taboles. The branching of (be primary collecting tubules continue'^ to the time of birth ; or until the fifth fetal month, according to Hamburger.
In the development of the secreting tubnles the inner zone of nephrogenic or metanephrogenio tissue alone is concerned.
This tissue preseDts little btid-like prulongatioiis, each such little bud of i-ells later acquiring u luiuen and M'jurating from the pureiit tissue (Fig. 12^,1, 6,c}. The buds are now small sacs, the renal veaicleB (Emery), of which there are at least two for each collecting tubule. Each vesicle now elongates and assumes an S-shaped form, the concavity of the up]»er part of the looking toward the collecting tubule, the vesicle on the right of the tnliule being, therefore, a reverse.1 S (Fig. 124). The lower limb of the^ S, from being simply tubular, becomes expanded, its upper wall being indented (Fig. 124, x), no Ihat it acquires the shape of a double- 1 aye red saucer, the space between its two layers being continuone with the remaining part of the lumen of tiie S tube. In the concavity of the saucer, beneath the middle piece
of the S, a strand of mesenchyme makes its appcamnce and into this tissue blooil-veKscls penetrate, hi that it finally becomes the glomemlns of the Malpighian corpuscle. The saucer-shaiied lower limb of the S becomes the capsnl* of Bowmaa; the lumen of the saucer, the apace of fiowm&n. Meanwhilii the upjwr limb of the S acquires continuity with the collecting tubule in close relation with which it has developed, and the two extremes of the S l)eing thus relatively fixed points, the ensuing elongation of the intervening portion neces^ii tales the formation of curvatures. A small part of the h)wer limb of the S, not being concerned in the formation of the saucer-shnjied anlage of Bowman's capsule, Wcomes a jmrl of the proximal convoluted tubule, the remaining portion of the latter, and tlic suc<vt'ding loop of Henle, the distal convoluted tubule anil the arched collecting tubule, being develojied respectively from the succeeding parts of the S. The outer zone of the metanephrogenic tissue gives rise to the capsule of the kidney and the supporting connective tissue, ineludiDg the columns of Bertini.
Iftnm Felix, after Bloofkl; Jir. ■mpulla af cnllvcUoR tubule ; oB. upper limb nf S; hB, lower IJmb of S (Bowmmn's cap(Die); X, pmllinn occupied by glomeru
The kidney acquires its characteristic features by the end of the second month of fetal life, and it reaches its permanent
position by the third month.
The Suprarenal Bodies
The development of these structures has been the subject of much discussion. It has been maintained, on the one hand, that the cortex of the organ develops either directlv or indirectlv from the mesothelium of the body-cavity and that the medulla has its origin in outgrowths from the sympathetic ganglia ; on the other hand, that the entire organ is a product of the mesenchyme — indirectly, therefore, of the mesothelium. Thus Minot, Human Embryology, 1892, remarks, "That both the cortex and the medulla of the adult organ are formed in man from the mesenchymal cells, as Gottschau showed was the case in several mammals, is, I think, beyond question "; but in his Laboratory Text-book of Embryology, 1902, p. 267, the same authority, in describing pig-embryos, says : "The sympathetic tissue gives rise to the so-called medulla of the adult organ." Again, Aichel, 1900, from his investigations concluded that both medulla and cortex arose from the mesenchyme. O. Hertwig, in his Lehrbiich, 1906, expresses his conviction of the correctness of Poll's ^ conclusions as to the double origin of the organ.
The cortex, according to Poll, whose work reaffirms in
many particulars the conclusions of some earlier investigators, arises from small bud-like masses of cells that come
from the mesothelium of the coelom in close relation with the
genital gland and the mesonephros, but distinct from them.
These buds lose their connection with the coelomic epithelium
by the fifteenth day in the chick. They are situated on each
side of the root of the dorsal mesentery and all those of one
side unite to form a single organ. This occurs in man by the twenty-eighth day (Souli6). In the lower vertebrates
this organ fails to unite with the anlage which represents the
medulla of the higher vertebrate suprarenal body and constitutes the separate interrenal organ of the lower vertebrates.
In some cases — e. g., in sharks — the anlages of the two sides
unite with each other, forming a single unpaired interrenal
organ, which lies l)etween the primitive kidneys. Failure of
union of some of the buds, it is believed, is responsible for
the accessory suprarenal organs of Marchand which are occasionally found between the layers of the broad ligament of
the female or in relation with the epididymis of the male>
these having followed the descent of the testis or ovary
respectively.
- H. Poll, Die Entwicklung der Nebennieren Sjrsteme, Handbuch der
vergleich. und experim. Entwicklungslehre d. Wirbeltiere, Bd. III., Abt 1, 1905.
The medulla of the organ, still following Poll's account, originates at a later stage than the cortical anlage from chains of cells that grow forth from the sympathetic ganglia and form groups which for a time retain their connection with the ganglia. The cells show a differentiation into two classes, the sympathoblasts and tiie phjeochromoblasts, the latter gradually becoming the phseochronie cells, so called from their staining darkly by chromium salts. In many vertebrates these cell-masses remain separate from the interrenal anlage and constitute the phseochronie bodies or suprarenal bodies of lower vertebrates. In birds and rej)tiles the union of the phfeochrome and interrenal anlages occurs by a mutual intergrowth of their cells, the result being an irregularly stratified organ ; in mammals and man, however, the cells of the sympathetic anlage gradually {wnetrate in the form of cell-coixls into the interior of the interrenal anlage to occupy their adult position as the medulla of the organ. This process of intergrowth continues in man until the time of birth.
In the early stages of fetal life the suprarenal body is relatively much larger than in the adult condition, and is situated chiefly on the ventral surface of the kidnev. At about the thinl month it Iwgins to assume more nearly its normal position.
The account of the development of the bladder and of the urethra mav be deferred until the evolution of the internal sexual system shall have been considered.
The Development of the Internal Generative Organs
The Indifferent Type. — The internal generative organs of both sexes, in the course of their development, pass through a stage in which there is to be found no distinction of sex. This stage is designated, therefore, the indifferent type of sexual apparatus.
While the Wolffian body is attaining its full development, there appears in its vicinity a tube, the duct of MUUer (Plate VII., Fig. 1), which lies parallel with, and to the outer side of, the Wolffian duct. In non-amniotic vertebrates the duct of Miiller arises by fission or longitudinal division of the mesonephric duct. Its exact mode of origin in the amniotic vertebrates is not as yet definitely settled. According to one view its upper or cephalic portion is produced by an evagination of the mesothelium of the bwly-cavity, while the remaining lower segment results from fission of the mesonephric or Wolffian duct. According to another view the lower or caudal portion is produced by the direct extension of the upper portion in the caudal direction by the proliferation of its own cells. In whatever way the duct may be formed, its lower or caudal end opens into the cloaca, which receptacle receives also the termination of the Wolffian duct. The upper end of the duct maintains a communication with the body-cavity or coelom by means of an expanded funnelshaped mouth. Its lower segment is closely associated with its fellow and with the Wolffian ducts, forming thus the genital cord. The function of this canal in lowly organized animals — that of receiving from the body-cavity the female genital products, the ova, and evacuating them from the body — foreshadows its subsequent metamorphosis in most vertebrates.
While the duct of Mfiller is forming, the mesothelial cells overlying that part of the free surface of the Wolffian body which looks toward the median plane and somewhat forward, its ventro-mesial aspect, undergo multiplication and thickening (Fig. 125, a), forming an elongated swelling or ridge. This is known as the genital ridge, which produces a projection upon the wall of the body-cavity. The genital ridge is still further thickened by the proliferation of
llio niPstKlermid tissue (£') beneath the niosothelial cells. The
genital rulgos ol' the liiimaii fetus apijesir in the fifth week.
Further differentiation of the genital ridge results in ita tninRformatiun into the so-cilUciI indiffeient sexual gl&nd (Plate VII., Fig. 1), a structure common to Iwth sexe.s at this stage. The essential feature of this process is that the thickened raesothelial cells overlying the genital ridge become modified in character and penetrate the ridge in the form of conls or Btrands of cells. These mesothelial e]eioent.s were called by
Fin. 1».— Crou-fcrtlnn Uirnugh
IcrlBn duct, and the leiual gland of vMcV nr i
mrunlnud 100 dliiineMn: m, iDeacDti>ry: L, Mimatopleun
germinal epllheliiim tnim wlilob the Milllprlan dtict (i) hi
Uilckcncd pari of the Rcrminal eplih.lium, iii wlilch the
r the UUlilsy (afttr WaldcyOT), a', [he nglon of the been invaglnated ; a, rlRiary sexual cvlli. C
Waldeyer the genninal epithelium, because, after their extenaion into the interior of the ridge or gland, they give ri.se to the genn-cells, namely, the ova or the spermatozoa as the case may lie. The eelt-cnrda include two kinds of elements, the smaller mesothelial cells and the primitive sexual cells, which latter art! laryir und less numerous than the mesothelial cells
I>ia?ramm«llc reprewnliillnn of the ilcmlopmeiil of [be genllim
WolIBiMi bod; and Its derlvittlTeg belni; colored red, the MUUerlBn
rtTBtlves, green: I. ludHRri-nt type; i ilnHBerent type, Islet Btage,
HfllleTlan ducta and tlic piimillve areter now openlog into ibe iirc^eultal ilnn*
e type, lower ooda of MaUerlMi dnole ftued lu futm Ibe ilniu pocuUirli: ~
B type.
Xom, the
.,»«,. I
nkn and ^M
awl kive hr^ niicK^»lati*tl iiurl<M. Tlir tirlmlHv«i m^^^At
or primitiye ota, tin' mi imiIIihI iM^Hiiiat' H Ihi that thoy ilovolop olllirr inli» \\u' n\ii tit 0\t* 1 filaments accimliii^ tt) \\\v **v\ nl iIh' i'ImI«i\i« t'lu* ceU-cords have boon so<mi in \\\v iiHlillrrtnl iiImmiI i«I tltr hmun embn'o a:!: oarlv as (In* lil'tli wv^U \\ \\s\^ \\\s\\ althoagfa thero are no ^ross st^xiiitl tliiliiiiliitn i ii t>i«iMti"*ti«ii it ia po^iblo to ilotonninr iVnni tlir lii^tolnpitiil i)fn*ti'ii \ M the organ whether it is tn Im* n Itwti^j m tiM«M*n\. \\\* l'M«it> oells bein^ far loss iniinrniiM itliiti\i>l\ in \\\* t^Mni t
se than in the hittor (Nii^rl).
The indiBbront soxnal ^IiiihI iMiinr : imIh 'Hi i-^pn i'lllx .In .• rehitiun with tho n|)|NT or si*\niil •:i'iii> nl tin- nii i^nipln* orWolffianb<Kly (IMato V 1 1., \'"\\\. 1 1. llM«'i|»niMi -Mh . .m \\!\\. U fact will a|>|)oar hit or.
The Male T3rpe of Hnminl FtriHiMM
The teSticlo llil-: IL <ImiiIi|i. Ill ik:iM ini. llii |«tiipti qiii'titiotV part of tho orpin i -. iniiiliniil In dti im ('ihiiii|ilin t - •«! ilio
indifToroiit scxiimI ^'IiiimI. \Jiili ii - :\ dm ni pfYptonl iturU i-i furnisli(;(I liv ilw Wnllli'in Imih Miiifmn h'l > liri-n nciilr
■
of tho O4;ll-r.onl"- and tho lar^^rr and Irrvi ninnrroiiM priinitivo srvnal (*olls. The mesothelial coIIn inrrraso in nninhor and Uooonio so grouped as to form (^ylindrioal masses known as sexual cords, each of which inolndos sonu* of tlu* primitive seminal or sexual cells. By the ingrowth of connective tissue from the surrounding mesoderm, the timica albnginea is formed and the sexual cords are divided into roundish masses, each of whicli is made up of many of the smaller elements and a less number of the large seminal cells. These follicle-like masses become hollowed out to form the seminal ampulla, which afterward, undergoing great increase in length, are transformed into the seminiferous tubules. During fetal life, however, and even to the period of puberty the "tubules" remain solid cords of cells. The small mesothelial or epithelial cells give rise to Sertolli's columns, while the primitive seminal cells produce the spermatogonia.
Spermatogenesis, or the development of the spermatozoa from the cells thatline the seminiferous tubules of the functionating testicle, has been considered in Chapter I.
While the sexual cords are bein": transformed into the cylinders that become the seminiferous tubules, the surrounding mesodermic tissue ix^netrates the genital gland and forms the connective-tissue septa that constitute the stroma of the organ and divide it into lobules. At the same time, also, marked chanj^es occur in the Wolffian bodv. From certain of the Malpighian corpuscles of this structure, cords of cells, the medullary cords, grow forth and penetrate the genital gland, their ends fusing with the primitive seminiferous tubules. The conversion of these cell-cords into tubes furnishes the initial part of the system of excretory ducts of the testicle, namely, the vasa recta and the rete testis. Somewliat later, in the twelfth week, the rete testis is extended to form the vasa efferentia, and still later, in the fourth and fifth months, the efferent vessels lengthen and become tortuous, producing thereby the coni vasculosi or head of the epididymis (Plate YII., Fig. 3; Fig. 126). The upper part of the Wolffian duct develops into a convoluted tube which constitutes the body and tail of the epididymis, while the lower |X)rtion becomes the vas deferens, thus completing the system of canals provided for the escajx* of the spermatozoa. Near the caudal end of the Wolffian duct a little pouch-like evagination grows from its wall and becomes
Fig. 126— Inlemal generative o
EBni of ■ m&le fetua of about fouruc
wiiekB (Waldeyer); (. testicle; t, ej
dldymla: f. Wolffian duct: u>. lowi
jlfflan body; j, guberoac
the seminal vesicle, the lower end of the duct, below the orifice of the semiDal vesiule, being the sjacnlatory duct. Since the "Wolffian duct terminates in the cloaca, and since the anterior part of tbc cloaca corresponds to a portion of the later urethra, the termination of the ejaculatory duct in the prostatic part of the urethra ig accounted for. Thus it will be seen that while the secreting part of the testicle results from the transformation of the indifferent genital gland, the secretorr cells having their origin in the germinal epitheliam, the complicated system
of dnctd with which it is provided is fumbhed by the mesonephros or Wolffian body.
The series of tubules connected with the upper extremity of the Wolffian duct, the remnant of the pnmspbros or headkidney, frequently persists as a little pedunculated sac attached to the upper part of the epididymis ; it is known as the stalked liydatid and sometimes also as the hydatid of MorgagnL The posterior or lower set of Wolffian tubules likewise give rise to an atrophic structure, the paradidymis or organ of Oirald^a, which consists of a series of short tubes closed at both ends, lying among the convolutions of the tail of the adult epididymis (Plate VII., Fig, 3), while a lateral evaginalion from that i>art of the Wolffian duct which forms the tail of the epididymis becomes thevaa abeirans.
The dnct of Uttller remains atrophic, in the male, throughout its entire extent, and in fact, with the exception of its two extremities, it usually altt^ther disappears. Its upper extremity persists as a small vesicle, the nnstalked or sessile hydatid, attached to the upper aspect of the testicle. The lower extremity of the duct, uniting with its fellow, becomes converted into the sinus pocnlarls or ntenu mascnlintiB of the prostate gland (Plate VII., Fig. 3). If the intervening part of the tube persists to post-natal life and remains patulous^ it is known as the duct of Rathke.
the change of location whicii the testicle undergoes is a conspicuous feature of its development. To understand this clearly, it is necessary to recall the relation of the mesonephros and the genital gland to the peritoneum. Since both of these bodies originate from the cells of the outer wall of the body-cavity, or, in other words, from what becomes the parietal peritoneum, necessarily they lie between the body-wall and the parietal peritoneum — that is, behind the peritoneal cavity. With the increase in size of these structures, they project toward the peritoneal cavity, the peritoneum passing over them and forming a "mesentery," which anchors them to the posterior wall of the abdomen. In the case of the testicle, this peritoneal fold or " mesentery " is called the mesorchium ; in the case of the ovary, the mesovarium. It is prolonged upward to the diaphragm as the diaphragmatic ligament of the primitive kidney, and downward to the inguinal region as the inguinal ligament of the ])rimitive kidney (Fig. 120), since this latter organ is the largest constituent of the projecting mass. When the primitive kidney has disiippeared as such, the inguinal ligament nientiontul seems to connet^t the ovarv or testicle with the inguinal region of the abdominal wall.
The inguinal ligament eontiiins between its folds connective ti>«^ue and unstriped muscular fibers. These become the gub«»)iUACulum testis in the male or the round ligament of the uteiHH in tl»e female. As the bodv of the fetus continues to )i}\^\\ while the tissues of the ligament remain stationary or i;ro\N lr.-»-' rapidlv, the testicle is gradually displaced from its pmiiiioM al iho side of the lumbar spine, and by the third uioutii Uiuhr.-x I he false pelvis. In th(» fifth and sixth months it it in roui.u'l willi the anterior abdominal wall, near the iniu'i' iJuloioiiial ring. In th(» eighth month it enters the in>;ninal lanal, and \\\^\v the end of th(> ninth month, shortly bi-fou' Iniih, it K'a\e« the inguinal canal and enters the
- .\i>M «Uv.ivui .>i \\w Uviiis'U>M. \^llli (H>nstM)Ucnt emptiness and flabbinesH
Before the testicle leaves the abdominal cavity, the parietal peritoneum pouches through the inguinal canal into the scrotum, the protruded part being the processus vaginalis. Since the testicle is from the first behind the parietal peritoneum, it passes into the scrotum behind the vaginal process, the latter then folding around it as a shut sac of two layers. Subsequently the connection of the sac, now the tunica vaginaJis testis, with the abdominal peritoneum is reduced to a slender strand of tissue lying in front of the spermatic cord.^
The testicle necessarily carries witli it, in its descent, its blood-vessels, the spermatic artery and vein ; its duct, the vas deferens ; as well as its nerves and lymphatic vessels ; and these structures collectively constitute the spermatic cord.
The Female Type of Sexual System. — While the indifferent sexual gland, in the development of the male generative system, undergoes metamorphosis into the testicle, it becomes, in the evolution of the female type, so altered as to constitute the ovary ; and while the Wolffian tubules and the Wolffian body become in the male the system of excretory ducts of the testicle, they produce in the female merely atrophic structures. On the other hand, the duct of Miiller, which gives rise in the male, to atrophic appendages, forms in the female type 'the Fallopian tube and, by fusing with its fellow of the opposite side, the uterus and the vagina.
The ovaiy results from alterations in the structure of the genital gland analogous to those that occur in the evolution of the testicle. The special features of these changes are better understood, however, than arc those of the testicle. As in the case of the development of the testicle, the mesothelial cells on the peritoneal surface of the genital ridge become thickened, these altered cells constituting the germinal
of the scrotum, is designated cryptorchism (hidden testes). The presence of but one testicle in the scrotum is called monorchism,
- Occasionally it happens that the funicular process of the tunica vaginalis — that is, the stalk of the sac, remains patulous throughout its entire
extent, a condition which allows of the easy and sudden protrusion of a segment of the bowel into the cavity of the tunica vaginalis, constituting the so-called congenital hernia. Or the funicular process may close only at one or the other end, givitig rise to other varieties of hernia.
epithelium (Fig. 125). Coiuciden tally, (lie primitive connective tissue — iiiesodermic tii^ue — u ml e Hying the germinal epitheliiim pmliferates, contributing to the thickness of the genital ridge. By the sixth or seventh ^vcek, the germinal epithelium consi!^t»of several strata of cells, gruu{>3 of which begin to penetrate the umierlying niesodermio tissue in the form of cordlike procesBSB (Fig, 128, f, sch). The indifferent mcsodermic tissue at the same time increases in quantity, in turn penetrating between the groiijis of advancing cells, so that what takes place might be described as a mutual intergriiwth. The presence of the growing connective tissue acceniuatfis the grouping of the cells into cylindrical masses. These latter are the sexual cords or egg-colunma (Pfluger's egfi-tulie,-). They ccntyin twit special kinds of cells, the large sexual cells or primitive ova (Fig. 128, xie), and the smaller hnt more numenins mesothelial cells. The connection of the sexual cords with the germinal epithelium is much more obvious in this case than in the case of the developing testicle, and the primitive sexual cells are much more abundant. The e^-colnmns, surrounded by young connective tissue, constitute the nucleus of the cortical part of the future ovary. This mass is later sharply marked off from the free or peritoneal aspect of the gland, the region of the germinal epithelium, by a zone of proliferating mesodermic cells which become the tunica albuginea of the ovar^'. An important change now takes place in the egg-columns ; the primitive ova, or large sexual cells, increase in size, their nuclei l>ecoming espi-cially well ilevelojKH], while the small
Fig. 127.— Internal or^ni of ft remale Ibtiuof sbtiut fourtti'ii werks (WnMeyer): 0, ov»ry; *, epoSphoron or ptrovnriiim : u-.WulfflKD duct: m. MUllerlftD duct; v. iQwcr put uf (he Wulfflaii body.
mesothelial cells become smaller and less conspicuous. It frequently happens that several of the large cells fuse into a single mass of protoplasm, while one of the nuclei outstrips the others in growth and, with the surrounding zone of protoplasm, becomes the ovtun. Each egg-column is now broken up into several groups of cells by the penetration of connective tissue, each group (Fig. 128, c, «;/(') containing a single ovum, but many of the smaller cells.
Fig. 128— PBrt of safflltal aeclfon of ati iivarx of a ehlld jujt born (after W«ldeyor). Hlfdily matniifled: te, Berminal epilhi-llum ; t. acft. PflQger's egg-lubes;
ue, primitive ova iylng on the germinal epilhclium ; e, kK, long PflOgert tubes. In
process of being coiiverled Into folll«les ; el, b. L-gK-balla (nests), likewise in process
of being resolved Into follicles: /, youngest follicle already Isolated: gu. bloodveBsels. In the tubes and egg-nests the primordial eggs are dlstingofshable from
tbe smaller epithelial cells, the future follicular epltheUum.
These groups are the young Qraafian folllclas of the ovary (/). The enveloping zone of connective tissue becomes the tbeca of the fotliote, while the single large cell constitutes the omm, and the smaller cells are the membrana granulosa. At first the graniitosa cells surround the ovum as a single layer of flattened cells which gradually assume the columnar type and become so numerous as to form many layers. They secrete a fluid, the liquor folliciili, which crowds the ovum to one side of the follicle where it is enveloped by a special group of graniilosa-cells, the discns proUgenu (Fig. 129).
The question of the origin of the lolliouhir cells ia still an unsettled one, though it seenib probable, thut the> are derived from the cells of the egg <. Itimiis jnd Minot believes that thej are probabjv ikscendt i from the prmiitue o\a.
Till formation of new Qraaflan follicles iiid consequently of o\!i, begins in the decjitr pirt of thi o\ar\ and advances toward tlic iiurface TIk production ot o\a and follicles is
Fin. tiB,— Section of human OTary. including cortex . a, geriolnal epiltiellum
or ft^e aurdicv: b, lunica Bltjuglnea; c, peripheral aironm canUlnlng loimBluTe
Gnuflnii follicles Cf I -. c. WHtl-advonced fullicle IWim whcsc wall memljninil granaloBii hu partialis Mimrsleil :/. cavity of liquor folUcull: g, ovum surruunded by
cell-miut coiistliullng illaciii proligenis (Pis reol).
limited to the fetal stage and to the early part of post-natal life, their formation not occurring, according to Waldeyer, after the second year.
What has been said above refers to the development of the cortex of the ovary. The medulla is produced by the growth toward the egg-columns of cord-like processes, the medollaiy cords, fi^m the epithelial walls of the Miilpighian corpuscles of the primitive kidney or Wolffian body, the cords l)e('oniing surrounded by connective tissue and forming a network. The fetal medullary cords are reprcsentwl in botli the cortex and the medulla of the mature ovary by the groups of interstitial cells di'-]>o«ed between the bundles of the stroma-tissue.
The Oviducts, the Uterus, the Vagina
The system of passage-ways that constitute the outlets for the ova and the means of nourishing them and evacuating the product of gestation from the body in the event of impregnation — namely, the Fallopian tubes, the uterus, and the vagina — result from the metamorphosis of the ducts of Miiller. These ducts, as stated above, lie along the dorsal aspect of the body -cavity, separated from it by the parietal peritoneum, and parallel with the primitive spinal column (Plate VII.). The probable method of their formation has been pointed out (p. 243). Near the lower (caudal) end of the body they approach each other, and finally unite about the second month to form a single duct for the rest of their extent (Plate VII., Fig. 4). The upper, ununited parts of the ducts become the Fallopian tubes or oviducts, while the lower portions, now fused into one, become the uterus and the vagina. The upper end of each single duct expands trumpet-like to form the fimbriated extremity of the Fallopian tube.
Until the fifth month there is no distinction between the vagina and the uterus, the two being represented by a single sac-like structure. The development of a circular ridge in the wall of the sac marks the division between the two organs, the part above the ridge acquiring thick muscular walls, while the part below it, the future vagina, remains thin-walled and more capacious. In the third month the uterus is bifid at its upper extremity, a condition which is permanent in some animals and occasionally in the human subject.^
The Wolffian duct, which, in the male, becomes metamorphosed into a part of the epididymis and the vas deferens, remains undeveloped in the female, producing merely atrophic or vestigial structures (Plate VII., Fig. 4). The upper series of Wolflian tubules, the remnant of the pronephros, frequently
' The formation of the uterus and of the vagina by the coalescence of two parallel tubes affords an explanation of the uterus bicomis or bifid uterus and of the condition of double uterus sometimes met with, as also of the presence of a median septum in the vagina^ since by the failure of union of the two tubes in greater or less degree one or other of these anomalies would result persists, as in the male, in the form of a small pedunculated sac, the stalked hydatid or hydatid of Morgagni. When present, it is to be found in the broad ligament, in the neighborhood of the outer extremity of the ovary. The middle or sexnal series of the Wolffian tubules with the adjacent part of the Wolffian duct, which, in the male type, develop into the epididymis, become in the female, an atrophic structure known as the epodphoron or parovarium, or organ of Bosenmiiller (Fig. 127). This structure, which is almost constantly found between the layers of the broad ligament in close proximity to the ovary, consists of a larger horizontal tube representing a segment of the Wolffian duct, and of shorter vertical tubes joining this at a right angle and representing the transverse Wolffian tubules. The lower set of small Wolffian tubules, those which, in the male become the paradidymis, give rise in this case to a similar atrophic body, the parodphoron. This is also situated in the broad ligament, usually to the inner side of the ovary. The Wolffian duct, with the exception of that portion of it that assists in the formation of the parovarium, usually entirely disappears. Occasionally, however, it persists as a small canal traversing the broad ligament close to the uterus and passing on the dorsal side of the upper part of the vagina to be lost upon the wall of the latter or, more rarely, to open near the urinary meatus. When thus persistent, it is known as the duct of Gartner.
The change of position of the ovaries is similar to, though less marked than, that of the testes. The inguinal ligament in the female (Plate VII.) extends from the primitive position of the ovaries in the lumbar region of the abdominal cavity to the groin, where it pas>5es through the abdominal wall, traversing the inguinal canal, to terminate in the labium majus. The upper part of this ligament, coiitiiining involuntary muscular substance, firmly unites with the ovary. In the third month the ovary descends to the lower part of the abdominal cavity and is now connected, by the succeeding portion of the inguinal ligament, with the uterus. This connection may be a factor in the fiiial change of position of the ovary — that is, its descent into the true pelvis. The part of the inguinal ligament that passes from the ovary to the uterus is the permanent ligament of the ovary, ivhile the remaining portion, which passes from the uterus through the inguinal canal to the labium majus of the vulva, is the round ligament of the uterus. As the inguinal ligament perforates the abdominal wall, a small diverticulum of peritoneum goes with it. Normally this peritoneal pouch subsequently becomes obliterated. Occasionally, however, it persists and then constitutes the canal of Nuck. Should the canal of Nuck be present, the ovary may pass into or through it, thus reaching the labium majus. A patulous canal of Nuck, as in the case of a patulous funicular process of the tunica vaginalis of the male, may j)ermit the sudden occurrence of an inguinal hernia in the female. The mesovarium or " mesentery of the ovary accompanies the ovary in its descent and constitutes a fold of peritoneum which envelops not only the ovary but also the adjacent part of the duct of Miiller and the remnants of the Wolffian body. Upon the uniting of the lower parts of the Miillerian ducts to form the uterus and the vagina these mesovaria unite with each other mesially to become the broad ligament of the uterus. Thus it comes about that the uterus, the ovaries and their ligaments, the epoophoron, and other fetal remnants are contained between the layers of the broad ligament.
The account of the development of the external genital organs will be deferred until after the consideration of the formation of the urinary bladder and of that part of the urethra that originates from the same embryonic structure.
The Bladder and the Prostate Gland
As stated in Chapter V., the urinary bladder and a part of the urethra are derived from the intra-embryonic portion of the allantois. In the same chapter the allantois was described as a sac which develo|3ed as a pouching-out of the ventral wall of the gut-tract near its caudal end (Plate II., 5 and 6). The sac protrudes from the still widely open abdominal cavity, enters the extra-embryonic pAit of the bodv-cavit*-, and reaches the inner >arface nf the fidse amnion, with which structure it intimatelv unites to form the true chorion (Plate III.L A? tht- «-all> of the abd<4DeD gradually Q\i^\ leaving only the umbilical aperture, it is, neceRsarilyy through tlii< aperture that the allantois prxH trudes.
We have seen p. 91) what become? of the extra-abdominal part of the allantoic — in what dejrri'C* it contributes to
the formation of the placenta and of the umbilical cord.
Obviously, with the j^evering of the umbilical cord after
birthy all this extra-embiyonic ]Kirt of the allantois disappearsy giving rise to no adult organ.
Its intra-enibiyonic ]K>rtion consists of a tube extending
from the caudal end of the intestine to the umbilicus (Plate
II., 5 and Gj. As early as the second month, the middle
segment of this tube dilates and assumes the form of a spindleslia(KHl sac, which becomes the minary Uadder (Plate VII.).
The part of the tube connecting the summit of this sac with
the umbilicus remains small, gradually loses its limien, and
constitutes in the adult the (usually) impervious cord known
as the urachns. Should the cavity of the urachus persist in
its entirety, and should there l>e at the same time an external
o{>eniug at the umbilicus, the condition would constitute an
imibilical minary fistula. The proximal {>art of the allantois
— that is, the i)orti*)n intervening l>etween the bladder and
the intestine — is designat(f<l the sinns nrogenitalis, while the
caudal end of the intestine, which is, in (fffi'ct, a jH>nch in
which lH)th the allantois and the intestine terminate, is known
as the cloaca (Fig. 96). The urogenital sinus receives the
terminations of both the Mullerian and the Wolffian ducts
(Plate VII.).
In the sixth week or slightly earli<»r, there api)ears uix)n the
surface of th(i IxhIv, in the region corres|M Hiding to the iK>siti(m
of th<» <'loaca, a (l(>pr(»ssion, the cloacal depression (Fig. 96),
which lat<T, except in man and the higher mammals, meets
the cloaca, and thus establishes a communication between it
and the exterior. In the Amphibia, in reptiles, and in birds, as also in the lowest mammals, the monotremes, the cloaca
is a permanent structure, and through it, in these groups of
animals, not only the fecal matters and the urine, but also
the genital products, the spermatozoa and the ova, are evacuated from the body. In all mammals, however, with the
exception of the monotremes, the cloaca undergoes division
into a posterior part or anal canal and an anterior urogenital
aperture. This division is brought about by the growth of
three ridges or folds, of which one springs from each side
of the cloaca and one from the point of union of the urogenital sinus and the intestine. These folds coalesce about
the eighth * week to form a complete septum, which continues
to thicken antero-posteriorly up to the time of birth and
constitutes the perineum.
It will be remembered that the ureters originally spring from the terminal parts of the Wolffian or mesonephric ducts (Fig. 118), Owing to alterations brought about by processes of unequal growth, the orifices of the ureters subsequently change their position so as to open into the urogenital sinus (Fig. 121), and still later, by the further operation of the same agency, they come to open into the bladder on its dorsal wall, thus gradually assuming their permanent relations (PL VII.). After the division of the cloaca the urogenital sinus, as stated above, opens independently upon the surface of the body. In the female it is transformed into a short tube, the urethra, and an expanded terminal recess or fossa, the vestibule of the vulva (PI. VII.). In the male it becomes the first or prostatic part of the urethra.
In the twelfth or thirteenth week, the future prostatic urethra acquires very thick muscular walls, and the original epithelial tube pouches out into the muscular tissue in the form of little sacs, the lining cells of which assume the characters of secreting epithelium. In this way is produced the aggregation of muscular and glandular tissue known as the prostate gland. This is a well-developed structure by the fourth or fifth month (Tourneux). The recess in the floor of this part of the urethra, the sinus pocularis or uterus mas
- Fourteenth week, according to Miuot
enlintu, Iia« l>een previously referred to as the homolc^e of
the atcrus, being the persirtent caudal extremities of the
ducts of Miiller (Plate VII.).
THE EXTERNAL ORGANS OF QENERATION.
In the early stages of the development of the external genital oi^ns no ^xual distinotions are apparent.
Reference has been made to the eloacal depression as a superficial fossa which makes its appearance at the caudal end of the body of the embryo iu the sixth week (Fig. 96). At
Fig. 1:10.— Pi:ur iiut'i:vulvi' lUufoa titAe\e\n\iiaenltit tliv i;!!!'^!!!! gciiical organr
(IndlR^rcnl typcl of Iho hurnun felui iif Sito M mm. {0.ie> to I J5 inuhl ITourneux) :
1, MEnlUl etnlneucG or lubcrck; 1, eliin<: 3. geoilal groorc; 4, gcnllal ridge; 5i
olilMBl ilupTWlxD : fi. cncr^Rcil emlaeace.
about the same period an ciieireling elevation, the genital ridge (Fig. 130, -"1,4), is seen to surround this depression. Within the gciiitul ridge, at the anterior part of the cloncal fbcsa, a email tubercle, the genital eminence, appears at the same time. On the under asjH'ct uf the tjonilal eminence there is soon distiiiguishftlile tin; genital groove (Fig. 130,3), which apjieare as if a continuation of the fis,sure-Iikc cloacsil dcpreession iji)^
and the groove very shortly becomes flanked by two ridges, the genital folds, one on each side.
The genital eminence becomes the penis or the clitoris^ according to the sex of the fetus. It very early acquires a knob-like extremity (2) which is the beginning of the glans penis or of the glans clitoridis, as the case may be. Further development of the glans is brought about by the appearance of a partially encircling gr(X)ve which serves to differentiate it from the body of the organ.
At this stage of development, the rudimentary organs, as described above, are precisely alike in the two sexes. Early in the third month — about the ninth week — sexual distinctions begin to become manifest. Since the female organs exhibit the less degree of deviation from the early indifferent form, they will be first considered.
The External Genital Organs of the Female
The sexually indifferent genital eminence which, as we have seen, presents even by the end of the second month a rudimentary glans and an indicaton of a prepuce, elongates somewhat and becomes the clitoris. The genital folds bounding the genital groove on the under surface of the genital eminence (Figs. 130 and 131, A) never unite with each other as they do in the male, but become prolonged in the direction of the future anus and constitute, by the fourth month, the lateral boundaries of the orifice of the urogenital sinus, or, in other words, of the vestibule of the vagina (Fig. 131, 3). These folds, continuous over the dorsum of the clitoris with its rudimentary prepuce, are the nympha or labia minora (Fig. 131, 2), 7) of the fullyformed state. The masses of erectile tissue in close relation with each labium minus, the pars intermedialis and the bnlbns vestibuli, are the homologues respectively of a lateral half of the male corpus spongiosum and its bulb. The genital ridge, which, from the first, encircles the genital eminence and the cloacal depression, and, consequently, the later clitoris and the aperture of the sinus urogonitalis, increases greatly in thickness. The part of it situated on the ventral side of the clitoris becomes the mons veneris, while the lateral parts of the ridge become the labia majora of the vulva. The several jiarls of the female g«niJ talia develop lo such a degree durhig the fourth month tlmn their aexual characters at this time are well marked.
Fig, ISI.— FouKUCcesalvt aUR*Tiiprilovlip|.iiii 'i' .r im. ■ m. iim! jii-uital
Of the human remsle fisliis {Tuuruvuxi^ 1, clHi.tiK. •;, tluii.s clllDriilli; \
genlMlflnun:: 4, labia majors: S. anui; 0, rwc jgeHl emlncDPC : 7. Uljlami
The reader is again reminded that in the stage when the 1 cloaca is present, the Miilleriaii ducts terminate in the siniia | I un^nitalis (Plate YII.). As previously stated, the stnus u gsnitalia becomes the female urethra, ita terminal portion expanding into the vestlbulum vagins. The openings of the Miillerian ducts fall widiin tliis latter veatihular region of the sinus. the lower purtidu of the two ducts by this time, however, have fused to form the uf«ru3 and the vagina, and the glans penis, wliilc the integuinentary fold that ])artially encircles the latter assumes more distinctive character as the prepuce. This fold gradually advances over the glans and adheres to it, the adhesion j)ersisting until, or shortly after, birth. All of the rudimentary penis, exclusive of the glans and of the genital folds, becomes the corpora cavernosa of the adult organ. The characteristic structure of the corpora cavernosa is forcshadowcKl as early as the third month by the appearance in the ])enis of capillary blood-vessels, which, in the sixth month, undergo marked dilatation.
The groove on the under surface of the penis becomes dee{>er, and the genital folds, which bound the groove laterally, increase in size. This groove extends from the orifice of the urogenital sinus to the glans penis. The genital folds, which in the female, remain distinct and l>ecome the nymphie, unite with each other in the male and convert the groove into a canal, which latter is practically an extension of the urogenital sinus along the entire length of the j)enis to the glans. The e^uial thus formcnl is the anterior part of the male urethra, or, in other words, it includes all of the urethra except its prostatic i)ortion, whicli represents the urogenital sinus. The orifice of this newlv-formtHl canal, situated in the glans, is the meatus urinarius. Failure of union of the genital folds, either wholly or in part, results in total or in partial deficiency of the floor of the urethra, tliis anomaly being known as hypospadias. If the defective closure involves only the glans, the condition is denominated glandular hypospadias.
The genital folds form not only the si<les and the floor of the penile urethra, but by an extension of their growth, also its roof, thus completely surrounding it. Upcm the acquisition, by the now united genital folds, of bhwKl-vessels and cavernous spa(!es, they become the corpus spongiosum of the j)enis, and thus is established the p<?nnanent or adult relation of these partes.
The genital ridge be(^omes differentiated into two prominent folds or pouches placed one on either side of the root of the penis. In the fourth month these unite to form the scrotum, the line of union being indicated by the raphe. Pailure of union of the two halves of the scrotum is one of the features of certain forms of so-called hermapliroditism.
The glands of Cowper, which correspond to the glands of Bartholin of the female, are developed, like the latter, as evaginations of the terminal part of the urogenital sinus.
The accompanying tabulation exhibits a comparison of the organs of the two sexes on the basis of their common origin. Male and female parts that develop from the same fetal structure are said to be homologous with each other.
Homologies op the Sexual System.
Fetal Structure. Female Organs. Male Organs.
Indifferent sexual gland. Ovary. Testis.
Wolffian body — Its middle series of tu- Short tubules of par- Vasaefierentia^rete testis
bules and ovarium and and coni yasculosi.
Corresponding part of Horizontal or long tube Tube of epididymis.
Wolffian duct. of parovarium.
Keinainder of Wolffian Usually altogether dis- Yas deferens, seminal
duct. appears ; if persistent, vesicle, and ejacula Gartner's duct tory duct
Upper serin of short Stalked hydatid of Mor- Stalked hydatid of Mor tubules (pronephros). * gagni. g^^i Lovoer series of tubules. Paroophoron. Paradidymis (org^an of
Girald^).
Duct of Miiller — Its upper extremity. Fimbria of oviduct. Sessile hydatid.
Succeeding portion. Oviduct Usually disappears ; if
persistent, duct of Bathke. Remaining portion, by Uterus and vagina. Uterus masculinua.
fusion with its fellow.
External Organs.
Fetal Structure. Female Organs. Male Organs.
Genital eminence. Clitoris. Penis.
Genital folds. Nymphae and bulbi ves- Corpus spongiosum, en tibiili. closing spongy part of
urethra. Genital ridge. Labia majora. Scrotum.
Urogenital sinus. Urethra and vestibule. Prostatic urethra, membranous urethra, prostate, Cowper's glands.
Glands of Bartholin.
Summary
1 . Th(» male and the female internal generatiye organs, as well as the kidney and the ureter, originate from the mesothelial Hning of the b(Kly-cavity, being directly produced from the Wolffian IxhIv and the duct of Miiller.
2. The bladder and the female urethra, but in the male only th(» prostatic urethra, result from the metamorphosis of the intrn-ombryonic jmrt of the allantois, and are therefore to be repirded as of entodermic origin.
3. Ikfore the estiiblishment of the j)ermanent kidney, a temponirily functionating organ, the mesonephros, performs the oflice of a kidney during a jxirt of fetal life, and this latter is preceded i)y the pronephros, an organ which, though represented in the higher vertebrates by a vestigial remnant^ is functionally active only in larval Amphibia and in bony fislu's.
4 The Pronephros. — The mesothelial cells of the outer or pari(>tal wall of the body-cavity become invaginated in a line parallel with the axis of the body, and the cord of cells thus Ibrined bectnnes hollowed out to constitute the pronephric or s('j;;iiiciitnl du<'t. At several points this duct retains its connection with the surface-cells by nieiins of cell-cords, which latliT beeonie tubes and acquire glomeruli. The long tube anid the shorter tubules with their glomeruli constitute the pronephros.
5. The Mesonephros. — The transverse segmentation of the middle plate, whi<'h connects the ]>araxial mesoderm Avith tlit^ parietal plate, results in the formation o( a series of ccll-mass<'s, llu' nephrotomes. Each nephrotome becomes a tube and a<M|uircs one or more glomeruli. The decjwr eniU of the tubes bcc<)me c<»imected with the pronephric
- ir M«n;iiicntal dnct, which latter is known henceforth as
the nicsonephric or Wolflian duct. These tubes, with the atljacent part of the Wolffian duct, constitute the mesonephros, which fnnctii»nat<'s, for a time, even in the human fetus, as an organ of urinary ex<»retion. The entire AVolffian duct, with the pronephric tubules and th(» mesonephric tubules, eonstitntes the Woltlian body. The AVolffian duct ojK»ns at its lower or caudal extremitv into the cloaca.
6. The metanephros or pennanent kidney develops in part
from a small diverticulum that pouches out from the lower
or caudal end of the Wolffian duct. The straight collecting
tubules and the pelvis of the kidnev correspond to the
dilated and subdivided fundus of this diverticulum, while
the ureter represents its stalk. The secretory tubules of the
kidney develop from the inner zone of the metanephrogenic
tissue. The surrounding mesodermic tissue furnishes all the
component elements of the ureter-walls and of the kidney
except the epithelial parts, as noted above.
7. The supraxenal bodies probably are derived, in part, from epithelial outgrowths which proceed from the mesonephros to form the cortical part of the organ, and, in part, from chains of small cells that bud forth from the embryonic sympathetic ganglia to form its medulla. The surrounding mesodermic tissue contributes the connective-tissue parts of the suprarenal body.
8. The sexual apparatus in its earlier stages presents no distinctions of sex. The elements of this earlv indifferent type are the indifferent genital gland, the Wolffian duct, and the duct of Miiller.
9. The indifferent genital gland originates in the mesothelium of the body-cavity. The mesothelial cells overlying the ventromesial aspect of the Wolffian body undergo multiplication in the fifth week and thereby produce an elongated elevation, the genital ridge. Further multiplication of its cells and the addition of other elements bring about the transformation of this ridge into the well-defined genital gland, which now lies in close relation with the Wolffian tubules. The mesothelial cells are the "germinal epithelium of AValdeyer, the cells that produce the ova or the spermatozoa, according to the future sex.
10. The duct of Miiller makes its appearance soon after the AVolffian duct. It lies parallel with and to the outer side of the Wolffian duct and also terminates in the cloaca. It is of mesodermic origin, being produced either by evagination of the mesothelial cells of the body-cavity, or by a splitting off from the Wolffian duct.
11. The generative systems of both sexes result from the
metamorphosis of the three structures making up the early
indifferent sexual apparatus — namely, the indifiei'ent sexual
gland, the Wolffian body, and the duct of MuUer.
12. The male sexual system is ])roduced by the transformation of the indifferent gland into the testicle, and the conversion of the Wolffian tubules and the Wolffian duct into the system of excretory ducts for that gland, the short tubules becoming the va.sa effcrc^ntia and coni vasculosi, while the Wolffian duct itself furnishes the body and the globus minor of the epididymis, the vas deferens, the vesicula seminalis, and the ejaculatory duct. The duct of Muller remains undeveloped and is represented in the adult by the atrophic sessile hydatid and the uterus masculiuus.
13. The female sexual apparatus is brought about by the development of the indifferent gland into the ovary, and by the metamorphosis of the upper segments of the ducts of Muller into the Fallopian tubes, and the fusion of the remaining portions of the two ducts to form the uterus and the vagina. The Wolffian duct and tubules give rise to atrophic structures in the female, the most conspicuous of which is the parovarium or ej)oophoron.
14. Both the male and the female external genitalia are developed from fetal structures conmion to the two sexes, the genital eminence, the genital ridge, and the genital folds. The genital eminence is situated at the anterior or ventral part of the eloaeal depression. The genital ridge is an elevation surrounding this pit and the genital eminence, while the genital folds are on the under surface of the genital eminence, on(» on each side of a longitudinal groove.
15. The Wolffian ducts and the ducts of Muller open into the cloaca, but when that a|>erture becomes differentiated into the anus and the urogenital sinus, as it does at the fourteenth week, these ducts fall to the latter apartment. The orifice of the urogenital sinus being at the base of the genital eminence, the sinus comes into continuity with th(» groove on the under surface of the eminence.
16. The female external genitalia are produced by the further development of the three structures mentioned above.
The genital eminence becomes the clitoris. The genital folds
on the under surface of the clitoris become somewhat prolonged to constitute the labia minora. The genital ridge
becomes, anteriorly, the mons veneris and laterally the labia
majora. The orifice of the urogenital sinus is represented by
the vestibule, and since the Miillerian ducts near their termination in the urogenital sinus fuse to form the vagina, the
latter passage opens in the adult into the vestibule. Since,
also, the urogenital sinus receives the termination of the
allantois, which becomes the female urethra, the latter canal
likewise opens into the adult vestibule.
17. The male external genitals represent a further development of the embryonic genital eminence, genital folds, and genital ridge than do the female organs. The genital eminence becomes the penis, the genital folds, uniting with each other so as to surround the groove, producing the corpus spongiosum. The groove itself, being thus converted into a canal which extends the now closed urogenital sinus to the end of the penis, constitutes all of the male urethra except the first or prostatic portion. The prostatic urethra represents the proximal extremity of the allantois. Since the Wolffian ducts open into the urogenital sinus after the division of the cloaca, the terminations of those ducts, represented now by the ejaculatory ducts, open into the prostatic urethra ; and since the Miillerian ducts also open into the urogenital sinus, the uterus masculinus, which is the representative in the male of the terminal parts of the Miillerian ducts, is found likewise in the prostatic urethra. The lateral parts of the genital ridge, which, in the female, become the labia majora, fuse with each other in the male to form the scrotum.
18. The condition of so-called hermaphroditism may be produced either by an unusual degree of development of the female external genitals, resulting in a clitoris resembling a penis and in labia majora which simulate a cleft scrotum ; or by the arrested development of male organs, whereby the genital folds and the genital ridges fail to unite, the urethra in consequence opening at the base of the penis.
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