Talk:Book - Human Embryology for Medical Students (1951)

Human Embryology For Medical Students © Popular Book Depot Bombay 1

first Publnshed 1950 Revi5Ed 1958

{§Enteg53Z§C?1Srdep:‘?:?§-I, Aéutx:§A!;°?Irl:1Dpl;)E1Ils2Li(Sh¢E,$ Y G 1; Bhaékal For The Popular Book Depot, Lammgmn Rang/Boml3Iy To The Memory Of Professor James Couper Brash Of Edxnburgh Preface

In this edition many aspects of the subiect which were concise have been revised and made elaborate

Since much attention has been paid of late to the diagnostic methods and surgical treatment of congenital heart diseases, the chapter on the development of the heart has been re~ written and an attempt made to co relate the congenital anomalies of the heart with clinical findings The chapter on the development of the urogenital system has been revised to explain the causes of various developmental anomalies Certain fundamental aspects of human genetics including the significance of blood groups have also been added as knowledge of same ‘IS considered desirable for understanding the problems of health and disease

Recent clinical and academic interest shown in the study of “Sex Chromatin” and its presence in the female cells is an important step in the knowledge of nuclear sexing Cytological determination of sex has proved a valuable aid in the diagnosis and treatment of intersexes and some related conditions A \irie'i account at the identification of “Sex E‘nrornatin“ is given as an appendix

The book has been rewritten with a view to make the subject correlated with clinical ohsenations thereby creating interest in the study at Embn elegy not only during the pre clinical )ears, but also in relation to the study of medicine in general If this ohiect is achieved, the book will have fulfilled its purpose

I am greatly indebted to my frienfi and colleagues in India and abmzid, especially to the late Professor James Couper Brash, university of Edinburgh, for going through the original manuscript and making helpful suggestions Professor Brash had been a friend and guide to me and I consider it a privilege to dedicate this book to his memory

SR NAIR FOREWORD

There is no longer any need to Justify the inclusion of Embryology into the medical curriculum E-nbryology pro vides not only an essential bridge between comparative and human Anatomy, Phylogenesis and Ontogenesis, but also enables the student to approach the man} apparently hap— hazard structural features of our body in a logical and orderly manner

While the earlier edition of Dr Nair's boo}. catered nhieﬂy for the essential needs of medical students the scope of the present edition is more ambitious Though still bearing in mind the needs of the undergraduate, it also oflers help to the post graduate student by giving within the limits of a book of its size the fullest possible description of many varied developmental abnormalities In many instances there is also a detailed discussion of the essential clinical methods necessary for the diagnosis of such anomalies at the bedside and thus putting the subsequent treatment on a firm and logical basis ln fact this appears to be the first serious attempt by a teacher of long experience to provide a book one could iustifiably call

The Essentials 0} Clinical Em‘oryo‘iogy“

Much effort and industry has been spent in bringing the present edition thoroughly up to date and in this respect the author has succeeded well Happily no mo Embryologists viill ever agree as to the exact requirements of an ideal text book on their subiect, and Dr Naii"s book is no exception from this rule In spite of this Dr Nair has produced a book which succeeds in fulfilling the aims set out by its author The book is concise, well informed, suﬂlciently controversial to have its own character and I am wnxinced it will gain well deserved popularity amongst students in many parts of the English speaking world

PAUL BACSICH M D . D so. r n s i:

Reader in Fxnbryolog) Uniuzrsity of Glasgoii M coN1'EN1s

CHAPTER V

THE EXTERNAL FORM OF THE EMBBYO Page Tm: Pnmmnx. D:vzx.o1=-xmm 40 Mrrnons Or Mzasukma Emmvos 41 Gxow-m Ox: Tm: Enuaxwo 41

CHAPTER VI DEVELOPMENT OF THE CIRCULATORY SYSTEM

Tm: HEART 49 Tm: Rmm Am: Lzrr Ann 55 Tm: Vmmcm 59 Tm: A‘l'RIO-VENTRICULAR Vanvss 62

THE S!-ZMJLUNAR VALVES Or Tm: Aom-A Arm Tm: Puuxommv Ax-rzmr 63 Dmtcrs Or Dmmwemmzr 05* Tu: Hum “ 64 Cross: 0'.‘ Cm<<‘.1=x<nAx. Max.-.-omunons 0? TH. Hum '23 Tu: Am-zmzs 76 Tm: Vans 88 D1-zrzcrs OF DEVELOPMENT 01-‘ THE Ewen VISSEIS 101 Tm: LYMPHATIC SYSTEM 102 Tm: SPLEEN 103 Haznxopotms 103 Bwon GROUPS 104 Emm-moamsrusxs Fomnus 106 TH: Fa:-mu. Cmcuunow 107 CHANG}: Iv Tm: Cmcuun-new An-an Burm 107

CHAPTER VII DEVELOPMENT OF THE ALIMENTARY SYSTEM

Tm; FACE 110 Tm: PALAT: 110

DEFECTS Or Dsvsmrmnrr 05‘ TH: Fae}: JAWS AND PALATE 117 THE TONGUE 119 T1-12: TE1=.'rH \ 122 Du-vzcrxv: Dsvsmnsnrr Or Tm‘. Tu-ru 12§ Tm: T1-nrnom Gum: 12' Tmz PAHATHYROIDS 129 Tm: 'I'mn\ms

THE Toxvsus 32 D1-:vz1.ow.xm1;u. Dznurrs Or Tm: P1-ummcaAL R5610‘! 1

Tm: Onsopxucus 133 CON'l'ﬂ‘('l'S xin

Page

Tu: Sromucﬂ 133 Tm: Snuvzmv GLAND: 135 TH: Duomzmm 135 TH: Lwm 135 TH: Pmcnus 139 TH: Snmu. In-u:sm~u: 142 TH: C/uzclm Am) Arpnxnnc 145 Tm: LARGE Imnsnuz 146 Tm: Rtcrum Am: ANAL CANAL 148 A REVIEW Or Tm: A-rrAcHM£N'r OF THE Msssznnznxzs 150 Dzrzc-rs Or DEVEIDPMENT Or THE Auwxznnnv CANAL 152

CHAPTER VIII DEVELOPMENT OF THE RESPIRATORY SYSTEM Tm: Lamnrx 157 THE TRACHEA 158 TH: Bxoucxu Am: Lxmcs 159 CHAPTER IX DEVELOPMENT OF THE BODY CAVITIES Tm: Pmuclumxunx 162 TH: Pu:uR.u. Cnvrmzs 165 THE Dumnumm 167 Darncus Or Dzxrsnonxmrr 01-‘ TH: Dmpnmxn-.3 169 CHAPTER X DEVELOPMENT OF THE UROGENXTAL SYSTEM THE KIDNEY 170 Dnrxcws or Dzvzwvmzm Or Tn: Kmm.-vs 177 Tm: Bumnzn 179 Dartcrs Or D:v:LoPm:1~rr Dr THE BLADDER 131 DEFECTS Or Ururrm 183 TH: Ttsns X83 Dmczm or Tm: Trzs-ms 181 Dtzric-ts Or Dzvtmntmr 01: THE Trsns 189 Tm: Owuw 1‘J1 Tm: Unaus 194 Tm: VAGINA 195 Dcrtms Or Dzvnox-nu:N'r Or Tm: OVARY, Unmus AM:

Vnunm 197

Tm: Pnosr.-rn: 193

Pm: Szmmnz. V1:xcLm 199 “V C0‘l'!‘EN'1S

Page T1-n: Ammmx Tss-ns Ann Pans-M-nc U1-nxcuc 200 THE MALE UEETHRA Am: Exn:x.~r.u. GENITALS 200 Dnracrs Or Dz:vz1.a1=m:m- Or TH: Uxurn-ma Am: PENIS 203 Tm; Fun.“ Uxu.-n-nu Am: TH: Ex-rzxmu. Gswrmzs 204 DEFECTS Or Davznopmmr Or Tm: Fawn: Ex-rsnmr. Gumus 205 CHAPTER X! DEVELOPMENT OF THE NERVOUS SYSTEM THE Sum; Conn 206 Tm: Mmunu 214 TH: Czmzazuum 217 TH: Mmaxmm 219 TH: Dmxcarmnon 221 TH: Tumcuvx-mm: 224 TH: HH'lNlIN'CEFHALON 226 TH: Nmvuuuv. 228 TH: Cumamx. Comnssuaxs 229 ‘mm Caaxm. Naavm 230 Dzvu.ox=wx1.m or Smut. Nzxwzs 234 Davsnonlnrt Or THE Au-ravcmc Nuwaus Svs-mu 235 DEECTS Or Dzvnnomzw-r Or THE Ssuux. Conn Ann Burn 235 THE Amuzuu. Gums 233 THE Hvromwsxs 241 CHAPIER XXI DEVELOPMENT OF THE EAR, NOSE AND THE EYE THE Elm 244 D::n:c1's Or Dzvzmmzm 09 ‘PH: EAR 250 THE Nos: 259 THE En: 251 Dnrzcws Or Dzvzwmaw-r or Tl-I2 En 261 CHAPTER XIII DEVELOPWXEINT OF THE SKELETAL SYSTE“ Tm: Sxznmux 254 TH: VLRTEIRAI 265 THE Rxas '1'!-In Snuuvuw 270 TH: SKULL 2.”

'1':-12 Vzscmzocmmuu conmwis xv

Page Tm: Murnxanz Am: MAx!u.A 273 THE Bows or Tm: Urvm Am: Lawns Emnmutxzs 275 TH: J arms 276 Dzrzcrs Or Dt:vELo1>vAr::rrs Or THE Sxmjzrow 275 CHAPTER XIV DEVELOPMENT OF TEL‘ MUSCULAR SYSTEM Tm: Muscu-5 01* Tue Tmmx 23X Tm: Musczss 0: THE: NECK 283 TH: Muscuas Or Tm: Ham: 283 THE Muscuzs DERIVED FROM Tm: PHARYNGEAL ARCH!-LS 281 THE Muscus Or Tm: Lnmzs 284 Com:t:vx'x-AL. 1'):n»:c-rs Or SKELETAL Muscms 285 CHAPTER XV DEVELOPMENT OF THE INTEGUMENTARY SYSTEM

Tm: Sun 285 Tm: NAILS 287 Tm: Hnm Z87 Tm: SEBACEOUS AND Swan Gumns 288 Tm: NIAMMARY Gmuns 289

Dznzcrs Or‘ D:vELoP1\u:m' Dr Tm: In-n:cuMm~rrARv SYs‘n:M 290

APPENDIX

Sax CI-mown-m 29'.’ Ixmrx 296

Page 119, lmc 1. read “Mm-ostomus" as “x\Iucros(omus" CHAPTER I THE FEMALE REPRODUCTIVE ORGANS THE Huwmv Ovunx

'90 The ovum 15 the female repmductxve cell produced 1n the ovary It 15 the largest cell Ln the human body measunng about 0140 mm m dxameter Under the mxcroscope the fresh ovum 15 seen as a sphencal mass of protuplasm can taming a granular cytoplasm and a nucleus wxlh the charac tenstlc chromatin netwn}-_l_( The cytoplasm contains a few

IONA PI:Ll.\7l:u>A

CELLS or Column unmrn

fiG 1-—HIm1:m am m (X 2570) 2 HUMAN EMBRYOLDGY

Y0“! granules and certain characteristic bodies known as the 1nItDL‘h0ildT‘l¢ and the Golgi apparatus The outer boundary of the cytoplasm forms the thin oitellme meinbfone Just outside this is a transparent thick membrane called the zona pelliicidc Outside the zona pellucida the surrounding follicular cells form irregular radiating Iayezs called the corona rizdiota

The process by which the ovum is produced in the ovary is known as oogenesis As early as the fifth week of date. lopment the genital gland assumes a position as seen in the urogenital system of the frog The germinal Eplfihellunj attached to the kidney (mesanephros) has the potentialities to form the germinal cells uhich are pushed inwards at first in the form of tubular masses of cells (Pﬂugers tubes) In the substance of the gezutal gland now the ovary these tubular masses become separated from the germinal epithelium and form groups of cells iihich grow in a peculiar way One cell of the youp grous bigger than the others and becomes the primary oocyte The remaining cells surround it as follicular cells The whole group is now called the prinordial follicle During the last months of foetal life about four hundred thousand of such follicles are present It is thought that these primordial follicles atrophy altogether alter birth to be replaced hi neii ones from the germinal epithelium Some im estigators belive that the primordial germ cells have migrated to the gernunal epithelium from other

regions of the embryo

The primary oocyte grows b gger acquiring the charac teristic cytoplasmic components of the ovum Its outer baun dary forms the titelliiie membrane The surrounding folhcular cells multiply enormously Those cells in contact with thvitelline membrane become a homogeneous layer the zona pellucida In this mass of proliferating follicular cells cauta Lion taltes place as a result of which a layer of follicular cells becomes irregularly attached to the primary oocyte and the remaining outer cells arrange in compact layers peripherally The ﬂuid that fills the cavity is the liquor folliciili The late!‘

attached to the primary oocyte is the corona flulmtﬂ Tb? oocyte is surrounded by the follicular cells at one pole of the THE FEMAIS REPRODUCTIVE ORGANS 2.‘

1'o1hc1e—!hey are the cumulus cell: The arrangement, thus of the falhcular cells and the ovum 15 called the G1-aafian folhcle Many of these In various stages of growth are scat tered about m the cortex of the young ovary The penphez-aI folhcular cells grow xnlo many layers to constitute the granular layer The hquor folhcuh contams the oestrogemc hormone

FOL’-lC!1l-Al CHE

°°’"‘ 11”“ run. each:

GROWTH O!‘ POLL CU.Ll LIQ FOLHCUU

CORONA IADIATK

Fxc. 2—Schcmcmc sectxon of ovum shousng oogenests and ouulanon

acstradlcl wluch controls the Integrity of the <ex organs cmauam lhe hquor loT11cu\1 Lncrenscs In qunnmy and lhc penphcral gnnular layexs become reduced to a Ihm In) er The Graafian folhclc mugrzues to the surface of the max} and due to the xntcrnnl pressure of the liquor [clhcuh the thxn 1139:‘ which scparnlcs the contents of the Iollncio from the 4 HUMAN EMBRYOLOGY

abdominal cavity is stretched out and ultimately the follicle ruptures Normally only one Graafian follicle ruptures es ery month Usually this happens either in the right or left ovary The cumulus cells to iihich the oocyte is attached separate the oocyte which with its covering corona radiata escapes into the peritoneal cavity This is the process of ouilcitioxi

Ovulation occurs only during the repioductiye life generallv

nwLoLmmstmatioiT‘“%‘

%—~‘..

The primary oocyte undergoes certain changes in the Graafiari follicle before the rupture This is the process of niiituratzon. The primary oocyte contauis as in the other cells of the body 48 chromosomes appearing in pairs 23 pairs of autosomes and the last: pair forming XX chromosomes Meiotic dl\lSl0l‘L of the nucleus of the primary oocyte takes place asymmetrically \V ith the formation of a secondary oocylc and a small bud of nucleus separated from the remainder of the cells as the first polar body It is probably at this stage that the rupture of the Graafian follicle takes place Again a further meiotic division of the oocyte takes place in an asymmetrical manner with the formation of the mature omui* and a second polar body Simultaneously the first polar body divides into two equal sized polar bodies Hence as a result of the meiotic division of the primary oocyte one mature ovum and three degenerating polar bodies are produced

During the process of maturation oiung to the meiotic divisions the chromosome number I5 halved The original 48 chromosomes arranged in 24 pairs separate longitudinally

Out of these one pair is a typical type of similar chromo somes or sex chromosomes called XX—chromosomes Thus, when the primary oocyte divides the secondary oocyte contains 23 plus X chromosomes and each polar body also contauis a similar number The mature ovum therefore con

tains 23 plus X chromosomes After the Graafian follicle ruptures the granular CCIL grow more and blood capillaries proliferate in bet“ een them

the cells later for-rniiig 3 Yell°\V Plgmemed ”‘3‘9“3l Th“ the corpus luteum which secretes a hormone called pwgestu

rang controlling fertilisation and pregnancy '1'!-E FEMALE REPRODUCTIVE ORGANS 5

TH: fi:~uAi.i: REPRODULTIVI ORGANS The female reproductive urgans consist of —

A pair of O\Ell1E:

A pair of uterine tubes Uterus and

Vagina

The Oumvts These lie in the pelvic portion of the abdominal mwity suppoited by the broad ligaments of the uterus and the ovarian and the suspensory ligaments Each ovary is elliptical inshape roughly measuring 14 xi X 1 3 A section of the adult ovary shows an outer layer consisting of a mesenchyma] netiv. ark containing many Graaflan follicles and corpora lutea covered by the tunica albuginea '.['he innei matrix is the medulla consisting of connective tissue blood vessels and nan es

UTITHXNE {VIZ DTIHYIVI ‘(VIE OVEIHNU INTO UTEKVS

CIVKTY OF UfiDS HMIIMI OVAIY ROUND l-lﬂ Of OVAM’ CEVIX IIOAD HGAMWT VAGIKA

fin 3 —l'-'cwriuli- rt-pro lii:-In c organs 0 6 HUMAN EMBRYOLOGY

The Ulennc Tubes are narrow tubes each about 4 xnches long enter-mg the upper part of the uterus, wxth a funnel shaped outer end Each lube IS sxtuated between the two layers of the upper edge of the broad ligament of the uterus The Irregular tunnel shaped opemng :5 near the ovary The walls of the tube are made up of muscular and fibrous covermgs “nth an mternal hnmg membrane contamxng many cxha

The Uterus ts a hollow pear shaped thtck walled mus Luiar organ It consists of an upper pox-non, the body, and a lower portmn the fixed term: The utenne tubes enter the upper porhon cf the body and the cervix xs ﬂexed at an angle of about 31) to 100 degrees to the uterus The uterus 15 kept m posmon ‘by the ‘broad ligaments, the round lugs ments the utero-sacral utero-vesxcal and recto-uterme hgaments The cervax proaects mtn the vugmu Much extends to the external gemtals (For detaxls of the reproductxve organs the student may refer to any textbook of Human

Anatomy) CHAPTER II

THE MALE REPRODUCTIVE ORGANS THE SPERMATDZOA

"A human nzermnlozaan is in tiny, 1-re: him-.nn nrgimmn It: helmviour is determined by Ilia maimer in which its living praioplaxmu: xystevn react: to the chemical, mechanical and other energy relationJlilps of its enmramnent. It may be said that the mavemeiiu a] the sperm are clxemctutiic or -rlieatactic when such it pmprr maul: and mechanistically detennlned rpm-ni fertilize: it proper human ovum in a suitable environment, processes start which sometimes lead to the development 0! it. new and infinitely complex but again freely muvmg, Inminn argunb-in Certain of these 0'l'ﬂll7lLT71lS so developed at definite stages In their live: per:-ewe, hue emotion: 1-cuan, talk, build air-planes and mm. and bomb other similarly consruttzd liuman inguvimns’ Carmichael, Leonard The Experimental Embryology of Mind, In Twentieth Century Psychology New York Philosophical Library, 1946

In 1677, when the Dutch naturalist Leeuwenhoek dis~ covered slender objects musing about in the ‘swarm’ or the seminal fluid, it was thought that these constituted special organisms or parasites called “sperm amma " or “spermatozoa " These were later recognised as the male reproductive cells, but the original name “spermatozoa" is still retained The human sperrnatozaon is about 60 microns in length and consists of an oval disk-shaped head containing a nucleus, a I\a‘l't‘0\'\ neck, i\ connecting picce (middle piece) and a tail The head is covered by the modified protoplasmic material called the iicrosame in front and by the post-nuclear Cllp behind In the centre of the head there is :i vacuole‘ A small granule in the neck is associated with the anterior cirntrtoliz. The connecting piece extends from the anterior (.'€1t!T\Dl2, to the poitcﬂor centriolc situated distally, in the form o’l_a"c;ig The central filament is surrounded by mitocliondriul granule: The filament ls continued as the tail. The spermatozoa are

produced in the lCSll5 3 Huxmn meavonoey

you xucuuu cu Acn¢s°M-g x-n:/u> Mnocaun saumx cu

ANY UENTROSDMI‘

com: nee: (menu: rm.-zy

NUCIIVS or srznnnm

HDWINT 0}‘ \‘ cncvusu

POST CDHRDSDM!

Fla 4 —Hurn2n sper-matozoon and :13 trans-Iormatzmt from spermaud

Tu: T:.s1r.s

The male repmducme system conszsts of the patted testes then ducts, the prostate, the s?mmal tesxcles, tho» _, ,__.._. eyaculatm-y ducts and the pent:

The testes are lodged m the scrotum Each tests as covered by a connectne hssue capsule called nmxca albugmea !t is dwxded mlo a number of lnbules by the conneclxve lxssue septa Each lobule contains one to four senumferous tubules Each of l.hese ends m a Straxght tubule at the medxasunwn The straxghx tubules gum together In a netxx ork called the rate testy: In the mlexsuces betueen Lhe semu-uterous tubules are found the mterstxtxal cells whxch secrete a male set her rnone The spermatozoa are produced tn the sex-nunferuus tubules from where they enter the reletesus By means of efferent ducts they are carrxed to the much coiled eptdzdynus, whxch xs contmuous with the vas dejerens The epldxdymns when stretched out measures about 125 feet The vas deferens extends from the scrotum through the mgumal canal and opens xnto the prostahc part of the urethra Near {he (enmnatxon of each ductus deferens xs the dnertnculum the scnunal vesicle The process of the productlan of mature spermatozoa by the semxmferous tubules 1s known as spermutogeﬂeszs

zrmmrnls

VA: ntrmtus Sn“

-rusuu:

com: nssu: SEPTA

Fm 5—Schematxc duzgram of a. sartwn ol rash:

51=:x.\1 xrocmsssxs

In the fifth neck of deweloprnent. the gemtal gland (tesus) ls attached to the mesonepln-as From the germmal layer \Vl'llCh covels rt, cells-cords grow mwaxd and ulhmalcly form young semxmfemus tubules In the early stages, the wall 05 each luhulc IS composed of one layer of cells restmg on a basement membrane These are the germmal cell: Among them, there are n {cw pyramidal shaped cells known as the Sc:-ml: or rupportmg cells Gradually, the germmal cells 10 H'U‘M.AN EMBRYOLOGY

undergo dwxs on first the spermalogoma are formed wlnch dlvuie Into the pnmury speﬂnutocytes A fun]-,9, dmswn resu ts m the formatxon of the secondary spermatocyte; wh.¢h contam only half of the ongxna! number of chromosome; These further dmde to form the spew-rnutwls \\I-nah are

xmmature male germ cells mostly found adhering to the semi cells

3:: sn:mA-roam:

srmnlnozox

PHJM. SJBHATDCYTS

com! 115517: srawnocomon (DNIDING)

Fm 6 —-Schematnc diagram of part of the wall of semxmferous tubule show-my sperrnatogenesu

Each spermahd transforms Into a sperrnalozoon During these stages of spermatogenesss the wall of the sermmferous tubule becomes thicker and the cells crowd in towards Ilne lumen By puberty the adult structure of the serum fem-I5 tubule xs estabhshed The spermatozoa are freed from the

Sertolx cells and move about u: the lumen The cells of the body of the male contain 43 chromo somes appeanng um dxfierent sues and shapes and an each THE MALE REPRODUCTIVE ORGANS ll

nucleus in pairs as III the female cells The last pair is seen differently in males and females In the male, the last pair consists of unequal chromasomes, one of the pair looks like that of the female cell and is known as X-chromosome and the other Y-chromosome Hence, the primary spen-natocyte contains -15 plus XY—chi:omosomes During spermatogenesis meiotic divisions of the primary s-per-matocytes take place and result in two secondary spermatocytes of equal size, each containing half the number of chromosomes That is. each secondary spei-matocyte contains 24 chromosomes, the last being X or Y—chromosome The second meiotic divisions occurring in these two secondary spei-Inatocytes produce four spetmatids of equal size containing in each 23 plus X or 23 plus Y-chromosomes Therefore, hall the number of spermatozoa contain 23 plus X-chromosomes and the other half 23 plus Y—chromosames

SEMEN

The semen or the seminal ﬂuid is a muctute of the secretions from the epididymis, the seminal vesicles, the prostate and the bulbc-urethral glands. in which the spermatema ﬂoat The quantity discharged at one eiaculatioii varies from 3 to 5 ml , normally containing from 100 to 200 million spermatozoa per millilitre It is alkalme in reaction The spermatozoa are not motile in the seminiferous tubules After getting themselves detached from the Sertoli cells, they pass through the epididymis and the ductus defcrens by the peristaltic muscuL'it action of the ducts In the semen, the spermatozoa become motile, especially due to the prostatic secretions Their viability is about 48 hours in the uterus, although they die at room temperature after 24 hours They die in the iaginal canal, alter 2 hours. as the iziginal secretmns are acid They move at the rate of about 2 to 3 millimetres a minute and reach the uteruie tube within 1 to 48 hours after insemination

It is estimated thiit only about 20 per cent of the spcrma— tueoa reach the uterus after a coitus. The concentration of the spermatozoa in the semen and the percentage and type 12 HUMAN simn\oLoov

of motility seem to have an influence on fertility Semen cuntairung from 25 to 100 millions of spermatozoa per ml with a high motility count is considered to be fertile and less than 5 million per ml with poor motility infertile

fi:n1ii.xz.Ima~r

Fertilization is the actual union of the ovum and the spermatozoon The preparations for this union in the ovum are the process of maturation of the primary oocyte the reduction of the chromosomes of the nucleus and the forma tion of the second maturation spindle The ovum thus prepared, soon after being expelled from the Graafian follicle enters through the fimhriatecl end and remains at about the outer one-third of the uterine tube, awaiting the spermatozoon Only one spermatozoon penetrates the corona radiala of the ovum, the others crou ding around it perish ultimately

The penetration is probablv due to an enzyme (l:iyaluronidase) contained in the seminal ﬂuid coming in contact with in substance (hyaluroxue acid) present in the binding cells of the corona Soon after the fusing, the surface la;er of the ovum forms a thin membrane known as fertilization membrane, which prevents the entry of more spermatozoa After the fusion, the head of the spermatozoon absorbs ﬂuid from the c) taplasm of the ovum and forms a pronucleus It is the head of the spennatczoon with the nucleus and the neck with the anterior centriole wluch enter first the tail being lost in the process The chromosomes of the tito sexes thus unite together to form a zygote and contribute to restore the origu-ial total number of 48 chromosomes At this stage. a centrosome proloablv denied from the anterior centriolo appears between the chromosomes of the two pronuclei and guides into two {oi-i-rung 3 spindle The chromosomes of the fused pronuclei arrange on the spindle and split into two equal halves towards the centrosomely EH9‘-'*|ﬂ§ 91! ll”! nuclear dimsion At the same time a furrow is formed in the cytoplasm at the equatorial plane of the spindle ditiduiil it into two parts ‘Plus is the first cell-dwmon Further cell

divisions constitute the stage of ¢l9M°9¢ THE MALE REPRODUCTIVE ORGANS 13

S23: Dr:u:.:umxA'c1mx

Set is a qualxty “Inch xs mhented m a xery snmpln manner, the contrasted characters of the male and female segregaung sharply from one another In the adult As men honed earher the genes controllmg the sex are earned on the sewchroxnosomes XY and XX The dxfferenuauon o‘ 1he l\\o sexes occurs by smtable combmauon oi the sex chromosomes dunng fertxhzauan The accompanymg figure represents dxagrammaucally the chromosomal determmauon of sex

M A L E FEMALE

FATHER

n M A 1 I c

cnnnuasuzs

MOTHER

X s o M . 1 r. culovasmss

mm: ovn

46 XY 46 XY 46 XX 46 XX M A L E FEMALE

Fxc 7—Dmgrmn of the clrronmsomal dcternunatnon of <e*

Gzmmcs mm Inurru-mncz

Mm is the product of his hercdxly and enuronmcnl Nexlher of lhese (no is constant at any time, and lheu‘ \ana~ nuns lead to dnflercn! forms of physical and mental characlcn Thc<e characters are controlled by hercdxtar} factor». or 391195, \\l\lv:h are prexenl m cu.-ry ull bung pxssed from 1-! HUMAN EAEERYOIDGY

parent to children through the medmm of the se‘( cells or gametes

The fertuhsatxon of the mum by the sperrnatozoon produces the zygote Whxch develops mto the adult human bemg The body cells contam a defimte number of chromosomes which are arranged tn pan-s Thetr number 15 constant {or each specxes bemg tweqtyfour paxrs or fortyexght chromosomes xn the human bemg The cells of repmductxon the sperm an e ovum contam one of each par of chromosomes so that only half of the ongmal number is present and thls process xs called reductxon dxvxsmn or mexos 5 Any one chromosome of a par can pass tnto each gamete at conceptwn The male and female then urute to form the fertxI.secl ovum or zygote \\h1ch agam coutams txxentyfour pairs of chromosomes as m the orngmal body cell 1e twentyfour smgle chromosomes of each parent The offspnng thus has a genel L composmon wluch IS dependent upon xts parenls

Durmg the mtta utenne and the whole lxfe of the mdnx dual the genes mamfest themselves as mhented characters A smgle gene represents one or many characters and many genes may represent a smgle charactgr The total gene content 15 known as the genotype of the mdnndual xxhereas the mamfested characters ax-Eknown as the phenotype The gene also exxsts In alternate or allelomorphlc form”s“ Thus alleles or allelomorphs are dxfierent genes occupymg the same locus as a homologous chromosome locus bemg the soot where the gene 15 sxtuated on the chromosome

If a character is demonstrated by D and Its allele demonstrated by :1, 1t :5 possxble to ha\e an znhentance of three types vxz. DD Dd and dd Thxs denotes a Slmpl’ mhentance When the alleles from both parents IS the same the ofisprmg IS saxd to be homozygous (DD and dd) but when alleles occupymg adyacent locx on a pan‘ of chromosome d-ﬂier the 55 san‘. ‘co be ‘rretemvygsvus (Dd)

All mhentances ho“ ever are not so sxmple smce at one locus 11 Is possxble to have multxple alleles Alternatnely the Inc: may occur so near to each other on a partxcular chromosome that characters become lmked and are passed on from generatzon to generatxon m thzs manner This :s called lmkagn THE MALE REPRODUCTIVE DRGANS 15

For genes to he linked they must be on the same chromosome When such characters are associated with sex chromosomes X or Y, they are said to be sea: lmlced‘ A common example

hentd abng_rmality H|Al’ﬂaX1_1S_l1Befn0yhllI3

During the reduction division, the chromosomes of each pair in the gamete twist round one another and then separate Sometimes a chromosome may break and regoin so that the top half of one chromosome in the pair yams with the bottom half of the other chromosome, thereby a crossover occurs no genes being lost in the process If the genes are situated very near to each other on the chromosomes the chances of a cross over become very rare In cross over there 15 a changn of chromosome material with a change in the gene structure and this is called mittatwn Mutation may be permanent provided the allotment of genes is not imurious to the oflsprino or temporary and may revert in a few generations

In a series of alleleomorphs not all the alleles manifest themselves with equal eﬂiciency some genes completelv masking the action of the other genes The masking gene is said to represent dommiznt character and the masked gene is said to denote the recessive character and will only be detectable when the recessive alleles come together in an other generation CHAPTER III CLEAVAGE AND GER‘! LAYER FORVIATION CLEAVAGE

Cleavage consists of a repeated series of mitolic divisions of {he zygote The fertilized ovum first divides into me cells the cytoplasm of both being in contact Each cell IS known a a blastomere The blastomere again divides into mo This pmcess is repeated in a systematic manner till manv blastomeres are formed The blastomeres crowd together within the zci-ia pellucida as a n mass called the mggtla In the monkey (Macacus rhesus), the first division takes place about 29 hours after ovulation and in about 72 to 96 hours the morula is formed

z1.As'rom:iu: ruxrﬂzn DYVISION

EXJSIOMSESE 1,.“ POLAR

wall. or Imrum: ms

Fm 8-Cleauage stages m_th2 uter-me tube FORMATION or win: PRIHARY Gum: LAYERS

The fertilized ovum grows as the moi-ula in the ute§1_e tube On its yaumey to the caviky of the uterus the zona pellueida is absorbed and cavitation occurs xi. ithm the morula in {mm an inegular vesicle or cavih Sunullaneousb the CLEAVAGE AND GERM LAYER FORMATION 17

remaining cells group together at one end leaving a thin layer as the outer cells This N the blastocqst stage The cavity of the vesicle expands more in size to form the iegmentatmn cizutu or the _ The heap of cells established at one pole is known as the inner cell mass which grows as the‘ embryo proper The outer layer of cells later help to absorb! nourishment from_the uterus and is called the trophoblust orl trophoderm About the same time, a few upper ce ls of the] inner cell mass become arranged to form a small cavity which is the b_egirining of the amnion \

The lower portion of the irin_or cell mass resolves mm a definite layer, extends beyond its margin and surrounds the blastocoele This layer becomes th and the cavity I-shclosed by its extension becomes the arclientei-cg or the primitive gut cavity or the math ML‘ The remaining cells arrange in a regular layer to fonn the outer primitive la) ei or the ectoderm The inner cell mass Whlch is thus modified into two layers becomes the bilmmnar em out dﬂtk which IS more or less ciicular in shape and lies between the amnion and the arclienteron The ectaderm is continuous with the! iiinnion and the endoderm with the archenteron l

At one end of the disk, there is an extra roliferatiovi at the cctoderm cells growing in a line to\vard of the _¢ﬂs_k This is the ¥pr1Ii1_lz£_e_xtr_eliL (fig 10 Si 11) The region of the disk, where the thickening of the primitive streak appears, indicates the caudal end of the embryo The multiplicalion at the eetoderm cells, restini: on the endoderm

out betiieen the two la ers 0 t e embryonic disk This is the Eeginnlng of the establishment of the third layer, the

~iic-sodm-Jvi Hence, along with the growth of the primitiic streak, the iiiesodeni-i cells spread to the sides and bsicltii ards. The collections of cells from both sides of the primitive streal. |"£\()\t2 foniard towards the_ front portion of the embryonic plate. iihore they meet together to constitute :1 future landiiizirk the ciirilingcmg m_qwoilmvi—tlie place of origin oi the heart (fig 11) A feii tresoclerm cells spreading backward enter the portion (bod) stall.) “here the caudal and

-3 13 HUMAN ZMIBRYOLOGY

WALL OF WNDUS

'fiO?HOD!lH

ausfunnruc veslcliinma

EHIIV ru7:{ §fi{,°°‘?,‘,'§’,‘, 5 '

YOLK SA:

Cl’ IH IHIIV nA'r:«{ {Nn%“D‘“

‘(DUE SAC WALL on ursnus

AHMON

scream-1 mun mu: ( mwnun

101.1 uc Ixrusnuv nssoutm

snnv run-s { {fi",,%'§,‘E'§';‘

YDLK SAC (SEC) TOLK {AC ﬂ‘IIH )

Fm 9—Dmgrams thowmg fonnatnmn of the pmnaru germ. Layers

of the embryo is attached to ihe trophnblast

Due to the growth of the pnmmve streak and the spread of the mesorle-rm the embryomc disk as stretched out In n lengthwise manner mto an oval and then mm a sandal shaped plate cunsmmg of the three pnmary layers Al the same tune _Ihe dlsk IS nused up on the ectodermel surface The esnmated age oi the embryo at ﬂue stage 3 about 15 to X8 days

The mesoderm thus formed In the embryo rs known as the mtra emba-yomc or secondary mesuderm The mescderm cells are pnmanly formed ‘between the tmphohlast and ML CLEAVAGE AND C-ERM LAYER FORMATION 19

atchentemn as the ertra—embryouv.c mesaderm Thxs later sphts mlo two layers—-one layer hes-an contact wxlh the archenleron and the other with the ‘frophoblast The sphttxng extends up to the roof of the ammon The space between these two layers consmutes the extra embryamc coclom extendmg In the xuof of the an-mmn

Thus TEE tnlammar embryamc plate at about the 15th to 18th day, 15 attached to the trophublast at the margm nemlhe prxmmve streak, whxch later, becomes the caudal end of the embryo The oppuszte free end 15 the regmu of the head

'1‘:-11'. NOTOCHORD

At the antenor end of the pnmmve streak there IS an extra prohferatxon of cells formmg the przmmve knot or

rxmmvt smznx I

cmmu urn / fiG 10 —Su.rIace ‘new of early bxlammar emlzruumc plate

uucconun umznlm:

HUM. smut

fiG l1—E1nbr1_/ovuc plates (cctodr.-rm and yolk sac removed) showing the spread of mesoderm cells from pnnuuec streak and notochord 90 HUMAN HIBRYOHJGY

Hen.se1i.’s node The cells in the centre of this node push in and grow anteriorly (towards the head end) in the form of ii cord of cells, between the ectoderm and the endoderm in the mid axial region and form the notocliordal process The small opening produced by the pushing In and subsequent growth of the notochordal process is the blostopore Collections of cells from the sides of I-lei-isen‘s node form mesoderm \\l’|lch 101115 the rnesodei-m derived from the prlmlllVE streak

The anterior end of the riotocl-iordal process extends EU the anterior portion of the embryonic disk According to come investigators the blastopore sinks into the whole length of the notochordal canal resting on the endoderm Later the floor of the notochordal canal “hich has meantime fused with the endoderm breaks at irregular intervals leaung the dorsal wall of the canal as the nntoclioi-dal plate Later. the plate separates from the endodermal lining and becomes throcl shaped notaclio-rd, serving as the supporting structure of the growing embryo Along iuth the formation of the notochord the embryonic plate elongates and by the txuenty-first day the primitive streak is pushed more towards the caudal end

At the caudal end the endoderm of the roof of the yolk sac comes in contact with the ectoderm of the emhry onic disl establishing a bilaminar area the primordium of the cloacal membrane Mesoderm cells extending backuard from the primitive streak pass through the sides of the cloacal mem brane At the cephalic end of the notoehord between it and the mass of cells (cardiogenie mesoderm) migrated from the sides of the primitive streak is an area “here the ectoderm of the embryo comes into contact with the endoderm of thcephalic part of the roof of the yolk sac This is the prunitne buccopliuryngcol membrane (fig 11) The notochord lies in the mid luie of the embno between the endcderni oi the roof of the yolk sac and the ectoderm uhich later forms the neural groove

TH: lN'mA riiniwomc Mssoivzim

The intra embryonic rnesoderm anses from the prirnitm streak Hansen's node and probably to a small extent, from cnmvmn AND G1-IBM LAYER roiuiiuiow 2i

PKIM IIKSYDPORZ STEZRK. I ,L_\//{ /1 1 Annie" EDDY STKLK QTR. EMEHY }d$UDEEM ‘Ll-ANTOYS

z<.'mi7n1M (BRAIN) Nmoci-ioiimii. i>Ui‘n:

YOLI SAC

fiG 12———Schematic section of Heuser embryo showing for manan of notochord (See fig 26)

ihe notochurdal process A’ first the mesoderm cells which lie an either side of the notuchord become arranged as a thick mass The lateral extensions of each mesodermal mass are called the lateral plate mescderm This extends between the

zciontau mrmuii. Misozizau (soMm:i

fit: 13—Trruis1iersc section of presmnitc embryo

(ma prfmarv lavers 0f the embryonic disk and comes in contact \\ ilh the extra embryonic mcsadcrm on the margins of the disk. A section of the embry cnic plate during the third vicek of development, shoixs lh'it the lateral plate mcsoden-n has dnidod ink: t\\o la) ers \VIIl’I the {oi-malian of a cant} called the uitrn-embryonic caelom and the cell masses on both S1dQ\ of the nolochoi-d remain undivided as the pai-axial 71II.$0LlE771| The l'i)er of mesoderm lying in contact “fill the eclodci-ni for-ins the comutic ur the parietal layer and the other in contact \\lKl\ endoderm forms the splauclmic or the visceral layer B} the end of third neck, the paraxizil mesodcrm 22 HUMAN EMBRYOIDGY

divides into paired masses called the samttes or the body segments In the centre of each somite there is a small cavity, the 'm.-yocozle which later becomes clused by cell growth The ventrolateral portion of each somite projects into the coelom as the intermediate mesoderm which later transforms into the urogenital organs (fig 116) The first somite tn be

?ARIE7AL

ECTODERPI "Sam."

4 0 e \ cotton ,;-° ,9 0*‘ 3‘ VISCEIAL

‘- ‘~ "4 nesanrxn

fit: 14—T'ransverse sectum of early smmte stage

formed is Just behind the cephalic end of the notochord and is called the first occipital Further somites are formed just caudal to the previous ones In all 42 to 44 pairs of somites are developed

(Further history of the mesoderm is discussed in chapters XIII and XIV)

Twivs

Twins are classified into two types, identical or monomgotic and fraternal or dizygotic

Identical or Monozygotic Turns Mono7ygotic' twins originate tram one ovum The fertilized zygote divides into two separate masses at the early cleavage or inner cell mass stage and each of them undergoes development into a normal embryo Since the ovum is fertilized by one spermato— zoun, the two offspring arise from identical germ plasrn They must therefore, be of the same sex, must show striking resein blance to each other in their physical form and must possess the same genetic character in duplicate

Sometimes the fertilized zygote divides into three or four entities, leading to the formation of triplets or quadruplets CLEAVAGE AND GERJVI LAYER FOR!\1A110N 2.1

The famous Canadian Dnonnes are of the monozygotxc type

Very rarely the dxvxsxon of the zygote 15 incomplete and both the masses are Jmned together at some area These mas=es undergo development and are born at full term as L‘0Tl]0H1t tum»: If the fusxon IS superficial each grows min a nearly complete Il'ldlVld|.Ial and these conjcunt hvlns are often called “Sxamese twms", after a palr of twms born In Slam In lhe 19th century

Sometimes the fuslon of the hvms IXIVOIVCS many regxons

~ T1

PM: 15 —A ' double mo'n.mzr' The body wall 1: cut open to shun: the duplncahon of the Internal organs (Courtesy/—-Pnncxpal, Gran: Mcdxcal College) 24 HUMAN EMEBRYOLOGY

such as the upper, middle or the lower part of the body on the lateral, ventral or dorsal side These are called double monsters, owing to their abnormal appearance These twins are, rarely, of unequal growth and present an appearance of unequal double monsters The smaller of the pair is referred to as a ‘ parasite ’ on the bigger member of the twin Most of these monsters die in the foetal condition.

Fraternal or DlZ)gﬂUC Twins This type is the result of the fertilization of two difierent ova by separate spermatozoa Two ova may be shed from the same ovary, from an abnormal follicle containing two or more ova or as a result of defective timing from two follicles of the same ovary, or separately from each ovary Their sex may be same or different, depending on the X or Y chromosome of the spermatozoon \Vl1lCl1 fertilizes the ova They are no more alike in their hereditary characters than their other brothers or sisters The tendency for a double ovulation and the production of twins in the mother is iiilluenced by heredity, age and parity Very rarely. triplets and quaduplets arse as products of multiple ovulation

Sormrorrrnnov J

Soon after fertilization, the ovum undergoes normal deve lopment, the different stages of which are given in subsequent chapters During pregnancy, further ovulation and fertih ution do not occur normally But in very rare instances owing to the irregular function of the endocrine glands, a further ovulation may occur one or two months after preg nancy has set in There may therefore be a chance of another’ fertilization and the formation of a second foetus in the uterus This may be cleln. ered a month or two after the birth of the first one, or may cease to grow and be expelled along with the first one This phenomenon is known as super-{aeration CHAPTER IV THE FOETAL MEMBRANES

It has already been considered that the product of the fertilized ovum by the second week grows into a trilaininar embn onic plate between the amnion and the archenteran and is enieloped by the trophoblast layer The embryo grows by the transformation and growth of the embryonic plate The accessory structures serve the fundamental functions of nutrition respiration excretion and protection of the embryo growing as a parasite within the will of the uterus of the mother These accessory structures are cast ofi after birth and hence are known as the foetal nicnibrmies Considering their origin from the early stages and their elaborate growtn activities they could better be designated as the ettraem bryonic structures instead of the traditional use of the term

the foetal membranes

The trophohlast layer grows enormously spreading in the inner wall of the uterus and undergoes many changes to form the placenta which functions as a medium of the mtercliange of material from the mother to the foetus The extraern bryonic i-nesoderm splits all around the mnet aspect of the trophoblast except at one region “here the ernbryonic plate is attached This region is the body stalk the future umbilical co-nl connecting the placenta and the foetus The growth of the primitive endoderm around the blastocoele results in the formation of the urchirnterorn or the yolk sac The superficial cells of the inner cell mass of the blastocyst become arranged early to form the amnion. containing the amniotic ﬂuid The ﬂuid increases in amount and the amnion later envelops the whole embryo A sm’ill diierticuluin at the caudal region of '.‘.-ie ',~i:-.-r.\.-.\'.~.e M-e.‘:ieri‘<-z=’:r.‘. §'o:'ms. ‘the r.'.'.n~r.'.o~u-'.¢1-it 1192-iyerti culxim which ex on in the earl) stages enter into the body stalk

li\ii>r.AvrA'rio~: Awn FORXIAHOV or rm: Dzcniult

A ten. d'iys after ovulation certain important structuml changes occur in the lining membrane of the uterus to accom 26 HU\IAN EMBRYOLOGY

modate the fertilized ovum The sexual life of woman is characterized by the periodic fioii of blood mixed with mucus from the uterus known as mznstruutiml. In a non pregnant woman menstruation usually occurs regularly once in 28 13335 lastuig for about 3 to 4 days but individual variatioiu and irregularities in the intenal and also in the duration are common The menstruation is followed by the post menstrual phase and after a short intenal or proliferative phase the pre~menstrual or secretory phase is established Before ovula tion the Graafian follicles ripen in the ovary and secrete oestrogenic hormone which produces prolifei-atite hypertrophy of the endometrium After ovulation the corpus luteum secrets progesterone which produces secretory hypertrophy of the endometrium

During the pre-menstrual phase the glands of the endometrium enlarge and become congested with secretions and grow tortuous with engorgement of blood \esSelS The “hole lining membrane increases in thickness These are the prepa rations of the uterus for implanting the de\ eloping ovum into the endometrium

If the ovum reaching the uterus is not fertilized during this period the blood How to the superficial layer of the endometrium diminishes white ﬂood corpuscles escape into the surrounding tissues the capillaries rupture and the blood escapes along with the secretions of the glands and the debris of the superficial layer This unclotted blood flow is menstruti mm The superficial layer becomes tiecrosed in many irregular patches but the underlying basal layer remains unchanged By the end of the menstrual period, the endometrium 1: reduced to a thin layer which slowly regenerates during the post menstrual stage

If the ovum is fertilized and passes through the uterine tube to the cavity of the uterus during the premenstrual stage its implantation into the endometi-ium takes place Usually this happens about a week after fertdization The blastoi-Y5‘ comes in contact with the congested eridometrium The zona pellucida is destroyed and owing to the activity of the outer ti-ophoblast lay er the whole blastocyst becomes buried in the superficial layer of the endometrium and comes in contact THE FOEFAL MEMERANI-3 21

with the compact layer The opening into the endometrium is later closed by a plug of fibrin material from the neigh bouring tissues Probably by the tenth day after ovulation the implantation of the blastoeyst is fully eﬂected

fit: 15 —D agrammntic mew of the cavity of uterus showing implantation of ferttlved ovum.

As a result of pregnancy the expected menstruation does not occur and the cndometrium continues to grow more in thickness as the endometrium of pregnancy which is even tunlly separated and cast off during child hirth and is therefore known as the decidim The growth of the implanted blastccyst produces a bulging in the casity of the uterus The decidua separating the blastoeyst from the cavity of the uterus is known as the decidim capsular-is or reﬂex-u That part of the dccldua between the blastocx st and the uterine muscles forms the dccidua basalts and the remaining part of the decidua of the uterus is the deciduri parxctalu

The factors that govern the changes in the reproductive organs of the woman both in pregnmt and non pregnant 23 HUMAN EMBRYOLOGY

condmons, are entxrely under the control of zzertam hormones These are progesterone from the corpus luteum, oestrogemc hormones (mamly oestradml) from the Graafian follicle and follxcle shmulanng and lutemxsmg hormones from Ihe anlenor lobe of the pxtuxtaty

(For detaxled study of these hormones refer to any boa}. on Human Pl-Aysxology)

usvuxus “cm” C mrcxnu usxus

zmzxvomz: PLATE

ntcmull MHIITM-IS AMNION

"W" or Wm": nzcxmu mum.-2u.1s

cmvxcu. CANAL

ronmx or VAGXNA

fiG 17 ——Schem.atzc secnan of uterus of about a month after fertzlxzztw-n, showmg the fm-nmtrzm of clecxdua

Tm: PLACENTA

Soon after the unplantanon of the blastocyst mto thv enclometnum, the cells of the trophoblast SP1‘?-"-‘d 0"‘ 1“ 3“ dnrecuons slowly destmymg the sunoundmg caplllanes and THE F0!-TAL MEMBRAN)-‘S 21!

enclasmg Irregular blood spaces By the end of the second week after fertnhzatxon the trophuhlast layer dxfierentxates Into an outer syncytuxl or plasmadxtroplzoblast and an Inner cellular or cytotroplfoblust or Langhan/s layer, spruutxng xnto megular projectxons called the 1&1: The trophoblast wxth the extra embryonxc mesoderm hmng It, IS known as the chorzon The mesoderm projects mm a cure m each vxllus to consntute a chorwmc mllus Thus by the end of the third week the chonurm: ulh grow all around the trophoblast The oute1 syncytnal layer corrndes the maternal tlssues xnnludmg the blood and lymph capxllanes and as a result large xrregular blond spaces are scattered about the rapxdly gruwmg chnnoruc Vllll At the same time small caplllanes are formed m thn mesodermal cores of the Vllll and become continuous “nth the blaod cnrculatmn already establxshed 1n the emhryp_ In the beginning the chormmc V111: are spread all around

but later there Eexcesswe growth of the vxlln m the regxon

2 cl-lemon (cur)

cxomcmc ﬂu! CHORIONIC noon VtSS!ILS

Fm 1S—Chor1o1nc xcsrclc 0] abuut /ire muck: (cut opal) (X 6') 30 HUMAN EMBRYOLOGY

of the decxdua basahs Wxlh the xncrease m slze of the chorlomc wesmle, the decxdua capsulans is stretched nut as a (hm sheet projecting more 211 the uferme cavxty Hence all the Vllll met xt slowly atrophy By the end of the second mamh

the decxdua capsulans approaches and fuses wxth the decxdua panetalxs The ramclly growing ull1 over the clectdua basalxs 15 known as the chorwn frandos-um. m contrast “nth the atrophxed vxlll over the decxdua capsulans knawn as the chorwn laeve The chonon frondosum together wxth the decldua basalxs constxtutes the placenta.

As the chonon frontlosum gums more, each \ xllus spreads out hke the branches of a tree By the thud month the cellular layer ox ex-grows through the syncytnal layer at many regxoxu fonmng the ‘ tmphohlasnc shell and remams ux contact wxth the decxdua as the anchormg null: The oiher vxllx 1-emam ﬂoating m the blood spaces whxch are Supplled by the uterme artenes and constantly bemg dramed by the

[ CKOHION mcxvosvn

fiG 19 —oCh¢mumc Leszclc of the second month (cut open) THE FOETAL MEMBRANES ‘S1

uterine veins By the fourth month, the cellular layer Sl0\Vl) atrophies Gradually, the trophoblast cells grow from the uterine side and divide the blood spaces into many sinuses In all the stages of pregnancy, the maternal blood -never comes into direct contact with the foetal blood, due to a thin layer of the tmphoblast always intervening between the two tissues

Normally, the placenta IS attached to the upper portion of the uterus During the third month, the amnion grows laiger in slze and surrounds the foetus and adheres to the inner surface of the chorion laeve, which is firmly fixed to the decidua capsulziris, which in turn is fused with the decidua parietalis Hence, the extra-embryonic structures separating the foetus and the uterus are the amnion, chorion laeve, decidua capsularis and the decidua parietalis, all fused to gether

Just before child birth. when the labour pains start the amniotic ﬂuid with the extra-embryonic tissues gust mentioned, dilates the cervix The tissues rupture discharging the amniotic fluid to the exterior Due to further uterine contractions, the foetus is delivered The umbilical cord is tied and is separated from the new born infant About 15 to 30 minute» after the foetus is born, the further uterine contractions expel the placenta with the umbilical cord and the ruptured portions of the amnion, decidua capsularis and the chanon laeve

msm AMNION 5 ciioriioii

fit: 20—Pliicenta at full term (maternal surface)

0 32 HUMAN EMBRYOLOGY

ummcu noon v'£ss:Ls

rusm AHNIDN 5 cuoaxorv

\ mIn:n.1=Ju. can

fiG 21 —Placenta at full term. (jaelal surface)

The full term placenta 1s a round dusk measuring about 6 to 8 mches m dzameter and about 1 mch In thxckness and welghmg about 1 lb ze about 1/72}: of the “eight of the foetus Its two surfaces the maternal and the foetal are charac terxstxc The maternal surface consxsts of about 15 to 20 xrregularl} raxsed areas called the cotyledons formed by the trophohlastlc shell covered by the decxdua basalxs Eetv. een the cotyledons are shallow sulcz whxch Lndxcate the lmes of attachment of the vxlh to the endometrxum The foetal surface cunsxsts of a smgle layer of the fused ammon and the chorlm Many blood xessels of the chonon on their way through ilk. umblllcal cord project out on thxs surface At the margn of the placenta the fused membrane consists of the ruptured ammon chonun laeve deculua capsulans and a few bnts of the decldua panetalxs The umblhcal card through whxcn blood vessels pass from the placenta to the foetus rs attached eccentrxcalb

Tu: Aumox

In the earl) blastocyst stage the upper cells of the inner cell mass become arranged to form the ammon As the remam mg cells of the mner cell mass arrange m a regular manner to constxtute the embryonxz: plate the amnion remaxns on the THE FOETAL MEMBRANE 33

ectodermal side of the plate Slowly, a ﬂuid accumulates in the amniotic cavity (fig 9)

It is already mentioned in chapter III that the extraemhry onic mesoderm between the trophoblast and the archeoteron has split into two layers This split has extended also to the roof of the amnion, except at one region known as the body stalk or the connecting stalk where the caudal end of the embryonic plate 15 fixed to the chorionic vesicle The roof of the amnion is thus covered by a layer of the extra.embrycmc inesoderm cells

Along with the establishment of the primitive streak, the

notochord and the embryonic mesodei-m, the shape and size of the embryonic plate change from a ﬂattened disk to a round and elongated body, prcuecting into the amniotic cavity The anterior and the posterior ends grow more to form the head and the tail folds respectively gy the end of the second month. the amnion grows larger in size obliterating the extra embryonic coelom and pushing the yolk sac nearer to the body stalk Suhsequentw the extra embryonic mesoderm covering ’the amnion becomes fused with the chorion (fig 19) Later the gllanto-enjei-ic diverticuluﬂ from the hindgut and the narrowed yolk stall’, enter into the body stalk which grows as the umbilical cord. At the same time the amnion is attached to the \vhol_e‘length at the umbilical cord and rcﬂeoted on to the inner surface of the placenta. Thus, towards the later stages of rgegnancy, the £oetus—l§' enveloped la) excessive growth of the amniotic fluid

Just before Chmlrlh during the ‘labour pains" the amnion with its coverings is pushed out into the cervix to facilitate its widening for the passage of the foetus Usually, at this stage the amnion ruptures and the fluid escapes There are many lnstances of the foetus being born without the rupture of the iimnion, but fully covered by the ‘bag of \\ titers "

The amniotic fluid is “derived primarily from the maternal scrum, which is modified during its passage through the amniotic epithelium" It is a clear liquid containing the products of excretlon of the growing embryo It is assumed th-it

the embryo constantly stxallows this ﬂuid uhich fills the

3 34 HUMAN EMBRYOLOGY

respiratory and alunentary organs It is excreted out and is re-swallowed Usually, the ﬂuid measures about a litre, but variations in the quantity are common Its function is not yet understood, except that of protecting the embryo and helping its free movements and also as a hydrostatic mechanism in the widening of the cervix of the uterus

Ti-ii: Your SAC

It has already been mentioned in chapter III, that the lower layer, or endodemi, of the inner cell mass of the blastocyst overgrows on all sides of the blastocele enclosing a cavity, the archenteron or the primitive yolk sac (fig 9) Later, extra-embryonic mesoderm appears between the yolk sac and the trophoblast As a result of the splitting of this extra emhryonic mesoderin and the formation at the extra embryonic coelom, the splanchriic layer lies in apposition with the primitive endoderrnal yolk sac, which now remains fur ther away from the surrounding chorion The distal part of the primary yolk sac (ie , the part away from the embryo) is detached from the main sac and wanders about in the extraembryonic coelom and attaches itself to the chorion and then atropl-iies The main mass of the yolk sac is now known as the secondary yolk sac

The mesodei-m covering the secondary yolk sac is important, because small irregular areas collect together forming “the blood-islands’ from which the first formed blood cel‘s migrate to the embryo through the vitelline vessels and establish emhi-yonic blood circulation Same imestigators believe that the primordial germ cells forming the testes oi the ovaries have migrated from the yolk sac wall

B3 the third week, due to the extra growth of the anterior or head region of the embryonic plate and its subsequent projection into the amnion, the part of the archenteroii included in the head fold thus formed establishes the tubular endodennal foregut anteriorly From the caudal side of the yolk sac arises a small diverticulum called the allanto-enteric diverticulum extending into the body stalk. Along with tlie growth of the hindgut, the yoll. sac becomes constricted as a narrow yolk stalk (vitelline duct) expanding into a yolk THE FOEFAL MEMBRANES 35

vesicle (fig 31) The vitelline blood vessels are seen passing through the mesodei-mal cm ermg of the yolk stalk (fig 52) By the third week, the eiidoderm of the embryo is demarcated into three regions in relatian to the yolk stalk, vtz , the foregut anteriorly, the hmdgut posteriorly and the midgut cominumcatmg with the yell. stalk (fig 94)

By the second month, due to the rapid growth of the embryo and the amnion, the yolk stalk becomes embedded in the body stalk and the vestigial small yolk vesicle at its terminal end is lodged between the amnion and the placenta

Tm: Ai.i.Ai~n-ozs (ALLANTO ENTERXC Divtzirrxcunuiix)

At the caudal end of the embryo of about three “eels, a small outgrowth of the primitive hindgut pmiects into the body stalk This is the nllanto-criteria iiiuerttculum At first. it extends the “hole length of the body stalk and as the latter grows as the umbilical card, it begins to atrophy heat ing onh a small remnant

I:t.‘i‘oi>i:iiM Roar or Imiuon

auccnvum MEMIKAICE PEIICABDXM. CAVITY Q} ” NOTOCHORD /Q "~<n.‘)’ \ i:xnuinmir..ni:soi7:im You 5,“;

fit; 22 —Sc7m-natic sugzttal section of early embryo

This allanto-cnteric dim erticulum is represented in embryos of the lower mammals as the allantois which is hen: much cvpmdt-d scning as a medium of exchange of nutrition between the foetus and the molhen It has no functional 35 HUMAN EMBRYOLOGY

significance in the human embryo, but exists 01113 as a \est.igial structure Its proximal portion, at about the sixth week, dilates and forms part of the urinary bladder The remaining portion attoplues in later foetal stages and becomes reduced to a ligament extending from the apex of the bladder to the umbilicus known as the median umbilical ligament The per sistence of the allantois with its patent lumen, leads to the occurrence of a in-mury fistula in the new born, after the umbilical cord is separated

Tm: UBIEIIJCAL Conn

The early formation of the body stalk (or the connecting stalk) is discussed elsev». here In the blastoc} st stage the extra-embrg oiuc inesoderin between the trophublast and the primitive yolk sac splits up to the rooflot the amnion, leaving a mass of cells connecting the embryonic plate with the ttophoblast This is the beginning of the body stalk Later, the mesoderm that is formed from the primitiie streak of the embryonic plate, also migrates to the body stalk Due to the further growth of the embq o, the expansion of the amnion and the subsequent appearance of the head and the tail regions the body stalk increases in length

At the same time the allanto-enteric diverticulum grows into it As the rapidly expanding amnion covers it on all sides, the yolk sac “lnfill is now transformed into the narrow yolk stalk, and the vesicle with the vitelline blood vessels also enter into it Thus, by the sixth week, the body stalk groiis as the umbilical card through which blood sessels from the chorion vi: the two umbilical arteries and a vein pass The mesoderinal cells of the umbilical cord are comerted into a gelly like mass 1-mown as W7'mrtou's jelly

During the second month, the intestine of the einbq 0, for a time is pushed into the proximal part of the umbilical curl as a transitory umbilical hernia (Eu; 100) Soon

the intestine is withdrawn By the fourth month, the 301k stalk and the \-itelline blood Vessels atrophy and the allaxito s

is reduced to a thin epithelial strand The full term umbilical cord is about Q inch thick and THE F01-ZTAL MEIUBRANFS 3’?

about 2 feet long more or less of the same length of the foetus Variations in its length and its coiling around the neck or body of the foetus often leads to many complications during delivery The cord shows many spiral twists, probably due to the circular movements of the foetus in the amniotic ﬂuid The cord is usually fixed to the placenta not m its geometrical centre, but to one side In rare instances, the card may he attached to the margin of the placenta

AB\0RMAL Srri-:5 or IMPLANTATIDN

When the fertilized ovum does not reach the uterus but develops at other places, the condition is known as extrautern-e or ectopic gestation The ovum may dexelop in the uterine tube (tubal pregnancy) or may rupture into the abdominal cavity and become attached to the peritoneum (abdominal pregnancy) In some rare instances, the ovum does not escape from the ruptured Graafian follicle but becomes fertilized there In all such cases the embry os do not grow for more than three months and surgical aid must be invoked for their removal

Sometimes, the Implantation of the ovum takes place near the cervix of the uterus the cervix being occluded by the developing placenta This is known as placenta PTGEDIII

Aiwominunss or THE Foimu. Mimaimnrs CHORION AND PLACENTA

Accessory Placenta (Placenta Siicccnturinta) Ari accessory piece of placenta becomes separate from the main mass 0“ mg to early separation of a part of the chorion frondosum

Fused Placentas Occur if nuns are implanted clo e together

Lohcd or Notched Placenta Owing to irregularities of growth

llydatitliform Mole (Vesicular ltlolc) Oiiing to dege ncrotion of the chorionic villi which are modified into small xcsicles containing a watery fluid The einbno Lll(‘S for \\ ant of nutrition 33 H'UM.AN EMERYOLOGY AMNXON AND Your SAC

H}dl'5.l'nhl0S There is excess of amniotic ﬂuid (over 2,000 ml) due to inability of the foetus to ingest and absorb the fluid This may be due to circulatory disturbances in the ioelus H}drocepl1a1us, anencephaly and spina bifida may be associated Wllh l-iydi-ammos

Olignrnnios There is less amniotic ﬂuid (less than 500 ml ) probably due to more absorption by the embryo

Meckel's Dherliculum of Ihe llenm This Is caused by the persistence of the proximal part of the yolk sac It may re. mam as a blind sac or extend as a cord to lhe umlnlicus

Fm 23—MecL2l’s iiwerm-alum (From Pathology Museum, T N N College)

Umbilical fislula The persisient part of the yolk sac extends to the umbilicus as a tube. discharging 739°“ maltei’ THE FOETAL MEMBRANE ma UMEILICAL Cox»

Variations in the attachment of the Umbilical Card to the Placenta The card may be attached to the margin or iii any abnormal position owing tn the irregular attachment of the body stalk to the choi-ion frondosum

Umbilical Hemin The intestines temporarily heimate into the umbilical card If the intestines do not return to the abdominal cavity umbilical hernia is caused Some times a secondary protrusion takes place after the intestines are withdrawn from the umbilicus (fig 105) CHAPTER V THE EXTERNAL FORM OF THE EMBKYO Tai: PRENATAL Devuopmem

Clmrcally, the prenatal development of the embryo is divided into the following stages ——

I “The Ovum Stage" The fertilized ovum undergoes the cleavage stages while passing through the uterine tube The blastocyst becomes implanted into the uterme mucosa and the primary germ layers are established during the first three weeks

ll “The Embryonic Period" From the beginning of the fourth week to the end of the eighth week, important changes take place in the embryo with the formation of the various organs and systems, the story of which is narrated in subsequent chapters

III “The Foetal Period” This is from the third month onwards till birth The organs that are already established grow more and the e*<ternal form of the body becomes characteristic

For descriptive purposes, the period of intra uterine development is classified as the presomite and the smmte stages, depending on the formation of the body segments The first twenty days of growth are in the pi-esomire stage After this, the first pair of somites appears on both sides of the noto» chord The subsequent ones are added on posteriorly By the thirtieth day all the somites are demarcated Usually, there are about 42 to 4-4 somites distributed as follows —Occipital 4, Cervical 8, Thoracic 12, Lumbar 5, Sacral 5, and Coccygeal

8 to 10 A few human embryos have been thoroughly studied

by serial microscopic sections It is difiicult to estimate the correct age of an embryo from the clinical history of men struation alone as fertilization may occur a day or two after ovulation The followmg descriptions are based on the study Tl-IE EXTERNAL FORM OF THE EMERYO 41

of a few embryos recorded by emment embryologlsts The ages mentioned are calculated from the probable tune of fertxlxzatxon

Mrmoos or 1\Iuso1uNc Evumros

The presomxte stages are measured 1n “the greatest length”, Width and lhxckness As the embryo after the fourth week shows a curvature of the body, measurements are taken in a lme from the top of the crown to the rump. usually known as “C R ” length In the later months, the foetus IS measured from the crown to the 111p and then to the knee and to the heel

fiG 24 -—I|Ieasurmg C R length of embryo by Lermer colllper (shmcmg embryo, 14 mm C R length)

Gxoxvrn or m: Emanvo

The followmg IS a short summary of the chief ewcnts occunng In the external form of the embryo The measurements are only appnromatxons and there may be many varaauons m the degree of growth 42 mmmn mmxvowcv

1} days 0 323" '3 z:::s;;*::.., °'!' ‘ X9 1: J "‘ 1

rm eewzee

.. 9 éfifi Q DJIIYQKIC PI-Afl »-Q-.

Iusrannmz: uanon VBICL!

fiG 25 —Secmm of human embryo, 7} day: After Hertxg and Roe!» from Am Jour of Obat & Gun 1942 Suze 0.3 x 0.3 mm The blaszocyat 1.: not round but flat and u enuelaped by a cove:-my of traphoblzm The Inner cell mass as nude :1: of ectuderm and endoderm layers A small ammatce cauty eruts between the trophablaxt and the cctoderm The cameu between the trophoblaet and the endodenn u the blustadermxc veszcle

13 d3):

AMNION

Ecrcnntu lam STKLI nmcbnlu

‘n 4,‘ mnaasm or from an:

. - :21-nu.-uunomc ,‘ ,I\ Mann:-.:u

YOLK nu:

Fm 26—SeeIm-n. of human. embryo, 13 days After Tot-pm, 1941 S122, 15 x 1 7 x 1 7 mm Embryonic P1539» 310 X 215 microns The chanmuc mil: are lust Itmnmg Arnmotu: cuuxty LS‘ Large The embryorm: plate I: made up of 9305977" ""1 endaderm The yolk sac I3 large THTZ EXTERNAL FORM OF THE EMBRYO 43

18 days Amman (cm)

‘tall sac nusrorou

fiG 27 —Surface mew of the emluryomc plate (Am-man cut ofi and yolk sac partly vxszble) Age about 18 days After Heuser, 1932 Sxze, 9 ‘C 14 x 15 mm Embryonic plate, 680 X 1,250 mtc-rous The embryomc plate Ls pear-shaped The body stalk IS dtstmct- The pnmmve streak and the bla.sta« pore are prmmnent The notochorcl :3 formed The mesazlerm 1.: s'p1-eadmg between the ectoderm a.7u:l emioderm of the embrymuc plate The neural groove IS begtmung to form The char-zmuc mlln are well formed

22 days

NEHML rem YOLX SAC

Immcm (cur)

ms? Ntuaoroal: ‘BODY s1'Al.x

£7 gf K ‘

fiG 28 —Human. embryo, about 22 days Illatlnficd from Payne, 1924 Sue, 21 mm long, 7 sommzs The embryonic plate takes a cl/lmdncal shape The head and tall folds are clear The pnmmne streak and the blastopore are pushed to the caudal and The -neural folds of the ‘neural groove between the 4th and the 7th somxtes have closed as the neural tube The neural fold: an-r the head regxon are large 44 HUMAN EMBRYOLOGY

25 da)s

YOLK SM:

Post ntuxopax:

Fm 29——Human embryo, about 25 days Modified from Heuser 2 5 mm long, (X38) I4 sormttes The fusum of the neural folds eztemlx antenarly and pastenorly The unclaserl antenor end is the antenor neuropure and the unclosecl pas

tenor end IS the posterior neuropore The head u slzghtly ﬂexed The heart and pencnrdmm are pfmnment The manllary mzndxbular and hymd arches and the 1 & II pharyngeal grooves are clearly seen The prxmxtne mouth camty ‘LS lzke n dxmple and the buccophury-ngeal membrane 1.: nbmzt to rap

zure The am: vesicles are clearly vmble THE EXTERNAL FORM OF THE‘. EMBRYO 45

30 31 days

You SM:

new SYALK

Fm 30 —Human embryo about 30$1 days (X20) 4 mm C.R length, 30 somucs The neurapcvres are closed The head

5 ﬂexed and the iorebmm IS p1-omment The tail is bent to

wards the umbtllc-us The [H pharyngeal arch rs formed The one vesxclcs are bur-zed under the surface ectoderm

5 \\ ccks

A human embryo about five weeks Snze, 5 mm CR length 40 to 44 smmte: shows the following —Thc head is more ﬂared and as prominent The fare and the hind limbs begun to appear The bulgmg of the heart and the pcncardmm ts praumncnt The out and the opuc vesicles and the olfactory pits are vmble The roof of the hmdbram Is thm The phamuneal arches arc dunnct 46 HUMAN EMBRYOLOGY

6 weeks

soar or mnnsum

one VISICLE

arm: vzsrcu: ronzsxam

UPPER Lnamfb

Fxc 31 -—Human embryo, about su: weeks (X7) 122 mm C R length All xmmtes are formed The head has Increased In SIZE The ueszcles of the forebram are apparent The I pharyngeal groove ts deepenmg and the rudunents of the external ear are seen us thwlcemngs of the mandxbular and the hymd arches The Lhgxts of the hands are begmnmg to form The belly :5 bulged out due to lmer and heart The

ll.1nb1llC1l5 ts dxstmct THE EXTERNAL FORM OF THE EMBRYO 4"

7 weeks NECK M) 31' EAR ( DIGITS OF HAND f DIGITS 01‘ FOOT UHEILICIIS

fiG 32 ——Human Embryo 7 weeks 17 mm CR length The head has mm-eased mun m SIZE and is less ﬂexed The neck begms to be formed The cxterrml ear is mstble The cyelrds

are just formmg The dtgzts of the hand are dvstmct and those of the feet are 51:11 umted

8 xx ceks

fiG 33 —-Human embryo 8 weds 30 mm CR length For -mutton of the -neck The eyelid: are clearly seen The inter nal ear xs dzstmct The face as human. :11 appearance The bulgmg of the belly zs lessened The umbnlxcal card becomes

uarrou. The dxgazs of lhr: hand and {out are dtslmct The [ml has atrophted 43 HUMAN EMBRYOLOGY

fiG 34 —Human embryo, 12 weeks 55 mm CR length The head becomes larger Forehead Is broad The euehd; are closed, the ltd: are adherent by thenr Bplfllﬂllﬂl cove:-mg Haw appears throughout the body The external 32:: charac ters are dutmct

16 Wed:

A human embryo, 16 weeks 100 mm C.R length, shows ... The face :3 null broad New growth of haxr appears other character: are as m the 12th week

20 weeks The foetus show: movements m the ﬂier-nu Nails are Just seen The eyelzds null 're17uu11, closed

2-! weeks The eyeltds are no longer adherent and the eyelashes are

Lmble The elem I-l‘ 1D1'HI.l<:led an appearance

28 weeks Hart on the scalp grows longer Submttanemzx fat makes 2.: appearance Verm: caseom covers the body

From 20th week to the full term foetus

The Increase In length of the foam: depends on the xndwn dual, the state of nutntxon and heredity The av. erage values are -20 week: 130 mm 24 week: 200 -mm 28 weeks 230 mm 32 weeks 265 mm 36 weeks 300 mm 40 week:

335 mm , lF\.r CHAPTER VI DEVELOPMENT OF THE CIRCULATORY SYSTEM Ti-ii: HEART

As soon as the mesoderm is established in the embryonic plate it becomes a loose collection of cells bets‘. een the ecto~ derin and the endoderm and as the latter extends out as the archenteron the mesoderm becomes continuous with the extra-embryonic mesoderin at the margins At the cephalic end of the notochord the mesodexrnal cells arising from both sides advance towards the extreme anterior end of the embryo in the form of an arch and appear as a crescentic mass of thickened mesoderm (fig 11) This region is the place of origin of the heart and the lining of the pericardium The mesodermal mass becomes continuous xuth the mesoderm over the yolk sac A circular space in front of the notochord is devoid of any mesoderm This region “here the ectoderm and the endoderm are still in apposition is the buccopharyngeal -membrane

The lateral plate mesorlernx new splits into two layerslhe parietal or the somatic in contact iuth the ectoderm and the splanchnic or the visceral in contact with the endoderm The space between these la) ers constitutes the primitive intra embryonic coelom The crescentic anterior portion of the mesoderm splits into a horse shoe shaped cavity—thc primi tiu: pcricardizil caiity, Vtllltll is continuous with the primitive coelom. The unditided mass of cells on both sides of the notochord later sub-divides lengthwise into the body segments

The splzinchnic mesoderrnal layer of the primitiie pericxirdial cwity on either side of the cephalic end is the site 1!‘. ‘origin ‘(.6 ‘i‘nt- ‘xnmr-. 2; uﬂrtlcmiii tfi 1.-ri€m‘i‘ire‘rrh t:e‘i‘d -pro

brihly arises in situ from the splnnchnic mesoderm and lies ht.-ti‘. een it and the endoderm on either side of the priirutive pcriczirdlzil unity The) acquire a lumen and form a pair of endolhelial tubes Soon the splarichnic mesodcrm around the

4 50 HUMAN EMJBRYOLOGY

fiG 35 —TmnsLe1se section of eaily embryo, showmg the pnrnordxa of the heart

endutlielial tubes becomes condensed and differentiates into myohlasts The endothelial tubes with their covering of myoblasts constitute the primordial heart tubes

The yowth activities of the cephalic part of the embryo, at about the end of the second week, are markedly rapid The expansion of the amnion, the acute bending of the head region, and the shrinkage of the yulk sac are all processes occurring in quick succession The endothelium and the xnesodermal covering of the two heart tubes are brought closer together and finally fuse in the midline Just ventral to the foregut, at the leiel of about the third or the fourth soimte

The heart at this stage is a double walled tube, functioning as a pumping organ and lying in the pericaxdial C3\'lf), being held in position at the caudal end by the common entrance of the vitelline Veins from the yolk sac, the umbilical veins from the chorion and the common cardinal veins from the body of the embryo The entrance of these veins causes a dilatation at the caudal and known as the sinus uenosus The cephalic end pmiecls out of the pericardium and becomes continuous with a rudimentary blood vessel formed in the maoderm of the first pharyngeal arch on either side This vessel constitutes the first pharyngeal arch artery At the same time, a blood sessel is formed from the dorsal side of the mesoder-in on each side to form the rudimentary dorsal aorta Hence, the cephalic end of the heart tube is contimiaus DEVELOPMENT OF THE CIRCULATORY SYSTEM 51

Fm 36 —SL-hcmatu: saglttul 1-epresentatwn of the cephalzc part :2] human embryos from the second to fourth week, show my the growth achvmes and relative pasmons of the bfdlll, pcncardmm. and heart (Mesodenn is not shown)

1 -—At about second week 3—At about tlurd week Z-—/U two and hall an-ck: 4—/lt {aurth week 5’ HUMAN EERYOLOGY

fiG 37 —Travm.e1'se seclurn nf embryo :1 owmg the fusion of the heart tubes

MIIOYHTI nmoennmn

DDIYOCARDHTM

fiG 38 —-Trmm.-er.re section of embryo ﬂwwmg further stages In the fusion af the heart tubes

mth the fix-st pan of aorhc arches pamng round the pharynx and connected to the dorsal aorta on exther sxde

The endothelxum becomes the endocardxum of the hear! The epuny ocardna] mantle dex elops mm the myocardxum and the fibrous nssue and the emveiopmg viscera) 1;; er at the

pencardaum Dunng the rapxd fusmg of the heart tubes :1 double membranous fold holds the heart (ubes to the daxsal

wall of the pent.-ardnum Thxs 15 the dorsal Inesocardxum \\hlCh masts only for a short time to disappear enuxe); along mth the further growth of the embna (figs 37 and 38) Thus DEVELOPMENT OF THE CIRCULATORY SYSTEM 53

itonric ARC)! V Bonus conbis

Fm ‘$9 —Vcntriil view 0! the early heart, after the

vznriiicu fusion of the primitive heart tubes ii-muivi . SINUS vziious

the heart is freely suspended in the pericardial cavity being attached to the caudal and cephalic ends by the blood vessels only

§\TllUNCDS \ AllfE'lUO:US

Fm 40—Heart, at about 3 weeks (Ventral mew)

Vnnnictl:

ATRIUM

By the third \\ eek, the simple heart tube rapidly elon gates and changes its shape and size, since the cephalic and the caudal ends are fixed by the pericardium The heart tube first bends into ti ‘ U ' shaped loop toward the right and then xnto ci double “S" shaped curve The caudal end where the veins enter is the sums Uﬂ7IOS‘llS A sheet of mesoderm at the junction of the embno and the yolk sac ai-iteriorly also migrates caudally along “fill the changed position of the heart This is the scptimi. trons-oerxiini Vthlch becomes interposed between the stnus xennsxis and the entrance of the veins at the caudal end of the heart

At this stage, the following regions can be demarcated in the growing heart The bmad sinus ueiios-us is continuous “llh the air-iiuvi the bend towards the right is the tentricle 54 HUMAN EMERYOLOGY

fiG 41—Hea.rt, comis

at about 33

weeks (Ventral mew)

|7Mmuciu. vmw

between the atrium and the ventricle is the narrow amoventrlculur space The ventricle 15 continued into a further bend upwards as the bulbus cordis, which tn turn is continued as the chief trunk of the artery or truncus arterzasus Ths trunk now bilurcates mto the first pharyngeal 4'-L|‘Cl1 arteries on either side to communicate with the dorsal aartae Gradu ally the loop between the bulbus cordis and the ventricle nan-ows into the bulbowentncular sulcus The sinus venosus is divided into a right and a left portion and merges into the atrium Soon the left portion of the sinus venosus becomes reduced in size and slowly atrophies into the coronary sinus due to the deviation of the ﬂow of blood stream entering into it through the growing liver by a newly formed Venous channel in the liver called the ductus venosus Consequently the right portion becomes prominent and the blood enters into the right side of the atrium The heart which was otigznally at the level of the third or the fourth gomite graduallv migrates more caudally to the level of the eighteenth to the twentieth somite

By the end of the first month there is a further increast in the size of the heart to accommodate more blood entering into it The atrium is pushed more towards the cephalic side and expands on both sides of the bulbus cordis The bulging of the atrium produces a groove in the middle “here the bulbus cordis rests The bulbo—venti-icular sulcus becomes less prominent and finally the caudal part of the bullius fuse: DEVELOPMENT OF THE CDICULATORY SYSTEM 55

with the expanding ventricle The atrio-ventricular space becomes Lonsti-icted like a collar to form the atria ventricular groove externally and a narrow canal internally

i Aoiiric Iuicii

LDT RTRIDM

IULBUS coimis

L1’ vi:m'iuCLi:

RIGHT VIIITXXCLE fiG 42 —VentrI1! view of the heart at about five weeks

Tu: RIGHT AND LEFT A-niIA

During the second month changes take place in the expanding atrium which bulges out on both sides of the bulbus cordis The sinus venosus has already shifted to the right side and drains into the right portion The opening of the sinus ienosus is guarded by two valves a right and a left one These valves fuse together superiorly as the septum spunwrn and inferiorly fade into the lower portion of the right side of the atrium At the same time a sickle shaped septum appears inside the roof the grooved middle portion of the atrium to the left of the septum spurium and grows towards the alno ventricular canal This 5 the septum prtmum Ah opening of a single pulmonary vein is seen in the left atrium to the left of the pnmary septum During this stage the endomrdial lining of the atria-ventricular canal becomes thick projecting into the canal as two elevations-a dorsal and a ientral cushion These endocardial cushions meet and {use together along with the extension of the pnmnry septum dlviding the ntrio-vcnlncular canal into right and left portions 55 HUMAN EMIBRYOLOGY

As the pnmary septum L5 ongmally slckle shaped 115 free border does not at first reach the enclocardlal cushmns and there remams a temporary opening called the foramen ‘prm-um between the two atrial cavmes This foreman IS fluwkly dosed bk’ further growth of the free border of the septum wxth the endoeardnal cushmns

1.1’ COMMON cub V11!

If IYIIVM

An. vntfn. aloov:

In‘ vnmucu:

fiG 43—Darml. mew of the heart at about the begmnmg of the math week (From a. model by Zuegler)

Sxmultaneously the upper part of the primary septum breaks down leavmg a commumcatxon called the foreman secumlum between the two atr al cavltxes Tlus 15 caused by a dnfierence in pressure 1n the mo chambers The nght atnum recenes all the venous blood uhxle the left atnum tecen es practxcallg. a negllglble amount from the undex. eloped lungs

By the seventh week, another crescentxc septum known as the second septum appears :1: the “all of the right etuum between the left \enou.s valve and the pnmar; septum As It grows xts concave edge comes tn the rxght sxde of the upper free margin of the pnmary septum and Sllghﬂ) beyond 1‘ Its growth stops here formmg an oblxque communication 1!! DEVELOPMENT OF THE CIRCULATORY SYSTB1 57

szrrvn snmm

mm D,‘ . I - YIILILVEIII smus vmosus > ATEIGVENTI J cusluon

Fm 44 —Schematxc semen of the heart at about the fifth week

tween the two atna This IS the Iammzm ouxle The foramen ovale IS thus bounded by the lower margin of the second septum and the upper margin of the prunary septum and penmls a ﬂow of blood from the right to the left atrium only

Pnmuutr nmum: snvrun s§é’5‘§:'=“€{'4 1sa:L‘:urx‘i':’c§ zunocn SE0“ SE1’ nus!-non munm

XHTEIIAPKML

GROOVE ~ YIIHAE FOIAMEN - SEPIUH nlmm '

ML vnrr cnoovz

I:2mucu:

EIVDOCAID cusmow

Fm 45 —Schc-mane stages I1: the for-nmtxon :7] the fornmen male (viewed from the nght nlrlnm) 58 HUMAN EMBRYOLOGY

SECOND s!:n'I7M Pm. Vmt

st? srlnutm

sums vzxosus

rmumna

RT V"-‘"3 srcmmuwl

at Amxovmrmcvux l.'r Arnxovxrnrxxcuux

cuuu. cnmu. F If VDITBXCLE n-r vumucu: \SEH'|!M fiG 46-Schenwtxz: sectxon of the heart at about the surth

week

As the left an-mm xs filled up the upper margm of the pnmary septum which forms the valve of the foramen ovale presses on to the second septum and prevents the back How of blood Hence blood from the nght atnum reaches the bulbo-ventn cular cavnty through two distinct c1rcuxts——one straight through the nght atno-ventncular canal and the other through the foramen uvale to the left atnum and through me left atno-\ entrlcular canal

At bnrth when the pulmonary cxrculauon :5 established the furamen male becomes closed due to the fuswn of the free margms of these septa with each other A small depresslon bounded by the cancave free edge of the eeptum secun dum known as the {asset ovulxs XS seen on the nght of the mter atrxal septum The lower margm of the second septum becomes the annulus mmhs Thus the xnteratrlal septum is completed after bxrth

The smus venosus has already merged 11110 the nght atnum The supenor and mfenor vena cavae and the care DEVELOPMENT OF THE CIRCULATORY SYSTEM 59

nary sinus open independently into the atrium The left talve of the sinus vcnosus fuses with the interatrial septum The right valve becomes thick by about the third month and gradually undergoes diminution in size and is ti-axistorrned into the valve of the inferior vena cava (eustachian valve) and the valve of the coronary sinus (thebesian Valve) The septum spurium becomes the cnsta termuialzs As the left atrium progressively grows in size the pulmonmy vein IS absorbed in the atrial wall and its four tributaries open mile pendent]; into the atrium

THE Vr:m'iiici.rs

By the fifth week the bulbus cordis and the ventricle have {used together The line of fusion is indicated by an oblique sulcus on the surface Internal to this sulcus a crescentic ridge grows toviards the endocardial cushions of the atrio-ventricular canal. This ridge forms the mterver mculur septum The area between the crescentic edge of the septum and the endocardial cushions is the inter ventricular forimicn through which the ventricles are in communication

By this time changes have taken place in the lumen of the truricus arteriosus continuous with the bulbus cordis The cndocardium has outgrown into thick masses or ridges in apposition with the corresponding ridges from the opposite wall These appear in E spiral manner and as the ridges come in appos tion and fuse together as a septum the original truncus arteriosus is partitioned into two spiral canals These separate from each other forming the aortic trunk leading to the fourth aortic arch and the pulmonary trunk leading to the sixth aortic arch The bulbar ridges extend as a right and a left ridge Owing to the oblique position of the inter \ cntricular septum the right ridge becomes situated above the right :itrio—vt-ntncular canal and the left one comes in contact viith the anterior flap of the inter ventricular septum It so lappcns that after the final closure of the inter ventricular foramcn the left \(.‘ntHClE communicates with the aortic trunk and the right tentricle with the pulmonary arter3

The intcr-Vcntncuhr foramen is gradually closed by :i 50 HUMAN EMBRYOLOGY

men 1'! vzm1ucu: Pl-Ion. E! v‘:x'rucu:

Hun. urn! Aoxmco mun smw Aczzm

Synuu. xoanconnuzu stzmm

27 3271321: xma: Y mums ma: zuxsow xx snow m H 3 ma‘: It vnrmcu a 3-1.! An vanCANAL H1’ APB. VDn'1L 3 cnmu. J ‘.3 ‘*1? vnrmcu: I11’ v:m1uc1.:

vtnralclruka szrnna

fiG 47 —Schematxc section of the zenlrxcles sltawtng the for mutton of the hulbar septum. and the establxshment nf Aornc and Pulmonary trunks (Lentrul mew)

mass of connective txssue denved from the nght and left bulba" ndges and fmm the endocardlal cushions of the atno-sentn

culax‘ canals and also to a small extent from the free border of the utter ventncular septum ltself As the left ndge comes tn contact “nth the antenor flap of the xnter sehtncular septum these mu become {used together by the grouh of ccnnectne nssue from the left ndge In a sxmtlar way the fight ndqe {uses with the postenor ﬂap of the mter xenmcular septum

Sxmultaneouslj, the mo ndges fuse together as the bulbar septum thereby etfectmg the closure of a large portxon of the mter tentncular fox-amen Further connectne tlssue growth from the nght sxde of the endocardxal cushxons together mth a small mass of ccnnectne txsme derned from the Inter

tentncular septum xtselt finally close the Inter \enh’|cu‘;aX‘ foramen Because of the complxcate closure of the mtet senm

cular fox-amen there may be possxlulxtxes of abnormalxttes In DEVE1.0P‘\lEN'l' OF THE CIRCULATORY SYSTKM 61

ll‘! Aaaﬂcomvunn

SDTUM M mm“! 1.1’ Agancvfinuunx L! wulux Inna: at RTBIOVDITB cmuu. Lr Ilmrc vnrm. cuuu. VENTR stpnm ll‘! nuaocruux Cusmnx

Fm 48~—Sch2matxc semen of the uentncles showmg the fusion of the bulbur rxdges -wnth the -Levilnculav‘ septum (viewed from fight ventricle)

L‘: sznuu

In’ IULIAR lune:

raster: or IULIAR nxxxzxs H’ mm” caavrm man at Ema:

nuulul lube: caowm man

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camu.

Gxowm no

DmocJuu> cannon: vnms SDWM

Fm -£9 —Schcmuhc sectum 0! the tentrxcles Sh0ll7l1lg H c later stages o[ the {nsxon of (I12 bulbar rxdgcs 1nIh the lrntnculnr septum (vunned from right ventricle) S2 HUMAN EMBRYOLOGY

the region of the ventricular outlet of the heart The composite mass of connective tissue which closes the mter ventricular {oi-amen later become: thin and fibrous and is called the pars membrariecei: septs Thus the ventricles are separated by the inter ventricular septum the right ventricle in communication with the pulmonary artery and the left with the aortic trunk The right and the left atria-ventricular canals open into the right and the left ventricles respectively

fiG 50 —Section of an embryo through the thorax Ti-ii: A-mo Vnwmwun Vzinvss

In the region of the atrio-ventricular canal the anterior and the posterior endocai-dial cushions have already divided the passage into two atria-ventricular canals In each canal from the endocardial cushions and from the adiacent wall: thick ﬂap-like proyections of the enducardiuvn containing ‘ll’ DEVELOPMENT OF THE CIRCULATORY SYSTEM 63

connective ussue elements under It appear These are the cusps of the aims ventncular valves-three on the ugh: and two on the left They are connected to the muscular wall of the ventncles by many bands of muscuiar txssue Gradually these bands are transformed mm the papluarj muscles and lhxck chordae tendmeae

Tm‘ Ssvnnumm Vuvr: or ‘me ADRTA um -me PULMONARY Axum!

wlm. or nuuns coams

cavm or ‘Hume. Am‘

—-—-lwunn mum:

ACCBSORY xmc:

HAM Jlnrzav

wnu. or rum ARTERY

fiG 51 -—T1a1I:Lcrsc sections a] the bulbus cordu shownq

the farmanon of the semrlumzr valves of the Aorta and Pulmonary trunks G4 HUMAN EKIBRYOLOGY

By the filth week, in the lumen of the truncus artcriosus and the bulbus cordis, two prominent endocardial hillock like elevations appear from the sides of the inner V\ all, in a qiiral manner The summits of these ridges meet and fuse together as a spiral partition dividing the original trunk into the aorta and the pulmonary artery The sides of these elexations still project as slanting masses in each partitioned trunk As the aortico bulbar septum fuses with the inter-ventricular septum to establish the ventricular connections with the aortic and the pulmonary trunls, the proximal portion of each trunk contains three enducardial elevations-—twu forming the halves of the main eles ations and a third smaller elevation arising from the luman opposite to them These are later transformed into the three cusps of the semilunar val; es of the aorta and the pulmonary artery The remaining portions of the endocardial elevations disappear due to the comparatively faster growth of the surrounding tissues

DE!-‘ECIS or DZVEIDPBXEYT or THE HEART

Aimrmtous SITUATXDVS or THE HEART

Higher Position of the Heart Normally the de\ elop~ loping heart migrates from the level of the third and fourth somites to the final position between the sexenteenth to twentieth sci-nites Sometimes there ma) be a defective descent

Ectopia Cordis The heart protrudes through the thoracic wall probably due to the defective formation of the sternum and pericardium

Dextrocardia with Situs liiu-rsus ahdnminalis The heart remains in the right side with the apex pointed to the right (Associated with complete or partial situs mversus of

other viscera ) Isolated Dextrocardin The heart remains in the right

side xuth the apex pointed to the right associated with normal position of other viscera Other mzilfnrmatiuns of the heart such as a single Ventricle transposition of the great

vessels are common DEVELOPMENT OF THE CIRCULATORY SYSTEM 65

L(!\0C€H.'l'l|a The heart remauis in the left side of the thorax with the apex pointed to the left The chambers of the heart and blood vessels show normal anatomical positions The abdominal organs are in position of situs inversus (Other associated malformations are common)

DEX-’E(.”K'5 01-‘ THE Ariuai. SEPTUIVI

Patent Foramen Ovale is due to any of the following causes —

1) Failure of anatomical fusion of septum primum and septum secundum

2) The opening of the foramen secundum in the primary septum may be larger than usual and the free edge of the septum secundum does not guard the opening

3) Incomplete growth of septum secundum and the con sequent mahility to guard the opening

4) The tear in the septum primum may take place at a caudal region and hence the septum secundum may not reach it

Premature Closure of the Foramen Ovale This is dun to overgroiith of septum secundum and the fusion of the valve of the foramen ovale to it

Absence of Atrial Scptum, Cor Ti-iloculiirc B“ enlricularc, Cor Ililocularc Absence of atrial septum is caused by failure of development or vestigial septum primum Cor Triloculare Biveritriculare occurs when accompanied by presence of ven tricular septum

Cor Bilociilare occurs when both atrial and ventricular septa are not formed

There may be other associated anomalies The prognosis usually depends upon the sufficiency and type of blood flow to the lungs

Climri l\et iioi-k Normally the right value of the sinus VEHOSUS is transformed into the valve of the inferior iena cava and the Val\|'.‘ of the coronary sinus The left valve l.S reduced in size and {uses with the septum sccundum Sometimes, these valves are reduced to a net-work of fibres extending from the region of the crista terminalis to the valves

5 66 HUMAN EUBRYOLOGY

05 the mfenor vena cava and the coronal‘) smus or to thratnal septum

Lu(emhaeher's Syndrome Thvs condmon 15 due to atrxal SEPWI defeﬂ (patent foramen ovale or defect anywhere else m {be atrnal septum) Tlus may be aSSOClalEd vuth narrowmg of the mxtral orxfice

SALn:z~:'r Ponrrs Bx -rm: Cumcm. Dmoxosxs or

A-nun. SEfiAL Drrrcrs

G-35111 and F611‘ (1956) state “A systohc murmur of only moderate xntensxty, maxunum over the second and thud left nntexspaees, wxth a reduphcated second pulmomc sound 15 present when (here 15 an anal septal defect Electrocardiograms almost always reveal an mcomplele ugh! bundle— branch block. The presence of a mxddlastolxc rumble IS com mon 15 the heart xs definitely enlarged and, m the overwhelmmg malomy Of 03555, does not mean the presence of assocxaterl rmtral stenosxs (Lutembacher syndrome) but denotes a relative stenosls due to xentrxcular dxlatatxon Fluoroscopy and roentgen films may he \\'lLl'A1n normal lxmxts 1f the septal defect and the left-to-nght shunt are small, or (here may be definxte cardxac enlargement, a bulge over the pulmonary area, m creased pulmonary vascular markings, and pulsauons If the defect and the shunt are large There Is no enlargement of the left alrxum and no dxlatahon of the ascendmg aorta

In pahents with ostxum pnmum and a splxt nut:-al valxe and or ventrxcular septal defect, this murmur is heard lower down, and there xs usually a loud blomng s}stol1c murmu" ox er the apex Dyspnea rs common, and the heart :5 almost always enlarged The electrocardlogram In (hese patxems usually shows left axxs devxalxon In the standard leads and nght xentncular hypertrophy 11: the preccrdxal leads "

‘ GASUL, B M, and FELL, E. H. (1936) Salient poxnfs In the clxmml dxagnosls of congemlal heart disease J Mner Med Ass I61

39-14. (By courtesy) . DEVELOPMENT OF THE CFRCULATORY SYSTEM 67 DEFECTS or Vimrrucuuxn SEPTLIM

Persistent lnterventricular Opening Tlus is due to defect» in the muscular portion (rare) or in the membranous portion (more common) The septum is normally formed from the downgrowth of the hulbar ridges and the atrioventricular canal cushions and from the crest of the ventricular septum Any defect in the complicate fusion causes persistent inter» ventricular opening

Absence of Septum The septum is not formed or may be rudimentary

SALXENT Pom-rs IN THE CLINICAL DIAGNOSIS or VENTRICULAR Saimii. Derec-rs

Gasul and Fell‘ (1956) state “Criteria for diagnosis of ventricular septal defects may be divided into those for small defects, those for large detects and those for defects with reversal of, or with, bidirectional shunt

Smnll Defects The diameter of the small defect Ls considerably smaller than one~half the diameter of the aorta, and the shunt from the left-to-right ventricle is small No symptoms are present, and there are no cardiac findings except those found on palpation and auscultation There is ti harsh loud systolic murmur maximum over the third and fourth left interspaces A S)SlDl)C thrill may or may not be present Often. the murmur is sof. “puﬂy", and short The second pulmonic sound is normal

Large Defects There is often a history of pulmonary infections when there is a large defect, especially during the first two years of life A hyperdynamie apical impulse, a thrust ox er the pulmonary area and. often. also ox er the lower parasternal region, and a pi-ecordial bulge signifying leftand-rlght ventricular hypertrophies are usually present A systolic thrill and loud systolic murmur is maximum Over the thlrd and fourth left interspzices and heard better oier the filth left intcrspace than over the second The second pulmonic sound is loud nnd strong 68 I-IUMAN EMBRYOLOGY

Fluoroscopy and roentgen films reveal a bulge over the pulmonary area marked increase in pulsations and size of pulmonary vascular markings and usually an enlarged left atrium Electrocardiogram reveals predominsintly left ventri K.‘JlHl' hypertrophy if the left to right shunt is large and ii there is no marked increase in pulmonary vascular resistance It the latter is present the shunt is smaller and the electrocardiogram shows combined hypertrophy

Large Defects with Reversal of or with Bidirectional Shunt‘ When there is a large defect with reversal of or with bidirectional shunt cyanosis may be evident There is no hyperdynamic apical impulse but the thrust over the lower parastei-nal and pulmonary areas is usualy present Systol c murmur over the third and fourth left interspaces is usually not as loud The second pulmoriio sound is very strong and loud Electraeardiagmm reveals predominant right ventricu

lar hypertrophy

Drrrcrs or PARTTHONINO or riii: Tiiimcus Airraiuosus

Tetralog} of Follut Normally the truncus arteriosus is divided hy the spiral aortico-hulhar septum into two xessels of about equal size If the ridges of the aortico<bulbar septum are located eccentricallv the partition of the truncus arteriosus mw be unequal resulting in a wide aorta and a narrow pul monary trunk This defect interferes with the fusion of the hulbar septum with the ventricular septum and results in the following conditions —

1) Defective formation of the membranous portion of the ventricular septum

2) The aorta arises entirely or in part from the right ientricle shove the ventricular septal defect

3) Narrowing of the pulmonary trunk (Pulmonary steno sis)

4) Enlargement and thickening of the right ientricle The enlargement is due to the large volume of blood entering

through the systemic veins to the right side of the heart 1'1 2 DEVELOPBIENT OF TEE CIRCULATORY SYSTEM 69

thickening is due to its function of supplying blood both to the aorta and to the lungs (Cyanosis occurs as a result of the diminished flow of blood to the lungs and to the ﬂois of venous blood from the right ventricle into the aorta)

CLINICAL DIAGNOSIS

Gasul and Fell‘ (1956) state “In tetralogy of Fallow, cyanosis is not evident during the first few months of life. unless very severe pulmonary stenosis or atresia is present In mild cases, cyanosis may not be evident even in later childhood, except after exertion Squatting is common, and, in severe cases, anoxic attacks characterized by sudden onset of inarlted dyspnea and unconsciousness occur There is usually a systolic thrill and a systolic murmur of varying degrees over the second, third, and, often, the fourth left interspaces In cases of pulmonary atresia, the murmur ma\ be absent The second sound over the pulmonary area is present, and often loud, and is due to the closure of the valxrs of the dextroposed aorta

Fluoroscopy and roentgen films reveal a heart that is usually not enlarged, with the apex elevated. presence of a concavity in the pulmonary area, and diminished peripheral pulmonary vascular pulsations and markings Houever, in many cases roentgen films reveal the heart either to be within normal limits or even to have a slight convexity over the pulmonary area, due to post-stenotic dilatation of the pulmonary artery In the presence of a severe anemia, the roentgen films may reveal a large dilated heart, and the pulmonais rnarlungs may be increased due to pulmonary congestion This finding, fortunately, is very rare The electrocardiogram reveals right ventricular hypertrophy of the adaptation type "

The Iiiscnmengcr Coniplovc The truncus artei-iosus divides into two vessels of almost equal size There is a deficiency of the formation of the hulbar septum in the lower part of the truncus nrteriosus This interferes \'\ltl’\ its proper fusion uith the ventricular septum, causing the aorta to communicate not T0 HUMAN EMBRYOLOGY

only with the left ventricle but also with right ventricle resulting in the following abnormalities 1) Defectiie formation of the membranous portion of the \ entnoular septum

2) The aorta arises in part from each ventricle above the defective ventricular septum or entirely from the right \Enh'.||'.‘le

3) The right ventricle is hypertrophied owing to its double function of pumping blood both to the aorta and to the pulmonary trunk The right ventricle functions as in the embryonic condition

4) The pulmonary trunk arises from right tentricle and is normal in size or wider than the aorta as there is no obstruction to the pulmonary circulation in the great \esseL This may cause excessive blood ﬂow to the lungs (Theie may be associated malformations of the heart Cyanosis is usually of a moderate nature and may appear late)

CLINICAL DIAGA DSXS

Gasul and Fall‘ (1956) state A history of late onset of cyanosu is present in Eisenmenger complex Systolic thrill and systolic murmur of varying degrees over the third and fourth left interspaces and a loud second pulmonic sound are present Fluoroscopy and roentgen films reveal a bulge over the pulmonary area and ixicrea-ed pulmonary vascular marl. ings The electrocardiogram shows right ientricular hyper trophy of the adaptation type

Stciiosis of Ostium Intundihuliim The outﬂow tract of the right ventricle uiferior to the pulmonary Valve becomes narrowed The force of blood through this narrow opening causes hypertrophy of the right ventricle below the stenosis and formation of a thick fibrous tissue above it owing to the iniury caused by the force of hlood

Sub Aortic Stenosis This is caused by a localised na: rowing of the outﬂow tract of the left ventricle The left \entricle becomes hypertrophied owing to its strong pumping action DEVELOPMENT 01‘ THE CIRCULATORY SYSTEM 71

Taussig Bing Complex On account of the defect in spiral ling of the lower part of the aortico-bulbar septum the aorta communicates with the right ventricle resulting in the follo“ mg conditions —

1) Defective formation of the membranous portion of the \€l"ttl'tCulE|!' septum

2) The aorta arises entirely from the right ventricle and remains ventral and to the right of the pulmonary trunk

3) The pulmonary trunk is wider than normal and arise» mostly from the left ventricle and partly from the right ven tricle due to defect in the formation of the membranous portion of the ventncular septum

4) Hypertrophy of the right ventricle

Complete Transposition of the Aorta and the Pulmonary Artery The aorticobiilbar septum dividing the truncus arte riosus does not appear in a spiral manner The bulbar ridges grow straight from the left to the right side Hence the aorta communicates with the right ventricle and the pulmonart artery to the left ventricle These two vessels run parallel to each other The membranous portion of the ventricular septum is absent and is replaced by the growth of the muscular portion Since the systemic and pulmonary circulations are separate this condition is not compatible \\ith life Some times communications between the two circulations may be established by associated atrial and or ventricular septal defects or other abnormalities These tend to prolong life expectancy

Persistent Truncus Artcriosiis Owing to the airest of development of the aortico bulbar septum only one arterial vessel leaves the \enlricular piirt of the heart The coronary and the pulmonary arteries arise from this single vessel There is marked cyano is and life is brief There may be asociated defects of the ventricular septum and also absence of the diictus artoriosus

Dicrecis or Tm: Viiwrs

Persistent lnti-rntrial I-‘uriimcn Pi-iriium The tree border of the sickle shaped septum primum ma) not {use with the 72 HUMAN EMERYDLOGY

ventral and dorsal endocardlal cushions, resultxng m the persxstence of the temporary opemng of the foramen pnmum Consequently, the following condmons occur —

1) The fatlure of the fusxon of ventral and dorsal endocanlxal cushxons causes the atrwventncular canal to remam as a common passage to both sxdes of the heart, as In the early ernbryumc condxtxon

2) The foramen ovale may he formed 11: a normal manner or there may be some defects m the growth of zts mfermr lxmb

3) The rxght an-tum, the nght ventrxcle and the pulmonary trunl: become dllated owmg to the extra stram on the nght sxde of the heart, m allowxng the oxygenated blood retumxng from the lungs to he recxrculated to the lungs (May be assocxatecl with ventncular septal or other malformatwns)

Tricuspid Atresia Absence of the tncuspxd onfice may be due to anomalous for-matwn of the atnal septum, or of the ventrxcular septum or due to foetal endocardxtxs. The atrial septum remaxns patent la) the persistence of the foramen ovale, (as m the foetal ClX‘C|.llalI0l"t) or by defimenc) of growth of the lower part of the septum. In rare mstances the septum us rudunentaxy Smce the blood from both atna enters the ventncular chamber, the mxtral onfice and the ventrxcular cawty remam wxder than normal A patxent wxth this detox-mxty IS usually cyanotxc

Stenosk of Mntral Valtc and Stennsis of T1-icuspid Valtc occur as a result of defectwe formatxon of the valves

Ebstein’s Malformation of the Tricuspld Valte Thus :5 caused by defects tn the attachment of the cusps of the valve Part of the tncuspxd valvular txssue 15 attached normally, but the remaxnder arxses from the “all of the rxght ventncle and from the ventrxcular septum (The fcramen at-ale 15‘ most};

patent)

Mitral Atresia Thus IS probably caused by xmproper tormahon of the valvular ttsues The mltral orifice 1s seen .15 DEVELOPMENT or nu: CIRCULATORY SYSTEM 73

a blind depression causing dilated nght atrium small left atrium and patent foramen ovale

Double Orifice ol the Mitral Valve or of 'l‘rii:uspnl Valic Double mitral orifice may be due to an irregular fusion between the medial and lateral ﬂaps dividing the original orifice into two distinct openings or the extra opening may cause as a result of the persistence of part of the embryonic common atno-ventricular canal According to Wimsatt and Lewis- (1948), before the fusion of the xentral and dorsal endocardial cushions, these fuse together and form an opening which grows into an extra riutral orifice

Pulmonary Stenosis or Atresia Probably mung to irre gular fusion of the valves after the establishment of the primordia of the valves

Bicuspid Aortic Viilie and Biciispid Pulmonary Vnlie

Deficiency of a cusp may be caused by defective formation of the valve or by inflammatory causes

Ciiusizs or CONGENITAL MAi.roiuiA-i-ions or me Hemvr

Tnussig (1947) in her monograph on congenital malior mation of the heart thinks that the aetiology of any case of congenital defect is attributed largely to genetic factors and also to the inﬂuence of environmental agents in the maternal uterus, acquired in pregnancy The exact role of genetli. factors that play a part in the cause of malformations of the heart is not clearly understood In situs inversus, deiitm cardia and other defects of the heart are commonly present Cockayanc (1938) in his review on cases of transposition of viscera states that this condition is transmitted as a recessnc gene He remarks “its familial incidence and general distri hution “lthln in family, its occurrence in both members of .1 pair of monuzygotic twins, and the high percentage of firstcousin marriages that give rise to it In aﬂaition the ratio oi affected to normal sihs in the fraternities agreed with that

3 \Vl'l\‘lSA'lT. W A, and LEWIS F T. (1518) Duplication of I-nitr‘|l valve and a rare apical lnterventrlzular lornmm In the heart ol yak call Am J Anne, 33. subs 74 HUMAN EMBRYOLOGY

expected of a recessive character From a study of seven} recorded series of cases of congenital heart disease in close relatives Herndon (1953) states that at least two genes or two sets of genes might be involved a recessixe gene or group of recessive genes producing familial cases within a single sibship and a conditionally dominant gene or group of genes being responsible for families exhibiting cases In multiple sibships

Deficiencies of several vitamins in the diet during the first two months of pregnancy have resulted in many malfnr mations Experimental studies on rats have demonstrated that a diet deficient In vitamin A or an excessive intake of ths vitamin in the maternal diet has resulted in increased inci dence of congenital diseases especially of the heart eyes and urogenital organs (Wilson and Barch (1949) Cohlan (1953) But there is no convincing evidence regarding the exact nature of deficiency of vitam ns affecting the incidence of congenital diseases

It has been emphasised by numerous workers that con genital defects are due to any non specific stress on the mother during early pregnancy The factors contributing to maternal stress include exposure to excessive heat or cold traum shock oxygen lack X rays or other radiation imury haemor rhage drug therapy or infections Any form of stress on the mother Just at the critical moment of the formation of the important organs in the embryo may disturb the embryonic primitive cells and tend to cause abnormalities of growth When the heart is being formed in the embryo during the first few weeks it is lkely for the stress factor to cause disturbances in the mesodermal bed of the heart and blood vessels Since the mesodei-mal cells also give rise to the skeletal system there may be associated abnormalities of the vertebral column or other bones in persons with congenital heart disease

The observations of Gregg (1941) in Australia have established that German measles or other viral infections contracted by the mother in the first two months of pregnancy could produce abnormalities of the heart brain eyes etc Murphy (1954) in an article on The birth of congenitally DEVELOPMENT OF THE CIRCULATORY SYSTEM 75

malforrned children in relation to maternal age‘ observes that children born late in parental reproductive life are more likely to haxe congenital deforn-iitics This is attributed to the influence of aging of the germ cells Ingalls, (1956) in an article on ‘ Causes and prevention of developmental defects" states “The meaning of the relation to maternal aging is that it probably signifies the contributory rule of chronic systemic and gynecologic disorders

Sex seems to have some partiality for the incidence of congenital malformations of the heart From many recorded cases (Polani and Campbell (1955) ) it is observed that coarctation of the aorta aortic stenosis tetralogy of Fallnt and transposition of the great vessels occur more in males than in females Patent ductus arteriosus is seen more in female: than in males

REFERENCES

CDCKAYNE E A (1938) The Genetics of Transposition at the Viscera Quart J Med 7 479

COHLAN S Q (1953) Excessive intake of Vitamin A as a causc of congenital anomalies in the rat Science 111 535

GREGG N M (1941) Congenital Cataract Fullnwing German Measles in the mother Trans opmh Soc Australia 3

HERNDON C. N (1953) Congenital heart disease Chapter 23 in Clinical Genetics edittd by Sorby A, Butter“ orlh & Co London

INGALLS T H.. (1955) Causes and prevention of developmental de fects J Amer Med Ass. 161 1047

MURPHY. D P.. (1954) The birth of Congenitzily malformed children In relation to maternal age Ann N Y Acad Sci 5'! 501!

POLANI P E and CAMPBELL, M (1955) Ann hum Genet 19 209

TAUSSIG H B. (1947) Congenital malior-mation: of the heart New York The Commonwealth Fund

WILSON J G and BARCH S (1919) Fetal Death nnd l\laldevelnp— men: resulting from maternal vitamln A deficiency in the rat Proc Soc exp:-r Biol 8: Med December 1949 75 HUIIAN EMBRYOLOGY

Airrizium

The Primitive Blood Vessels and the Arch of the Aorta Irregular plexuses of blood channels are formed from the mesoderm of the chorion and the covering of the }ollr sac Similar plexuses are formed in the mesoderm of the embryo These constitute continuous blood channels by the elabora tion of the primitive capillaries and bigger vessels In the early stages the blood vessels are simple tubes made up of a layer of endothelium Later the surrounding mesodermal cells envelop them forming the muscular and the CUDHECUVE tissue covering

In an embryo of about the third week a pair of vitelline and a pair of umbilical veins enter the sinus venosus at the heart along with the common cardinal veins formed by the union of the anterior and the posterior cardinal veins draining the body of the embryo Due to the early muscular contrac tions of the tubular heart the blood is pumped through the triincus aiteriosus into a pair of aortic arches passing round the foregut in the mesoderm of the first pharyngeal arch to reach the paired dorsal aortae lying doisolateral to the gut Each dorsal aorta supplies branches to the plexuses between the somites to the yolk sac as the uitellme artery and before it terminates in the caudal most region gives a big branch to the chorion as the umbilical artery

The events in the early developmental stages of man and the higher mammals can be compared to a rapid motion picture of the conditions elaborately exhibited in the dewlopment of the lower forms One striking example is the arrangement of the aortic arches in the human embryo as in the adult fish In the embrvos of all vertebrates six pairs of aortic arches are present connecting the aortic truiil. with the dorsal aortae

During the fourth week the postion of the heart is shifted more caudally more pharyngeal arches are formed and con sequently more aortic arches develop to connect the blood stream from the truncus arteriosus to the dorsal aortae In the human embryo all the six pairs of aortic arches never exist at the same time Each pair is succeeded by further ones appearing caudally in the pharyngeal arches By the DEVELOPMI-INT OF THE CIRCULATORY SYSTEM 77

time the second pair of arches develops in the second pharyvi geal arch (hyoid arch) by the end of the third week. the fiist pair atrophies During the early fourth week, the second pair also atrophies along with the appearance of the lhkrd pair The fourth pair arises by the middle of the fourth \\ eek and the fifth and the sixth arches arise by the fifth vseek The fifth pair is transitory and disappears early Thus by the sixth week of development, the third the fourth and the sixth aux-tic arches persist in a symmetrical manner By this time, the tnincus artericsus has divided into the pulmonary artery and the aortic trunk The forrner becomes connected

fiG 52 —-Schematic diagram of the pnmxtiie circulation of blood at about the fourth week .i., iicimc Ana: r. muvnaui. youcii ii. nun 78 HUMAN E\IBP.YOLOGY

to the sixth pair of arches and the blood from the aortic trunk is distributed to the tlurd and the fourth pairs

During these deselopvriental changes, the dorsal aortae fuse together in the iiudline hey oncl the arches, from the level of the seventh cervical segment to the third lumbar It has already been mentioned that the first two aortic arches atrophy early, but the dorsal aortae at the l9V€l of these arches persist while the portion of the dorsal aortae between the third and the fourth arches on either side atrophies

The aortic tmnk becomes the ascending aorta The fourth aortic arch on the left side becomes continuous with it and forms the arch of the aorta, which enlarges rapidly alloismg the blood to reach the dorsal aorta The portions of the dorsal aortae between the tlurd and the fourth arches on the left as \\ ell as on the right side atrophy The fourth aortic arch on the right side becomes the proximal part of the right subclavian artery The left SubCl.3V)3n artery starts as a branch of the arch of the aorta, at the level of the fourth arch The portion of the aortic trunk on the right side between the fourth and the sixth aortic arches forms the innominate artery \\l1.lCl1 is the common stem of the right suhclaviari and the right common carotid arteries

THE Ex'i1.iumi. CAROTID Airrziui:

The first tivo aortic arches atrophy early, but the xentral connections of the aortic trunk persist as the external carotid arteries on either side These arteries supply the derivatives of the first (mandibular) and the second (hyoid) pharyngeal arches As the muscles from this region haie migrated to the head, the external carotid arteries extend further to form the occipital and tempoml branches

Tin: I~ri'ini.\IiL CAROTLD Arrziufi

The dorsal aortic roots on either side connecting the first and the second aortic arches persist esen after the dis— appearance of these two aortic arches. On each side. thae aortic roots extend to supply the d€V€ll7p1!1g brain as the intemal carotid artery As the dorsal aortic connection DEVELOPMENT OE‘ THE CIRCULATORY SYSTEM 79

EXT CAROTTD A.

V Ileana ARCH rmANsrro1'n')

I! Aoxnc urn

bonslu. Aonrr.

fiG 53 —Schemanc drmung of the pnmmve aorlxc arches

between the third and the fourth aorlxc arches haxe alreadx dxsappeared on both sxdes the internal carotxd artery becom°s a contmuous vessel wxth the thxrd aortic arch Venlnlly the

13111 of the mam aurtxc trunk continuous xuth the lhxrd nor! 1: arch forms the common curotxd artcr_/

Tm: PUL\lOVl\l'lY An'n:m'

The orngmal lnmcus artenosus has already split nlo tho. pulmonary and the aortic trunk: The spiral septum started its growth In the hulbus cords and progressed tmxards the truncur arzermsus 1! the lane) of the sixth acme arch Islahlnshnng the cunnectnon of the right xcntrxcle mlh the 30 HUMAN EMBRYOLOGY

I AOITIC ARCH MTIDIHYI

ii iioiiri: Aiioii ‘ urrliovrnr)

111 -KORTIC ARCH

iv Acme ucil Mei-i or Aermu

___vr KOITIC Aiicii (but.-ms Julmuosws

l.'l' IUICLAVXAN A. IN‘! MAMMAIIY A

iaonsni. Anni-A

Fm 54 —Tra-nsformatioii of the aortic arches

pulmonary artery and the left ventricle with the aorta By the fifth week the sixth aortic arches give 03 branches to the developing lungs Slowly the right arch gets disconnected with the dorsal aorta but the left one still remains connected with it as the ductus iirtenosur throughout foetal life After birth when the lungs expand with air the ductus arteriosus atrophies

Tue VERTIHHAL AND Biisiuiii ARTIRIES

Soon after the foi-iriatian of the body segments the dorsal aorta distributes many patted branches to supply the dorsal iegions of the body of the embryo As these are arranged DEVELOPMENT OF TH2 CIRCULATORY SYSTEM 81

symmetrically between the body segmenis, they are known as the mtersegmental artenes They extend as paired tessels, one on either side of the neural tube The terminal ends of the fixst six pairs anastomose together to form a distinct vessel on each side known as the vertebral artery Gradually, the aortic connections to this newly formed artery atrophy Tl-ie vertebral arteries now extend towards the head region and at the letel of the medulla of the brain bend medially and ventrally and yoin together to form a single median vessel called the bastlar artery This artery further extends towards the infundibulum, where a branch of the internal carotid artery from either side Joins it to form the “circle of Willis “

{ ANT COWWMCAVMD

\ ,, Mmbu: CEREBRAL v C c1n<:i.i: or wnus

Pic 55 —Sclu:mut:c drawing of the lm-mutizm at the “Circle of Wxllis" 6 E HUMAN EMEEYOLOGY

The intei-segmental arteries at the let el of the 7!}: segment grow forward to supply the developing upper limb buds as the siibclauuzn arteries Due to the early atrophy of the aortic connections with the vertebral artery the latter is joined to the subclavzan artery on each side

Ti-is IN-riiiicosrar. Ami Iiniiiiwii. MAMIIARY Airriziurs

The paired dorsal intersegmental arteries arising from the dorsal aorta beyond the seventh segment to the sacral region also extend to both sides of the neural tube passing around the body of the vertebrae Along with the extensive growth of the body wall a lateral branch arises on each side of the intersegmerital artery at a point Ventro-lateral to the bod) o’ the vertebrae In the thoraeie region these lateral branches extend towards the body wall between the developing ribs to constitute the intercostal arteries In the lumbar region the lateral branches form the lumbar arteries Gradually the intereostal arteries grow bigger in size and the original doisal continuation of the dorsal intersegmental arteries becomes smaller in size to supply the spinal cord The intercostal arteries reach the ventral body wall where a longitudinal anastomosis takes place between the terminal ends of these and the lumbar arteries to form a vessel on each s de of the nudline These are the mteniol mammary arteries in the thoraeic region continued as the superior epigosti-w arteries in the upper abdomen and anastamosiiig with the inferior epigortnc arteries in the lower abdomen

The rupenor iiitercostcil artery is formed by the early anastomosis of the first and second intercostal arteries with each other and with the subelavian artery, of each side in the preccstal area The origuial attachments of the first and second intercostals to the dorsal intersegmental arteries disappear Hence the superior intercostal artery appears to b a bi-.inch of the subclavian The umbilical arteries fuse with the fifth lumbar inte~

segmental arteries in the sacral region to form the Artenml and the internal iliac arterier DEVELOPl\{EN'I‘ OF THE CIRCULATORY SYSTEM KI

post now: lumsf

POST cosru. Jumsi srmiu. animal

PEECOSTAL

iuuisr msorzmmos sur mssnrrziuc Am. D” or nrrmsnc u-muu. BRANCH iumu. A

INT MlLmuu1\'

F1: 56 —-Schematzc secnon of the trunk regwn, showing the

formatcan of the mtersegmental artenes from the dorsuluorta

THE RENAL SUPRARENAI. Trs-ncuum on Ovmww Am-zmas

In the early embryo the dorsal aorta dxstributes three groups of branches 1:: a dorsal lateral and a ventral dxrection The distributions and the modifications of the dorsal mter segmental branches have already been considered The part of lateral branches extends laterally, in association with the urogenital system which are derived from the intermediate mesoderrnal cell mass In the early stages many pans of arteries arise hierally at the thoracic and the lumbar levels to supply the mosunephros When the mesonephroi atrophy, these vessels form the renal suprarenal and the arteries sup plymg the gonads—viz, the testicular, in the male and the umnan in the {emzile

Tm: Cozunc Surziuon Ah!) I-zrsxuon ]\’[!'sEN‘rEﬂlC Airruus

The ventral group of branches from the dorsal aorta consists of tsm pairs of arteries one the ulellmc arteries 8-] HUMAN EMBRYOLOGY

dxstnbuted to the mesoden-n of the yolk sac and xts denvatwe: and the other the umbzltcal artertes to the allantmc mesoderm Along with the narrowmg of the yolk sac and the pmgressue development of the different parts of the gut the paxred vxtel

lme artenes are hmught close together tn the dorsal mesenten supportmg the gut Saon they fuse mto a smgle xessel By the fifth week three such mam vessels can be dxstmguxshed to be ongmatmg ventrally They are the coelmc the s-uperuor mesente-nc and the mfenor mesenterzc artenes Later they assume their defimte posltxons due to the changes In the growth of the gut A few ventral branches cephahc to the coehac artenes form the brzmchml and aesapl ngenl arteries

noun nu‘ EPKGASYIUC

mznum sncruu.

Fm 57—A-rterzes of the caudal regzons 21)‘ the embr_1o of ubmlf the seventh weak DEVELOPMENT OF THE CIRCULATORY SYSTEM 85 Tu: Ul\‘lEILICAL ARTERIES

These arise ventrally from the caudal end of the aorta as a pair of vessels tawards the mesoderm surrounding the allantcis and course through the body stalk to the choricn Each umbilical artery fuses with the fifth lumbar intersegmental artery Gradually, the original connections of the umbilical arteries with the aorta atrophy Hence, the umbilical arteries appear to arise from the fifth lumbar mtei-segmental arteries The intersegmental artery on each side branches as the external and the mtenzal ilzac arteries The umbilical artery, therefore, appears as if it 15 a branch of the internal iliac artery

After birth, when the umbilical arteries atrophy, their continuations from the internal iliac arteries are represented by the superior veszcal arterzes and the lateral umlnlical ligaments

As the mternal iliacs drain away most of the blood from the dorsal aorta at the sacral region, the terminal portion of the aorta narrows into the median sacral artery

Tm: BRACHIAL ARTERY AND rrs Bimrzcrim

The upper limb~l:uds appear at about the filth week as lateral projectians of the sucth, seventh and the eighth segments The intersegmental arteries from the dorsal aorta. at these levels send branches to the prnnitive blood plexuses formed an the limb-buds on either side These plexuses saon form a blood vessel in continuation vuth the seventh intersegmental branch Thus, the subclavxaii artery is continued as the brachial artery of the arm and the mtcrosseaiis artery 0] the forearm and hand The terminal end of the mterosseous is broken up into a plemis of capillaries which later farms the digital bi-aiiclics

The median artery starts as a branch of the brachial .2:-._~3 terminates in the vascular plexus of capillaries at the L: During the sixth and the seventh week, the ulnar and :52 radial l111€l'l(’S develop as branches from the brachial are-; to supply the forearm nnd the hand 55 HUMAN EMBRYOLOGY

ll-ACHIM. A. ‘

Fm. 58 —Deuelopment of the arteries of the arm.

Tm:Fnaom1.Am'm7nm1-xsﬂmmmns

The umbilical artery said: a branch to the developing lower limb-buds on each side. This is the axial artery which ends at the distal and of the limb in I plexus of capillaries DEVELOPMENT OF THE CBCULATORY SYSTEM 3'!

terminating as digital branches From the plexuses of the thigh a new blood vessel arises which becomes continuous with the external 11.12:: and Jams the axial artery This new vessel is the femoral artery Gradualy the axial artery degenerates due to the enlargement of the femoral artery which is continued in the leg as the poplneal and the posterior nbml arteries The remnants of the axial artery are a portion of the perm-zeal artery and a branch of the mfertor gluteal artery

The untznor tibial artery arises from a few plexuses near the original axial artery and ]0lnS the root of the postenor tibial

In the foot the early terminal plexus from the axial arten 15 now Jamed by the poster-zor tibial artery After bzrth when the umbilical arteries atrophy their proximal portions remain on each side as the mternal tlxac or hypogastnc artery

n:MonAi.

DIGITAL Plzzusfi

vmiuuu.

31‘ mar “OMMON lulu‘.

rmoﬂllu. I-0s1i'mAi. IOPLITEKL luau

HG 50~—De-velopment of the arteries of the leg 88 HUMAN niianronoar

Tiii: Vzms

The p“"“‘“’° “"15 °f l-he embrso as in the case of the arteries originate from a series of capillary plexuses scmemd m the m°5°d°"“ Th"? Pairs of xeins are established in 5 symxnetrical manner in an embryo three iveels old The, are the cardinal veins from the body the \|§5u|ne Wm, {mm the mesoderm over the yolk sac and the umbilical vein, {mm the chononic villi All these drain into the sinus xosug cf "39 hear‘ ‘}“‘°UEh1l1e septum transvei-sum

iiziinr Ii‘ Jun can v if Iiirr ciuin v ' 5 I vmosvs coamov cneiiuu rm: vmi H‘ rosium I‘! rest cub VI?!-L VIN

fiG 60—Scheme of the pmnitiie veins entering the heart

TH: Suruuon Vzzsli CAVA

The Veins draining the anterior portions of the body of the embryo are the untenor Cdfdtﬂdls and those from the posterior are the posterior cardinal! These vesels meet together as a common cardinal Vein (duct of Cuvier) on each side to enter the sinus venosus Tributaries of the anterior cardinal veins arise from the neighbouring parts The external jugular veins originate from the mandibular region and the eubclainan from the body segments concerned “Rh the upper limb-buds The anterior cardinal is known as the tum-rial

jugular ‘H2111 in the adult During the eighth week the two anterior cardinal seiiis

become connected by an oblique cross vein formed by the

anastci.-ncsis of the thymico-thyroid Vents This connecting veins allows the drainage of the left cardinal vessels through DEVELOPMENT OF‘ THE CIRCULATORY SYSTEM 89

it to the right side Subsequently, the portion of the left anterior cardinal between the heart and the connecting vein atmphies, leaving a remnant as part of the highest mtercostal DBUL The left common cardinal is transformed into the oblique vein of the left atrium The connecting vein grows larger in size as the left innmimiute vein The venous channel forming the continuation of the right cardinal vein with its two tributaries viz , the right external )ugular and the sub clavian, up to its Junction with the left innominate is called the 1-zghc mnommate vein Beyond the innomiiiate the chief vessel runs as the superior vevw. cavu. to the sinus venosus

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um V onuani: vtm Dr H’ ATMUM

(If you‘ calm )

I-‘ic 61--Transim-iimtion of the anterior cardinal wins and the formation of the siipcnor venzi caux

Tin: VENOUS Smvsss or -i-iii: Dunn MATER

The tenninul pattinns of both the anterior cnidinals are distributed ienti-all} and laterally in the region of the growing brain The vessels that are formed in the dura and in the brain produce three distinct groups of plexuses known us the anterior, middle and the posterior plexuses The anterior plexus is situated between the semilunnr ganglion and the optic stalk. The middle plexus remains near the midhrain 90 HUMAN EEBRYOLOGY

between the semilunar ganglion and the capsule of the internal ear The posterior ptexus lies m the hmdbrain, caudal 10 the internal ear vesicle All the plexuses of each side empty mm

the vena capztis p/rmuz, which 15 the chief tributary of the anterior cardinal vem

Mm iiuzxvs vm cums

mam. GANGL

l.1‘Im'rc)uinv

fiG 62 ——Sxde view of venous sinuses 0] duru mater in an embryo of the fourth week (After St-reeter)

By the end of the eighth week, the middle and the postenur plexuses J01!) together by a canriecting channel situated over the internal ear and known as the sigmoid 5-mus The antenor and the middle plexuses fuse together longitudinally to form the transverse smus in the dura between the cerebrurn and the cerebellum and a straight sinus In the middle The anterior plexuses fuse in the middle line in the dura between the two cerebral hemispheres as the superior and the inferior sagittal smuses

Mmruzxvs

fiG 63-Side view of venous sinuses of dura mater in an embryo of the sttth week (After Streeter) DEVELOPME'NT OF THE CIRCULATORY SYSTEM 91

MID uzxus

HAND ml‘ PLTK sums

Fla 6-4 -——Sxde view of venous sinuses of dxmz mater in an embryo of the exghth week (After Streeter)

The ophthalmic vems from the developmg orbxts and th-= muidle cerebral vems dram mto the vena capms pnma The pox-tum of the vena capms puma medxal to the tngemmal ganghon becomes the caverrums smus This cornmumcates wnth the transverse smus through the s-upenor petrosal xmus \VhICh develops as a modxficatmn of the nngmal basal stem of the middle plexus, and through the Inferior petrasal smus

denved from the pnmmve tnhutanes of the anterior cardinal vem

sum-nu. sums

cnvnmovs sum:

1-‘ta G5—Sudc mew of venous smuses of dun: mater m an embryo of the eleventh week (Alter Slreeler) 92 HUMAN EMERYOLOGY

'I‘i-ii: INFERIOR VENA CAVA

In the fourth week, the poﬂefwf cardinal veins extend as longitudinal vessels from the first cervical segment to the fifth lumbar, dorsolateral to the undivided ma-5 of mewdﬂm between the splanchnic and the somatic layers The lateral portions of this mesodermsl mass develop into the mesune phros The caudal portions of the posterior cardinals mm; with each other and with the veins from the legs to form the common ilmc uems

fin 66 —Schematic diagram of the veins of an embryo at about the fourth week

Another pair of veins called the subcardinalr arises on the ventromedial aspect of the mesonephrcs, probably by the longitudinal anastcmosis of the connecting channels from the mesonephros to the posterior cardinals Thus, the subcardinsls are situated parallel to the posterior cardinals with which they are interconnected by many channels passing through the mesonephros on each side During the fifth week, the mesaDEVELOFMI-ZN'L' OF THE CIRCULATORY SYSTEM 93

nepliroi grow larger in size, so that their medial borders nearly come in contact with each other, the sulicardmals consequently approaching and eventually anastumosing with each other at about their mid parts to farm a median subcardmal smus The blood from the hinder !'EglOnS of the body is drained by the terminal portions of the posterior cardinals and passed on to the subcardinal sums to reach the sinus venusus Hence, the cephalic portions of the posterior cardmals beyond the level of the subcardina] sinus slowly atrophy

iurrclmiav

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ms‘! clulmmu.

SUI CARDINAL ‘ KNIST ,3 I’ \\ / // /I V 1 COMHON IUKC LT EXT It-ll‘: HT :11’ “J15 '" "" "'"‘° 11- mr HMC XNAST MTDIIN SICRX1 .

or ii.uic

fiG 67 -—Sc!icnia!ic diagram of the 'Ll.‘l1l3 of an embryo at about the sixth urek 54 HUMAN EMERYOLOGY

Durmg thxs stage, the hver lvucls growmg m ‘be sepmm transversum come Into contact wxth the upper part of the ugh; rnesunephros, where H fold of dorsal mesentery called the canal plwa arxses The blood vessels pnmanly formed m an, meseniery become COXIHHIIOUS wxlh the blood vessels {mm the nght mesonephros and the plexuses of vessels 1n the lxver

/“\

Fm 68 —-Diagram shounng the formzmon of the mlennr vena ‘ mm, m an embryo of seven weeks

The blood from the subcnrdmal smus therefore ﬂaws through the rxght subcardmal vem to enter the mesentenc vascular channels wluch nnw enlarge Into a ‘bug vessel Th‘; 15 the DEVELOPMENT OF THE CIRCULATORY SYSTEM Ba

mesentenc part of the inefnor vena cava Soon after, due to the large volume of the blood ﬂow, the hepatic plexuses organise into a large vessel to enter the sinus venosus, as the hepauc part of the inferior vena cava

A Pall‘ of longitudinal vessels called the s1ipra—cardtnalx (or the thoracmlumbar veins), arises dorsomedial to the postenor cardinals and terminates anteriotly in the posterior cardinals Near the middle of the mesonephros the supracardirials your the subcardinal sinus and therefore the cephalic portions of the supracardinals atrophy The right supracardinal entering into the sinus enlarges in size to constitute the immediate post-renal part of the inferior vena cava

In the caudal region just where the supracardinals ]01n the terminal parts of the posterior cardinals where the right and the left iliac veins have {used together by a common anastomosis. the direction of the ﬂow of blood is through the right supracardinal towards the subcardinal sinus As a re sult, the caudal part of the right supracardinal enlarges to form the post-renal part of the inferior vena cava

Thus by the second month the inferior vena cave is form ed asymmetrically due to changes in the direction of the blood ﬂow In brief, it is made up of the following derivatives 1 lnteranastomosis, lietween the supracardinals and the posterior cardinals in the iliac region

2 The post-renal part, from the right supracardinal

3 The renal part from the subcardinal sinus (towards the right suhcardinal)

4 The pre renal part, from the right subcardmal 5 The meseriteric part, in the caval plica G The hepatic part, in the llVEY

Tue RENAL. SUPRARENAL AND THE TESTICULAR on Ownuiw VEILS

On the right side they dram directly into the inferior Venn CHVKI On the left, they win into a common vessel uhich Oflglllilll) formed a part of the left subcardmnl vein, “hilc anastnmosing with its fellow of the opposite side 96 HUMAN EMBRYOLOGY Tm: Coiiozwur SIN!!!

Due to the change in the direction of the blood flow from the anterior cardinals to the right side and the atrophy of the cephalic part of the left posterior cardinal, the left common cardinal tein becomes disconnected from the systenuc circulatian But its proximal part in the early stage is carried along with the definitive shift in yosition of the sinus venusiis and lies in the atno-ventricular sulcus on the dorsal wall of the heart Slowly, the venous channels in the heart muscles become connected to it and form the coronary 8’H|\lS

L‘! PUT-M. V iii‘! PULML V

IV! Vﬂll CAVA

Ell-WV! VH1!

OP MABHN-L If ATINII

LI‘ ATHPUM

colleiumv Inn: (near Mn! :9! 1.!‘ com; cup)

fiG 69 -Dorsal view of the heart of an embryo of the snow! month

Ti-ii: Pui.Moiuin' Vi-ms

They arise from the lung buds and enter the left atrium by a common vein Later, this common vein is absorbed :1) DEVELOPMENT OF THE CIRCULATORY SYSTEM 91'

the rapidly gm“ ing left atrium and its four components open independently

The Azvcos VEINS

Medial to the supracardinals, a pair of longitudinal plexuses of VEIIIS arises as the azygos venous system Each ]0l!1S cephalically the posterior cardinal and caudally the subcardinal vein. In the thorax they drain the intercostal spaces Anastomosis takes place between the two vessels by cross channels dorsal to the aorta, below the level of the third intercostal The continuation with the left posterior cardinal vein atrophies along with the atrophy of the left common cardinal vein Hence the axygos system 15 continued through the region of anastomosis to the right posterior cardinal vein The second and third intercostal veins of the left side which have already mined the posterior cardinals are out ofi from the main azygos system Along with the proximal part of the left anterior cardinal, the intercostal veins become attached to the left innominate vein

Tm: Pormii. Vzm

At about the third week, the paired vitelluie veins from the mesoderm of the yolk sac pass along the splanchnic mesoderm of the foregut, through the septum transveisum to the sinus \enosus During this stage, the liver buds arise as endo~ dermal autgrawths from the {oregut and crowd into the septum transi ersum The vitelline \ElI1S break up and intenninglo uilh the liver cords to form an irregular mass of sinusoids The terminal ends of these xeins still proxect out of the liver and enter the sinus venosus So, during the fourth ueek, the vitelline wins are divided into three parts, -oiz, a part distal to the liver. :1 part in the liter itself and a part proximal to the liver

A series of anzislomoscs take place between the vitelline \L‘tns In the liver, the two wins hate already intci-mingled togethr in ii position xenLral to the duodenum Distally, near the lt\ or. there forum an anxistomosis dorsal to the duodenum

7 95 HIHHAN EMERYOIDGY

Fla ‘IO —Scheme of the vztellme and the umbxlncal I721!!! of an embryo at abwt the fourth week

and still further a third anasiomasns ventral to ﬂ Gradually the left xein between the 1lVE! and the clonal anastcmosu and the rush: vem hem een the dorsal and the ventral anasto masts (dxstal) atmphy The ma ‘ems run as a camhmed S shaped vessel around the gut towards the Inex By this tune ihe yolk =5: and the pan of the vntellme vem: associated wzth at atrophy The remamder of the vale! hne xems persists as the supeuor mesentenc vem In the dorsal mesentex-5 of the Intestine Along wxth the sp1en.c van: 1: enters unto the neutral (dzstnl) anasiamasxs oi vztellme DEVELOPMENT OF THE CIRCULATORY S‘1bTEM 99

PLZXUS IN uvtx

HT mam ‘ Mus: m uvtx

DORSAL Iumsr

vnrmlu. ANASI’

LT VITELLINE

LT uymuclu.

111' 1111311.!

Fm '11——D:agram showmg the zrnnsformauou of the uxtellme and the umlulxcul -uems of an embryo at about ﬂu.

filth week

vcms ustabhshxng the portal Hem. The nght wlellme vem distal to the nnaswmoszs dcgencrates later

In the hver further changes take place The umbxhcal vems passmg along the panetal layer of mesodcrm of the embryo on 01011’ \\ 21) to the smus v4-nosus moo! the ovpandmg nght and icft lobes of the lncr at [he SldQ§ Consequently the lxvcr tap: all the blood from these wems to be freely mixed \\1lh the blood from the vncllmc xems m the smusolds. Dut:l,'§|'l 100 HUMAN mmnvonoav

m V” c"‘ H’ HDATICV

I1‘ lfilﬂc V

FORTH. Vlllt

manmnuc -—._uuu«:n to IPLDHC

fiG 72 —Du1g1-um. sholung the for-mutton of the portal tsem m an embryo uf about seven weeks

to tlus the ox-lg nal connections of the umhxhcal Vems to the sums venosus atrophy By the sxxth week the nght umbxheal vem becomes reduced in sxze and dnsappears All the blood from the placenta 15 therefore drained by the left umbdxcal vem Into the live!’ Slmultaneausly the opemng of the nuns venosus u shafted to the right Side The large quantity ol blood entenng the liver thruugh the left umbnlncal vem pur DEVELOPMENT OF THE CIRCULATORY SYSTEM 101

sues a diagonal passage across the sinusoids towards the right side of the sinus venosus This new channel is the ductus uenosus

The part of the right vitelline vein between the liver and the sinus venosus becomes the chief pathway of the veins entering the heart Its size is increased as it forms the ter minal part of the inferior V8132 cave The corresponding proximal part of the left. vitelhne vein subsequently disappears

After birth, when the umbilical vein ohliterates its rem nant, the hgrzmentum teres extends from the umhihcus to the liver as a fibrous cord The ductus venosus is transformed into the solid ligamcntum venosum

Darzcis or DEVELOPl\‘ll?.NT or rain Buoon Vsssus Aairnc ARCH SYS'l‘E'\‘l

Patent Ductus Artcriosiis The sixth aortic arch on the left, which becomes the pulmonary artery is joined to the dorsal aorta as the ductus arteriosus lt starts closing with the onset of respiratory function at birth and is obliterated within about two months In some cases it remains patent throughout life The direction of blood ﬂow through the patent ductus arteriosus causes certain functional disturbances re sultmg in almormalities in the circulatory system The shunt of blood through the persistent ductus arteriosus is from the aorta into the pulmonary artery, on account of the higher pressure in the systemic circulation The shunt causes dilatation of the pulmonary artery as it receives a large volume of blood At the same tune, as a result of the shunted blood carried to the left side of the heart, the pulmonary veins the left atrium and the left ventricle also become dilated

Co.-irdalwn of (lie Aorta The lumen of the aorta in the arch or iust beyond becomes congenital]; narrowed. There are usually tun types, "infantile" and “mlult" In the former the Y|ﬂl'l’0\Vlng imulvcs a large part of the aorllc arch \\hcrca.s in the latter the constriction is near the origin of the left subclaunn nrtery 10’ HUMAN LHIBRYOLOGY

M981!-‘fir origin of the Carotid Subclauan am: vm-1..},,9) Arteries This may be caused by the persistence of the 30,1", arches “hich normal]; atrophy or by the atrophy of those which normally persist

Virixs

Considering the factors in the formatton of the due, xeii-is of the body many gross irregulanties are probable eg persistent left superior vena cava Pulmonary veins may dra n mto the right atrium

Tm: LYMPHATIL‘ Srsrim

The lymphatic channels are developed as sachke out growths from the primitive veins at certain aefimie sites During the sxxth ueek two jugular lymph sacs anse m the neck near the Jugular xeins mo siibclavian 13;-n-iph sacs arise from near the upper limb-buds and two Xllac lymph glands arise from the inguinal region Later a series of small sacs along the dorsal body wall join together and form the thoracic duct In the lumbar region zi large lymph sac 15 {om-‘ed xetm. pei-ituneally as the cistema chyli Ventral to this develops the retroperitovieul lymph sacs extending to the mesenter} and also to the kidneys and genital organs

From the ninth week onwards due to further extensions and growth these lvi-nph sacs drain all the tissues of the bed) The thoracic duct enters the lelt iugular lymph vessel and drains into the left innominate vei.n The right gugular lsn-iph sessel opens into the right innominate vein Communications are established bet“ een the iugular and the subclauan lympn sacs and further to the superficial parts of the neck In the caudal region the mesenteric and the iliac lymph xessels drain into the cisterna chyh uhich becomes continuous ttith the thoracic duct By the thin! month the main lymphatic chan nels and their extensions are established The vahres of the Iyrnphatic channels are fully formed by the fifth month.

In the connective tissues surrounding the lymph channels groups of lymphohlasls accumulate which break up the DI-IVELOP‘\'lEl‘ﬂ‘ OF THE CIRCIILATOIIY SYSTEM 103

lymph channels into irregular smaller Channels The lymplio— blasts proliferate more in the C0l1!‘lECll‘vE tissue to form lymphocytes and establish the medulla of the lymph glands The capsule of the gland originates from the condensation of the em eloping connective tissue

THE: SFLEEN

In an embryo between the fifth and the sixth week the spleen is developed as several masses of mesodei-m cells which accumulate and groii. in the dorsal rnesogastrium attached to the stomach Along with the rotation of the stomach the mesogastrium is pulled to the left side and the mass of accumulatuig cells is shifted to the left with the mesogastrium and further moved dorsolaterally to lie against the body wall

Gradually, the cells arrange into a network of traheculae containing isolated cells wluch are differentiated into erythrohlnsts. myelohlasts and lymphocytes By the third month the adult condition is established During the later stages the splenic cells produce only erylhroblasts and lymphocytes

The splenic artery, which 15 ii branch of the coeliac artery distributes itself to the sinusoids in the substance of the spleen

The spleen, in the course of development, may be partially or totally subdivided Sometimes, the several masses may not fuse, leaving parts separate from the main substance as :icces— sor) spleens.

Hiizxioimii-sis

The primitive blood cells appear in the Blood islands '— \\lllClI are collections of mesenchymal cells in the extra-embryunic mesodcrm over the yolk sac of the emln-vo at about the third week The centml cells in these islands round then'i~ S0l\l!S to form the first blood cells. whereas the peripheral cells become ﬂattened to become the endothelial cells lining the first formed blood \ESSl2l5 These blood cells are free from haemoglobin and are called hat.-mocytohasts Later. they dmelop haemoglobin \Vl'-hln them to form the primime red blood cells iihicli are carried to the heart of the embryo At this stage lcucocydcs or white blood cells are not [armed IN HUMAN nranvomav

About the third month, the liver acts as a haemopoieu; organ producing blood cells until the time of birth Here the leucocytes and megakaryocytes are present but lympho. cytes are still not seen

Haemopoiesis and particularly erythropaiesis (fa;-mamm °f ‘ed 51°03 09115) 5981113 in the spleen two months after it has started ui the liver and ends at about the seventh month The other organs involved are the thymus where it is of ii Very Short duration, and the lymph node, where it start at about the third month and persist throughout life Hepatic haemopoiesis duninishes in activity from the fifth month 01Wards -ind d15aPP€3-H at birth. In the meanwhile the bone marrow, the spleen and the lymph nodes taken over this function

The blood of the foetus consists. fii-st, chieﬂy of primitive erythroblasts (red blood cells), and of these ninety-two per cent are very large and belong to the “first generation”, the si-naller cells of the “second generation" become numerous during the hepatic period of haeniopoiesis and gradually replace the more primitive cells The erythrocyte count in the foetal blood increases rapidly at ﬂist and then more slowly and it is only after birth that counts attain normal level Leueocytes appear in the embryo during the second month and number about 1000 per mm They consist mainly of primitive myeloblasts and as the‘count gradually rises, more mature cells begin to appear Lymphocytes are first seen in the fourth month of foetal life and monocytes appear at about the fifth month

The platelets which are formed from the megakaryocytes are fewer in number in the new born infants and attain adult

normal level at the age of thiee months

Biioon GROUPS

The red blood cells contain (on their surface) certain substances which characterise an individual Many typa of substances have been discoi ered on the surface of the red blood cells and these are responsible for the grouping of 191004 These characters are inherited and remain a constant factor 103 HUMAN EERYOLOGY

ERYIHSOELASTOSIS Forraus

This is a condition of anaemia occuring in the foetus, caused by haernolysis of the foetal blood, resulting in yaundice It ovies its name to the crythroblasts (immature blood cells) present. in the blood stream The condition manifests in the following forms —

1 H} drops foetiilis This is the severest form of the disease, where there is generalsed oedema with aseites The foetus is born either dead or dies soon after birth.

2 Ictcrus gravis iit.-ountorum This is a fatal condition associated with severe jaundice, appearing soon after birth

3 Congeiutal hacixiolytic anaemia This represents mildly affected cases ui which a moderate to marked anaemia appears a few days after birth

The basic aetiological process is the same in all the above terms This was first detected in a vioman who had a still birth The serum of this mother was found to contain an antibody \\h.l¢-l1 agglutinated the foetal and father's red blood cells It was, therefore, inferred that the mother had produced an antibody against a Eactor present in the red blood cells of the foetus. which she lacked and was inherited by the latter from the father This factor was soon identified as the Rh factor The mechanism is therefore, an antigen antibody reaction, the antigen responsible being associated with certain blood groups

In l’l’l8JOX'tly of cases of erythrolilastosis, blood of the foetus is Rh POSIIIVE and that of the mother Rh negative The blood of the foetus 3 haemolysed by an antibody from the blood of the Hh negative mother The antigen from the Rb positive foetus reaches the mother through the placenta and produces the antibody in the blood of the mother, when it returns through the placenta to the Rb positive foetus, a reaction takes place causing haei-nolysu oi the blood cells This

antibody may also be produced as a result of Rh positive blood transfusions git en to the mother

Associated with severe anaemia, regenerative erythropoietic activity becomes very marked in the bone marrow DEVELOPXIENT OF THE CIRCULATORY SYSTEM 101'

and wxdespread toe: of extramedullary erythropmesxs xn varmus organs such as hver spleen kidneys etc

Tm: For-:'rAx. CIRCULATIOV

The factors msolved tn the dynamics of the [metal cxrcu latter: have been thoroughly studxed by Barclay‘ and his assoclates at Oxford The oxygenated blood from the umbxhcal vem passes through the cluctus venosus 1n the lwer and umtes wxth the hepatlc vems and the mterxur vena czna to enter the right atnum At the same tune the de oxygenated blood from the supenor vena cavn passes to the nght atrmm The How of blood thmugh the umbxlzcal vetn :5 larger In amount and reaches the nght atnum earlner than the other blood ﬂow The drrectlon of the opemng of the rnfenor vcna cava and the foramen ovale 15 such that more blood 15 at first du-ected to the left atnum A large amount of oxygenated blood therefore passes through the foramen ovale to the left atrxurn where to a small extent xt rs mxxed wtth the venous blood from the pulmonary wems Thxs blond as passed through the left atno-ventricular openmg to the left ventncle to be pumped to the aottlc arches reachmg mostly the head the neck and the upper limbs Thus rt appears that the heart and the head of foetus recewe blood supply wxth more oxygen content than other parts of the body

The stze of the passage of the foramen ovale bemg smaller than the mfenor caval opemng the remaining blood In the rzght atrmm passes to the rxght ventr cle From there It flows through the pulmonary trunk enterlng the descendtm, anrta through the ductus artenosus The pulmonary trunk grves out a few branches to the developmg lungs

The dorsal aorta below the level of the ductus artcnosus therefore contams less oxygenated blood. Tlus blood is supplled bv the branches of the dorsal anrta to the lower regmns

of the body It as then contmued as the umbrlrcal aztenes to the placenta

Cm\r~cr.s xx -rm: Cmcvmnou Arrra Em-m

The follow mg 15 the summary of the changes Vshxch take place after hxrth 103 x-mmm umsozonocv

INXOM. V

’°°'"- ‘’ , muommm: A.

L1 co-men clmonn IL

at suscuw V L! suI1:a.Avu.v

svr vtlu CAVA

umau. v ‘\cm:7.ulc Av

)u:NIu.v_

coma. gum: A

at xunc A. nu‘ Gum:Iu. A

nrr xmu: L (Im:en.xcIU-)

Fm 73 —Plan. of Foetal Cwculuhovn. before brrth

1 Immediate contraction of the two umbilical anenea 1n the umlnhcal cord and late contraction of the umbxhcal vexn If the umbxlxcal cord 13 not separated the blood can ﬂow back from the placenta to the {mains to an extent of about 50 to 100 ml. m about half an hour

2 Contractxon and narrowmg of the duccus artenosus wxlh the commencement of the pulmonary funchons DEVELOPM!-‘.N‘l‘ OF THE CIRCULATORY SYS'l'El\l 109

3. The pressure of blood in the left atrium becomes more than in the right atrium and hence the septum primum of the faramen ovale is pushed in apposition with the septum secundum. The fox-amen ceases to function and gradually [mm 6 months to 1 or 2 years, the opening becomes closed by tissue—growth.

4. Obliteratiun and disappearance of the ductus venosus. leaving a remnant, the ligamenzum venosus. The left umbilical vein leaving a remnant, the ligamenlum teres.

The umbilical arteries leaving a iemnanl, the obliterated branch of internal iliac artery (hypogastne). The ductus arteriosus leaving a remnant, the ligamentum arlenosum.

‘BARCLAY. A. E; BARCRGET, .l’., BARRON, D H., FRANKLIN, K. J. AND PRITCHARD, M. M. L. (1941): Studies ol the foetal urculahon and of certain changes that take place after birth: Am. J. Anal. 69.

‘BARCLAY. A. F... FRANKLIN. K. J. AND PIUTCHARD, M. M. L (1944): The foetal rirculauon and c:u-diowascular system nnd the chzmgcs ﬂu! they undergo at birth: Blackwell Scu-nlliic Publicanovu. Ltd, Oxford. CHAPTER VII DEVI-ZLOP\IE.N’l‘ OF THE ALIMENTARY SYS'1‘E\I '1‘:-xi: FACE

ln an embryo of two to three “4 eelcs, the developmg head

5 made up of a neural part and a vnsceral part The neural

part develops min the bran), eyes, mtemal ears and the olfactory organs The vtsceral part consxsts of the cephalu: pan of the endodermal gut or pharynx and the structures developmg fmm the prxmmve pharyngeal arches to form the face

In the fourth ueek, there )5 the promment fmntal emnence of the bram (fig 29) On its lateral wall are seen the outgrowth: of the optxc vesmles and postennrly thxck depressxons of the one or audxtory vesxcles Below the frontal emmence, LS the buccuphaz-yngeal membrane whxch later break: down to put the oral cavxty m contmuatxon vulh the pharynx On the mat of the oral cavxty, m from of the huccopharyngeal membrane, a small ectodermal dweruculum known as Rathke's pouch ‘IS seen growmg tn the chreetmn of the iorehram to form the pnmordlum of the antenor lobe of the hypophyss Below the oral depressxon, a few mesodermal elevatxons called the pharyngeal or vxsceral arches are {armed

mono NASAI. moms

NLSAL 9&1

F1: 7-} —Dmg1-am showmg the fcn-matum of face an an embryo of about five weeks DEVELOP\‘lEl\'.l‘ OF THE ALIMENTARY SYSTEM Ill.

on the ventral and lateral wall of the pharynx Each pliaiyn

geal arch consists of a mass 0£_l‘t'lQSO(lEl"K11, lined internally by the endoderm of the pharynx and externally by the tick» derm The mesoderni of each pharyngeal arch is modified into branchial muscles and a bar of cartilage Between the pharyngeal arches externally there are deep ectodermal depressions or grooves, where internally the endoderrn projects out in the form of pouches Each ectodermal lirzixicliial yaove comes into close Contact with the endodemial pharyngeal pouch, with only a thin layer of niesoderm between them The musculature of each pharyngeal arch is supplied by a special visceral motor nerve and a pharyngeal arch artery arising from the aortic trunk and Joining the dorsal aorta The first arch is situated immediately caudal to the oral cavity and as it takes part in the fan-nation of the mandible is called the nwndibular arch The next is the second or the hymd arch which becomes

MANDIIULAI iuici-x HYOIXI Aiicii om: visicui \

\ awazfl/‘\

fia 75 -—Srde men: of face of an embryo a] about five weeks

transformed into the hyoid cartilage The succeeding third fourth, fifth and a sixth arch become less prominent and {ad'out into the neck region

ln the fifth week, the head is acutely ﬂgced and the frontanaml process becomes pmmment and bent downwards On either side of it, local thickening of ectoderm known as nasal placodc: appear Gradually owing to the rapid growth of the surrounding regions these plaoodes sink down as the olfactory or nasal pill The mesenchynial tissues on both sides of the olfactory pits project out as meiliol and lateral nasal pro112 HUMAN EMBRYOLOGY

cesses At the sxdes of the fronto nasal process near {he angle of the oral cavity, are rudn-nentary projecnons of the maxxl lary procases from the first arch Thus the oral cavuy xs bounded above by Lhe overhangmg fronto nasal process and the medial and lateral processes of the nasal puts bounded laterally by the maxxllary processes and below by the mam}: bular processes {using In the mxddle lme to form the xnandxbh

urr NASH.

nous: FRDNTO-NASH. Pnocrss

um Nltslu. noes;

Pm 76—Dmgra.m. shcvmng the fomuztwn of face m an enlbnlcv of about :1: weeks

In the sxxth week the maxillary processes gl’0Wln‘.§ across the lower part of the frame nasal process push mto the nasal proceses toward the nuddle Inn: At the same tune the oral cavxty becomes hlghly vaulted

mkxmaun Anni puuwmxvllux ucu

mo M.\:mmn.An cu}? mom Ancx-I

rmuzaw uznul. LAT nnsu. vnoc. HASH. rnoctss

Fm 77 —Sxde men: of [ace of an embryo of about :11: weeks DEVI-1.0P\ﬂ-INT 01-‘ THE AUMENTARY SYSTEM 113

By the exghth week the maxrllae approach the middle lxne by mcetmg the lower part. of the medxal nasal processes thereby formmg the prumtwe Lmtenor and posterzor "are: The deep portxon 0 the maxtllary process extends backward below the pastes? ‘ares to form the prtnntxve palate The nasal puts then deepen to form the prunxtwe nasal cavztzes and are connected to the oral cavxty by the postenor nares Th deep narrow portion of the fronto-nasal process between the nasal cavltxes becomes thm and forms the prumtwe nasal septum Musodermal extensrons form the maxxllary proceges meet together medxally and form an antero-postenor progec tron Joxmng the pnmltwe nasal septum Thxs xs the cnrtxla

mom mm:

Flt: 78 —Dxagrnm showing the formattan of face 111 an embryo of about seven weeks

_)l1t0u8 part of the nasal septum The lronto-nasal process aboxe rt forms the bndge of the nose The lateral processes form the alae of nose Due to drlferentlal growth of the medxal and the lateral processes the nose now looks ﬂat wtth nostrils wide apart

Where the maxxllary process of each sxde meets the lateral nasal process, a narrow groove xs seen dxrected upward and laterally This 15 the 1Iaso—ma1~xlln11_/ groove It was thought that tlns gro\e fanned the naso-lacnmal duct but the ongm of the 11350 Iacrunal duct ls from an epithelial mgrowth medial to the nasosmazullary groove m a lme parallel to It

From the thnrd month the contour oi the {ace acqunres

9 114 HUMAN DIBRYOLOGY

OH’ DH? 1’! M. NAB. SDTUM

MAX. TUB) HAS LAC]. DDITI’

LAT PALR1’ PIDC.

rouam: fiG 79 —Schemanc sc-non of the head of an embryo of about

memo Nun nous:

M12) nun mucus: trusma)

F1: 30 —D:agmm shounng the jormnmm of face m an embryo of about eight tceehs

a dlstmcﬂy human appearance The porbon of the face between the eyes grows more and bulges outward The eye: sxtualed laterally assume an antenor posxtxon and the external part of the nose px-o5ecLs out The upper part of the front!» nasal process becomes the forehead. The maxtllae and mend. ‘ble grow to proper stzes The outer openmgs of the nose become temporanly closed by the prohferahon of the hmng epxthelnxm.

The lower hp 1: formed {tom the growth of the free edge navznommr or rm: ALIMENTARY sysnr: 115

MAIN. SIIPTDM

ETHHO-TUE3

MAX. TURB. mu. uxcn. "“"“" DUCT MOUTH CAVITY ILECK. CREW

TONGUE

fiG 31 ——5chc1nutxc sectxon of the head of an embryo of about two and ct half months

of the mandxbular process In the dxrcctxon of the arch of the yaw Thxs growth cunsxsts of a mesodermal thxckemng covered by the eptthelnum of the oral cavxty Its antermr part becomes separated from the rest by a thzck mgrowth of epxthelxum called labml lamnw. whxch grows deep mm the mescnchymc In a dtrectmn parallel to the arch of the jaw Later, the mare ccnu-al masses of this lazmna dzsmtegmle and a groove called the tabla-gmguml groove 15 formed separatmg the ongmal growth from the mnndnbular process mto the lower hp and the gums

The upper ltp ongmates laterally from the growth of the free edge of the maxxllary processes and medially from the louer part of the [rant-nasal process The fonnatmn B lzke that of the lower hp Some mvestxgatnxs belxe\c that both the supczficxal and deep portxon of the mocha} part anse from the {roruo-nasal pracess (Kath, 19.23)’ But from studzes of congemtal anomalnes xn tbs regtcn, 1: Is evident that only the deep parhon at the medial part nnses from the fronto-nasal

‘KEXTH. A. (1933) Hurnnn Embryology and Morphnlagy Anwld. London 116 HUMAN EMBRYOLOGY

process, whxle the surface epithelium covering the medial part is derived from the medial extensions of the epithelium of the maxtllaxy processes (Boyd, 1933)’ The phxltrum of the upper hp is produced, probably by the crow ding of the maxi] lary mesoderm of both the sides The muscular part of both the lips takes origin from the mesoderm of the second or hyoid arch, which migrates to the face, along with its nerve supply, the seventh cramal nerve The epithelial coxenng of both the lips is derived from the surface ectoderm The nerve supply of the epithelium of the lips is derived from the labial brandies oi the maxxllary with twigs from the infra orbital plexus The lower lip receives branches from the mental part of the inferior dental

As the upper and lower lips fuse at the angle of the mouth, there is a thick spreading of the mesoderm of hyoid origin which gives rise to the buccimitor muscles of the cheeks The cheeks also are hned by ectoderm

The eyes arise from the prunary optic xesicles of the forebram and occupy the surface ectoderm by the sixth week, behind the closed eyelids By the seventh month eyelashes and eyebrows appear (See Chapter XII)

THE PALATE

In an embryo of about six weeks, the oral cavity or stomo daeum is bounded above by the maxillary processes (using with the ftonto-nasal process, and below, by the mandibular processes fusing together in the middle line The palate arises from the maxillary and frontmnasal processes

By the end of the exghth week, shelf like out growths called the palatine processes arise from the deep ponion of the maxillary processes and extend backward and medially deep to the opening of the stomodaeum, below the posterior nares At the same time, at the extreme rostal portion, a backward horizontal PYOJECHDD from the fronto nasal process forms the premaxillary region of the upper Jaw An extension of the

- BOYD, .1 n (1932) The classification of the upper up m mammals J Am Land, 67 DEVELOPMENT OF THE ALIMENTARY SYSTEM ll?

premaxillary region arises posteriorly as a small triangular process called the median palatme process When the palate is beginning to be formed the tongue which is developing from the floor of the mouth projects between the lateral palziune process and the roof of the primitive oral cavity Just at this time, the mandibular arch grows in length and width placing the tongue at a lower level

Due to further growth. the shelf like outgrowths which become the lateral palatme processes meet and fuse together and in the rostral area, they {use with the median palatine proccss The fusion progresses from before backward and ]OlnS the lower free edge of the nasal septum oushiug the posterior nares liackivard The nasal region is therefore sepa rated from the mouth cavity The palate that is thus formed is osified later (membranous) and becomes the premaxillae, palatine processes and palatine bones Posteriorly the palate extends beyond the nasal septum to form the soft palate and uuulu

Di:ri:crs or D£vi:LoPi.ii.N-r or THE FACE

JAWS AND PALATE

Cleft of the Upper Lip Lateral to the midline extending along the upper lip and the anterior part of the alveolus This is often called Complete hare lip" and is due to incomplate or anomalous fusion of the medial and lateral nasal processes with the maxillary process

Medial cleft of upper lip is due to lack of fusion of the l0\VEX‘ part of the two medial nasal processes

Cleft Palate May be unilateral or bilateral, complete or partial Cleft palate is due to failure of fusion of the palatme processes with each other and the nasal septum It is associated with cleft upper lip Genetic factors probabl\ play :1 part in the aetiology of cleft lip and cleft palate

Oblique Facial Cleft This takes place from the uppci lip through the nose or to its later side, extending over the cheek to the medial part of the eye Failure of fusion of the maxillziry process and the frorito-nasal process was considered 118 HUMAN EMBRYOLOGY

i

Fm 82-—A case of bzhzternl cleft hp (By the courtesy of the Prmmpul G M College)

as the cause of lhxs defect But Orbon‘ (19-14) Lhmks that 1t 15 caused by a traumatic mjury to the face of the embryo by adhesxons of the ammon m the region of the nasa lacnmal duct

Micro-Guathus (Small Jaw) xs ﬂue to dxmxmshed growth of the mandible The external ears may remam at the neck

11 the ongmal hyo mandxbular cleﬂ

'OR.BAN 3.. (19-14) Oral Hxslology and Ernhnolugy Henry Kanpton Landon DEVELOPMENT OF THE AIJMENTARY SYSTEM 119

Mici-ostouius (Large size mouth) may be caused by ii-i~ complete fusion of the upper and the lower lips

Tue TONGUE

The tongue originates from the ventral ends of the bmnchial arches, and is divided into an ai-iteriar two-third and a posterior one—third part

In the pharynx of an embryo of about the fifth week, on the ventromedial part of the first arch, paired thickenings are seen These are the lateral Imgunl swellmgs. Between them is a small projection called the tuberculum tmpar The middle part: of the second and third arches are fused to gether by another PTDJECUOII called the copula Below the copula, there IS the liypobranclizal enunence and the epiglattts Between the tubei-culum impar and the copula there is a small pit called the forumeii cuecum. which is the site of origin of the endodermal outgrowth forming the thyroid gland

WEIR Huvux rrumm CAECUM LA‘ LLNGDAL swI:u.n«:

sopuui A KYPDBBANCHIAL Emu.

Ikmrntnom swuuna

Fm 83—Sclienuztxc men; of the floor of the pharynx of an embryn, between three and four week:

The Anterior Part During the sixth meek, the lateral lingual swelling: grow large and unite with the tubei-culum impar The tuberculum lﬂlpill‘ itself dues not grow adequately and becomes msignificant, fusing finally with the lateral lm gual swelllngs. The line of fusion of these lateral swelling; forms the median sub’.-us of the tongue The anterior part 120 HUMAN ERBRYOLOGY

hecnmes covered by ectodennal epithelium from the oral cavity. which develops papillae on the surface Since this part of the tongue is mostly derived from the mandibular arch, the nerve supply of its epithelium Ls from the mandibular division of the trigeminal nerve (of first arch) and from the chorda tympani branch of the facial nerve (of second arch) The forsmen caecum is the chief landmark in the early development In the adult tongue a ‘V’ shaped sulcus tertnirialis with the foramen caecum at its apex, marks the boundarg between the anterior and posterior part:

TVIE. XMTAI FDIAMIN CAKIUK LAT L-DIGUAL IWELING

Fm a.A——schemanc view of the floor of the pharynx oi an embryo, between five and ti: weeks rhoumig the pmnordia. of the tongue

The Posterior Part (Rant) The copula grows encroach ing upon the second, third and also the Iourth arches anu forms the root or posterior part of the tongue It is cnvered by the endoderm from the pharynx. The third arch mesoderm fuses with the first arch mesoderm obliterating the first and second pharyngeal grooves ventrally The nerve Suppl) of the surface epithelium is from the glosophsryngeal nerve (of third arch) A small area at the posterior most part of the tongue near the hypohranchial eminence ls supplied by the superior laryngeal nerve, branch of the ugus nerve (of fourth arch) DEVELOPMENT OF THE ALIMENTARY SY5TEA/ 121

REGION or UK‘! um: swi1L Mnmul suicus /

/

Nzaioiv or Twin iMi>iiii

PORAMXN CAIICUM

fiG 85 —D1.agram showing the development of the tongue in an embryo of about the second month

The Muscles By the seventh week, the tongue becomes a prominent. organ with the growth of muscles inside The mesodenn arises mostly from the ventromedial parts of the first and third pharyngeal arches The muscles arise from the same regions, but from a study of the development of other mammals, it is evident that the striated muscles arise from the occipital myatomes It is possible that three iir four occipital myotcmes migrate mm the lingual region. carrying the hypcglossal nerve with them

The Surface In the early stage, the surface epithelium grows forming a layer of cuboidal cells (2 or 3 cells thick) By the seventh week, papillae develop on the dorsal surface The circunwallate and {oliate papillae are supplied by the branches of the glossophzii-yrigeal nerve Later, finigilann papillae gmw with the branches of the facial nerve The Idifar-in papillae arise during the third month They are not associated with any nerves

Summing up, the tongue arises from the ﬂoor of the pharynx with zi cmenng of ectodermal and endodermal epi« (helia The epithelium of the anterior 2/3 of the tongue is supplied by the lingual nerve from the mandibular division at the trigeminal nerve and by the chorda tympani branch 122 HUMAN EMBRYOLOGY

of the facial nerve The epithelium of the posterior 1/3 of the tongue is supplied by the sensory fibres of the glosso pharyngeal nene and an area in front of the epiglottis by the superior laryngeal nerve a branch of the vagus nerve

Tl-LE Turn-i

The teeth of the primitive vertebrates are simple conical pX‘0)ech0nS consisting of an outer covering of modified sur face epithelium and an inner core of connective tissue layer from under the skin both of dermal origin The same structural plan is maintained in the formation of the teeth of certain lugher mammals including man In the human teeth the enamel is derived from the ectozlerrnal la) er of thprimitive mouth cavity growing as a thick layer into the underlying mesoderm The mesoden-nal mass forms the dentine and the pulp of the tooth All the teeth are attached to the bone of the ]a\VS by the cement produced by the acti vity of the mesodermal mass of cells near the bone of the )3WS

It has already been considered in the formation of the lower lip that the labial lamina appears as a tluck ingrmvth oi the primitive oral epithelium in a direction parallel to the Jaw During the sixth week, there appears a thickening of the oral cctoderm on the inner aspect of the labial lamina of each Jaw By the eighth week a band of this thick ectoderm grows into the underlying mesoderm parallel to the arch of the Jaw on the inner aspect of the arch formed by the labial lamina Ths thickening is called the dental lamina The dental lamina proyects from the concavity of the labial lamina and extends into the region of the gum Along with the establishment of the dental lamina knob-like round or oval thickenings called the enamel organs arise from it at regular intervals corresponding to the origin of deciduous teeth. Each enamel organ grows deep into the mesenchyme and becomes connected to the dental lamina bv a narrow stalk The enamel organs for the permanent teeth arise later and remain inactive for a long time till the Jaws are sulficientl) strong to hold them DEVELOPMENT OF THE ALIIMENTARY SYSTEM 123

!'MHI:CUl.A ar mumuu:

Fm 86 —Sectlon of the Jaw showing the development of teeth (seventh week)

B00421. ORGAN nnruu. LAMINA

fiG 37 —Secnan of the jaw showing the development of teeth (exghth week)

The enamel organs, gradually became hollowed out fomung a convex outer wall and a concave mner wall The cells fonmng the convex outer wall are at firs: cubmdal In shape, but later become ﬂattened and conshtute the outer wall of the enamel organ The IIIIIEI‘ layer 15 made up of columnar cells and as may gaze 01-.-gm 19 2.59 enamel. .15 kzwwzz as the tuncloblastlc layer Between thus layer and the outer layen of the enamel organ there IS a louse mass of mlchcellular lluld called the stnllale rehculum The outer wall of the enamel organ LS ulmnately reduced mto thm folds allowmg the surmundmg mesa.-nchyme cells to grow between them 124 HUMAN EMBRYOLOGY

mm or raw. 100114 DENTAL PAPILLA

onomoausnc

uumu or “"3

Ducmuous ‘roan!

Auzzoausnc LAYER

“ g ourm wnu. or numu. ‘ exam

Fm 88—Sectmn of the jaw showmg the development of teeth. (thlfd month)

Wxthm the concave part of the enamel organ there forms a prmectnon of a mass of mesenchyme cells called the dental papxlla whxch later forms the pulp of the tooth The cells of the dental papilla gradually grew mto a duck mass and the cells on the outer part xn contact with the ameloblasuc layer become columnar m shape and assume the funcuon of secretmg dentme They are called the odontoblasts Blood vessels and nerves appear :11 the central poruon of the papilla Gradually the hzghly vascularxzed dental papdla grows more In the concave part of the enamel organ and projects mto the stellate retxculum Consequently the dental papxlla wxth the odontoblast comes xnto close contact wxlh the amelublastxc layer of the enamel organ m the regmn of the up of the future crown of the tooth.

Owxng to the mtxmate contact wxth the blood capnllanes of the dental papxlla the ameloblasts and the odontohlasts be gm to secrete enamel and dentxne respectxvely The odcnto— blzwts secrete dentme toward the enamel organ Along wxth the progressive accumulatxon of dentlne the odontoblasts recede toward the pulp While receding the protoplasrmc strands of the cdontcblasts become embeded 1n the dentme as dentmal fibres These p1‘D1.0pl3Sl1\lC fibres probably help m mamlznnmg the normal physxology of the dentme The odnntoblasts re ma.Ln throughout separating the dentme and the pulp DEVELOPMENT OF nu: ALIIVIENTARY SYSTEM 125

Iznu. -roam Lutuu 5 numzl. onclm

ELVISSLN

oDom‘o:u\s'nr LAYER

MANDIILE

AMZLOBLASTIC LAYER Dunn! wnu. or IMAM. om:

Fm 89—Sect«m of the Jam showmg the development of the teeth (about fifth month)

avluiolmbum

Kfimcggdxg ODONTOBLASIS 2;::‘*:~:'~‘==*-“’ *1, ~

W “gag! :5

aura WALL AMELOILASTS MBENCXYMZ CELLS or :muu:l. or PULP cnam

fiG 90 —-Seetmn of emzmal organ (fourth month)

Dunng the sxxth month whxle the denltne ts bemg formed the ameloblasnc layer secrets an organic substance called the calco-glabm which later turns mm a hard mass In the fomx af mmute rods or pnsms held together Thae enamel pnsms are arranged and to end {mm beneath the ameloblasnc layer toward the dentme The growth of the tooth ts from the 126 HUMAN EMERYOLOGY

dentine enamel iunction The ameloblastic layer graduall) recedes toward the surface and finally disappears leaving xi thin membrane on the enamel called the No.rmutb'.s menbrane

The formation of the enamel and the dentine starts from the tip of the crown, slowly progressing toward the root of the tooth. The crown is fully formed and remains beneath the gum in the full term foetus By the tune tooth begins to erupt, the root gradually elongate; and gets fixed to the bone of the Jaw by the cement

During the early stages of the formation of the tooth there forms a mesenchymal condensation of cells between the dental papilla and the developing bone of the Jaw Later, this mesenchymal mess spreads around the whole tooth bud as the dental son The portion of the dental sac over the crown disintegi-ates during the time of eruption of the tooth The remamuig portion wluch accumulates around the root of the tooth, transforms into a pennsteurn lts fibrous layer becomes continuous with that of the bone of the Jaw Hence the two periosteal layers form the pendevitul membrane The peridental membrane around the root of the tooth secretes the cement by which the tooth is fixed to the bone of the yaw

The development of the permanent tooth is exactly as the development of the deciduous tooth mentioned above The enamel organs of the permanent teeth arise from the dental lamina as secondary buds near the origin of the deciduous teeth The dental larmna also extends beyond the last deci duous tooth producing the enamel organs of the permanent molars Slowly the dental lemma loses contact with the sur face epithelium and disappear:

Dana:-rrva Dzvtmriiiurr or ‘ll-ll 'l‘r.I‘.'rH

The following are a few defects in the development of teeth 1 Irregular shape and variations in number at the roots. 2 Irregularities in number, size and position

3 Absence of a few or all teeth

4 Fonnation of extra teeth in other regions in the mouth

especially m the palate DEVU..0P\1ENl‘ OF THE ALIMENTARY SYSTEM 127

Defecuve enamel formauon due to the dxsluxbancc of calcxum deposmon

6 Formalxon of cysts due to the remnants of the dental pnmordxa. collectmg together m the gum

on

Tm: Txrmom Gum»

On the ﬂout of the pharynx of an embryo of the founh week, between the first and the second pharyngeal arches m the nudlme, Just below the tuberculum nmpar, 15 seen an euzgmalton of endoderm Th|s evagmatmn forms a dwert: cnlum, \VhJCh is the pnmordxum of the thyrmd

By lhe fifth week, the dwernculum grows mm a b: lobed

emu. CIwn'Y

I Pxluna POUCH

Mtnuul nfrnoxn

n POUCH DIVER‘!

Ill POUCH

XV POUCH

Fm 9L—Pnmxhu¢ pharynx of an embryo of the fourth week‘) (Vcntval mew) 128 HUMAN l-DVIBRYOLOGY

mass and the cunnezztioii to the pharyngeal part is thinned out into a narrow duct called the thyrwglossal duct The paint of origin in the pharynx is marked by a depression called the foramen caecum

The duct soon elongates and the bi lobed diverticulum of the thyroid migrates caudally ventral to the pharynx By the sixth week, the narrow duct ruptures and the Lhyroid diverti culum occupies the region of the IV pharyngeal arch near the upper part of the developing trachea The two lobes of the thyroid elongate on either side of the midlirie and are ]OmEd together by a narrow isthmus of cells, probably derived from the ventral portions of the fourth and fifth pharyngeal arches

mun. uwrrv I mock!

mvnocwsslu. nucr

ATRDPHY or piuuiv CONNECT

THYIIOID

IHYMUS riulrri-nmom 11-nmus

DESOPHAGUS

LUNG-BUD

sroiuicu

fiG 92 ——Fo4-nmtwn of the thyroid, parathyroid and thymus Ul an embryo of the seventh week DEVELOPMENT OF THE ALLMD\"'I‘ARY SYSTEM 129

By the seventh week, the elnngalmg Jayrold lobes meet and fuse wxth the ventral part of the fourth pharyngeal pouches On the caudal part of the fourth pharyngeal pouches there anse small dnerttcula (one In each pouch) called the lateral 17:1/told bodies or postbranc-hull bodxes These fuse wlth the medxan thyrmd dlverttculum on erther Slde Some behave that the postbranclual hoches transform Into the thyrord tissue, but most mvestxgators behave that they exther atrophy or bccome transfonned mm bemgn thyrozd cysts

Structure of the Thyrold The nngmal dnertxculum 15 made up of endcdermal cells arranged around a lumen These cells grow and as the surmundu-lg mesenchyme Invades mto them, become arranged tn the form of Isolated cords of cells Dunng the thud month these curds prollferate mto cell masses to form the thyrold Iollxcles By the fourth month collozd appears 1n the centre of the follxcles The nelghbounng mesmchyme becomes transformed mta the t'ibro—elastzc txssue and blood vessel: of the gland

DEFECTS IN 1111-: DEV!-1LOMl‘.N’l' or THE Tmmom

Perststcut Thyro-glossal Duct The \vhoIe duct may persxst leading tron: the foramen caecum to the larynx Parts of the duct may be converted mto cysts, accessory thyrolds or fistulae

Tm: PABATHYROIDS

The prunordla of the parathyrmd glands arse from two pairs of pharyngeal pouches One paur, dexelopmg as a Solid bud from the dorsal part of the thnrd pharyngeal pouch u calla.-d parathyroul Ill, and the other, from the con-espondxng [nrt of the IV pouch 5 called parathyroid IV

Parathyrotd Ill arises near the orrgtn of the thymus (also from the thud pouch) Durmg the seventh week, both these pnmordm assocxate together and mxgrate caudally dctachmg themselves from the punches at ongm. As the thymus rmg rates sull more caudally to the thorax, the parathyrmd Ill L) left attached to the thyroid and becomes the xnlenar para ‘I 130 BU MAN BBRYDLOGY

POUCH

131301»

I1 P0753

PARATHYIDTD LU

N 90176)! r

um»:-us Iv

MING-IUD

msonuaavs

Fm 93—Scb2mcmc plan of the denuzmes 0! the pharynx (Ventral mew)

thyrond of adult anatomy

Paralhyrozd IV anses from near the ongm of the post branch al bodxes and m assoctanon wxth them becomes at tached to the Lhyroxd capsule at the same level Tins :3 thu. supe-nov paruthyroad of mink anatomy

The two pnmordna are sumlar an scruclure Both anse .5 sohd masses of cells and later break up mlo cords of cells and cell nests wxth capxllary spaces between them In foetal hie there 15 xery lnme connecme txssue and sery mile for I manor: of a capsule DIIVELOPMENT OF THE AIJMENTARY SYSTEM 131

The common defects in the development of the parathy rord are —

The rmgratory changes 1:: position and the formatxon of supernumerary parathyroids Small glands may be found m the connective tissue around thyroid and thymus

THE 'l'inauus

The ongui of the thymus is from the III pharyngeal pouches In some lower animals thymus also grows from the IV pouches In the human embryo the primordia from the IV even if they are actually formed become msigmfieant and dxsappear early

In the sixth week on the ventral aspects of the III pouches near the ectoderm of the pharyngeal clefts ewaguiatlons one on each side appear as clul>—shaped hollow structures These are the pnmordia of the thymus By the seventh weel. the» elongate and come in contact with the parathyroid III and the hollow lumen becomes solid by the rapid prolxteratxon of cells Soon they lose connection wrth the pharynx and the broad dxstal parts meet together towards the mid] ne Just be low the devclopxrig thyroid By the beginning of the second month the distal ends youimg together grow into the thorax behind the sternum and remain tn the mediastmum near the parietal layer of the pericardium Later the two components fuse together as the thymus gland

Structure The primord um of each gland consists of thxck cords of cells During the third month these th ck cells break up into thymic corpuscles. The surrounding mesenchyme cells Am ade the gland and form the reticular connective tissue in which lymphocytes appear The lymphocytes rapxdly proliferate and predommate the substance of the gland Mean “Jule the thynius becomes Jnbulatod and each lobule dc: 4-lops Into a cortex and medulla

T1115 1:. the characteristic appearance of the thymus m the full; grown loctiuz and in young persons up to puberty After puberty the lymphoid tissue gradually dogenerates liemg re placed by (at or connective tassue or by both 132 HUMAN EMBRYOLOGY

Dsrzcrs or DEVELOPMENT or 1H: Twnxvs

The cords of thymlc tlssue at the upper ends ma3 persnst

‘P11: Tousxns

The palatme tonsds anse from the regressing H pharyn geal pouches By the end of the third month the connective hssue contalnmg lymphocytes mvades mm the II pouches As the lymphocytes grow more the endodermal covermg grows mto the lymph substance as solxd lammae which later on break down to form crypt: Dunng the sesenth month the lymph substance organises mto defimte iolhcles and the crypts branch xrregularly from the pharynx mto the mass of the tonsxl Gradually the tonsnl acqulres a capsule by the com pressxon of the surroundmg connectne txssue After lnrth the tons] contmues to grow 1;: bulk mxgratmg to xts defimtne posxtmn

The pharyngeal tonsxl (adenolds) on the dorsal wall of the pharynx and the lmgual tonnl under the dorsum of the tongue «use sumlar to the palatme tonstls bexng xnvaded by lympho cytes

Di:V1-:LOP1o.u-INTAL Dsrzcrs or me P1-x.uxn~.crau. Rzmov

1 Cervlcal cgst Ls the result of an imperfectly closed pharyngeal cleft usually occurmg between the second and thmi cleits A closed sac 15 formed m the cleft either from the ectoderm or the endnderm.

2 Cervical smus IS an openmg from the pharynx to the surface of the neck along the medial border of the sternemastmd muscle The pharyngeal cleits are mcampletely closed to form a pouch from the ectodermal gmowe towards the pharynx or from the pharynx as an endodermal pouch towards the surface Thus the pharynx and the surface of the nee}. are m commumcatxon due to the Incomplete closure of the pharyngeal cleft. DEVELOP\!EN'1' OF THE ALIMENTARY SYSTEM 133 The OESOPHAGUS

Just hehmd the hypobranchial eminence, on the lowei part of the pharynx at the level of the IV pouches, where the pharynx narrows into the foregut. is the median ventral groote of the trachizel diverticulnm The foregut extends to the posterior margin of the septum transvexsum and the mesa‘phagus, stomach, part of the duodenum, liver and pancreas are derived from it

The oesophagus at about’ the fifth week, is a short tube extending from the tracheal diverticulum to the dilatation of the {oregut which forms the stomach The short oesophagus lies on the posterior part of the septum transversum, in the visceral mesoderm, between the pericardio-pentoneal passages Due to the caudal migration of the heart and the stomach, the oesophagus elorigates In the process of the rapid elongation, it is possible that its lumen may be temporarily obliterated The lining cells arrange as a stratified epithelium and the muscles and connective tissue are derived from the surrounding usceral mesoderm

Tm: Sroiiuicii

The primordium of the stomach is seen by the and of the fourth week, as a fusifoi-m dilatation of the caudal part of the oesophagus Its original position is in the midline of the body and is attached to the posterioi wall by the dorsal ma seiitery called the dorsal mesogastriuiii The anterior part coming in contact with the septum transversiim becomes the Leritra! inc-sentery called the central mes-ogastrium The primitive stomach at about the fifth week, is a symmetrical dilatation of the gut with a slight curve, the convexity being to the dorsal side, and is attached to the dorsal and ventral mcsogastria

By the seventh week, due to the changes in the growth of the €I‘J)l'JX')0 and of the surrounding organs the stomach changes in shape and position The dorsal wall grows faster than the wntral “all and forms the coiiwx greater curtuture at the place of attachment of the dorsal mesogastrium and the 131 HUMAN EMBRYOLOC-Y

nucoo nutavmz MEMBRANE

Fm 94 ——Sch2matxz: sagxtfal sectxon of an embryo of the fourth ‘week

concave lesser curvature at the place of attachment of lhe ventral mesogastnum Near the cramal part a local hrulg mg of the gut establishes the fundus The stomach now be comes dxagonally placed At the same ume the stnmach retake: 90° along wﬂh 115 long axxs The convex pan in when the dorsal mesentery ongmally was attached to the mndlme now shxfts to the left smde and the greater curvature of the stomach comes to he on the left at a caudal level The doxsal mescgastnnm IS pulled out by vxrtue of the mlauon of the stomach and forms a pouch called the omenlal bursa As the lxver buds grow mm the septum transversum the ventral mesogaslnum xs cunverted mto the gastro hepatic omenlun (lesser omentum) and the falmlonn lxgament DEVELOPMENT OF THE ALLMENTARY SYSTEM 130

By the second month, the stomach migrates from its on ginal position dorsal to the heart, into the ahdonunal cavity, due to the general growth of the einhryo and the comparative lengthening of the oesophagus The vagus nerve which estah lished connection to the stomach in the early embryo (near

the primitive heart) also descends along with the development of the stomach

Mucous glands appear by the second month and the

gastric glands begin to appear from the fourth month onwards

Tm: SALXVARY GLANDS

Parotiil Glands At about the sixth week the epithelium of the inner surface of either cheek (ectoderm) g1D\VS inwards into the underlying mesenchyine as a solid cord of cells Bv the eighth week the ingrowth elongates tow anis the ear in the region posterior to the angle of the mouth The main duct branches into cell cords which are converted into the branching ducts and the ulueoh of the gland

Subniandibulzir Glands By the end of the sixth week, two cards of cells one on each side arise near the middle line between the tongue and the gums Each cord grows along the floor of the mouth in the mass of me-senchyme and bends tcntrally near the angle of the mandible By the second month the ducts and the alveoli are formed

Suliliiigual Glands By the end of the seventh week on either side of the root of the tongue there forms a row of cell-cords growing towards the floor of the mouth The group ol cell cords lateral to the origin of the submandiliular gland foriiis the greater xublmgual gland with its major duct About five to ten smaller glands each with a small duct, form the lesser sulilmgiiol glands in the alveoltrlingual groove As the submandibular duct of each side elongates, it includes within it the duct of the greater sublingual gland also In the adult, the two glands hate a common opening

The structure of the salivary glands is similar to that of other racemose glands. The solid cords grow towards the 136 HUMAN EMBRYOLOGY

underlying meserichyme and break up into cell nests from which the acirii and the ducts develop The surrounding mesenchyme supplies the blood vessels and the connectiie tissue By the sixth month, the ducts and their acini are well established

THE DUODENUM

In an embryo of the fourth week, the intestine is .1 simple tube extending from the slightly dilated part of the foregut which forms the stomach to the cloaca The duodenum is formed from the caudal end of the foregut and the cephalic end of the midgut The lumen, at the early stage, is \ery narrow and sometimes may he obliterated for a time

It is attached to the dorsal body wall by the dorsal mesentery called mesacluodenum Along with the rotation of the stomach to the right, the duodenum forms a loop at the apex of which are seen the liver and pancreatic ducts The dorsal mesentery elongates in the intermediate portion‘, but as the stomach rotates, the terminal part of the duodenum comcs nearer to the dorsal body wall and the dorsal meseritei-y becomes shortened and finally disappears, fixing the part of the duodenum to the posterior abdominal “all from which a fibromuscular suspensoi-y ligament develops by the second month

THE LIVER

In an embryo of the third week, bet“ een the Ioregut and the yolk sac, the endodei-m of the gut thickens and bulges out ventrally forming the hepatic bus! or dwerticulum By the fourth week, this part of the gut becomes the apex of the loop of the duodenum The hepatic bud grows into the substance of the septum transversum From the beginning, the bud grows in a cranial and a caudal direction in the xentral mesemery The cranial portion forms the substance of the liver and the hepatic ducts, and the caudal portion lengthens to form the cystic duct and the gall bladder The cranial portion subdivicles into a left and a right branch and cords of cells grow out in the septum transversum From these cards DEVELOPMENT OF THE ALIMENTAEY SYSTEM 137

fivwcus AETBIDSUS

Vnvmicu:

ronncur

Miscmznmos VOL! SIC

xmnaur

fiG 95 ——-Schematic ventral view of an embryo of the fourth

week showing the hepatic dwerticuluni. and the as socuited parts

of cells branches arise at right angles and still smaller branches radiate from these and form a network of cells can stituting the secretory portion of the liver From the mesa dermal cells of the septum tmnsversum grow the capsule, the connective tissue and the muscular walls of the hepatic ducts ..nd the gall bladder The proximal part of the original cephalic portions of the hepatic diveruculuni is canalised to form the hepatic ducts Smaller ducts also arise by canalisation and become continuous ducts draining the two pi-irnai-\ lobes into the hepatic ducts The caudal portion of the hepatic dncrticulum lengthens into a long solid tube Soon, by the process of vacuoliuilion of the central cells, it acquires a lumen and the terminal portion becomes a sacular gall. bladder and the proximal portion becomes the narrow cystic duct into 138 HUMAN BIBRYOLOGY

whxch the hepatxc ducts open. The cystzc duct ccntmues 3: the common bde duct, whxch along wnth the duct of the Ventral pancreas, termmates In the duoaenum

stomutcx-x sruzut

PANERIAS DCBAL

nossu. M1:nm:nY

fiG 96 —Schemutzc sxde men: of the alnnentary organs of an embryo of about the fifth week.

As the hepatic zlxvertzculum IS smxated close to the vxtel hne VELDS 1n the septum transversum, the network of cells comes xn contact xuth the vems whxch break up xnto xrregulzr dxlated caplllanes called smusonds m the substance of the grow mg card of cells Thus, the smusmds and the [wet cards form the substance of the lner By the filth week, due *0 the rich blood supply, the liver grcms enormously large wxth a nght and a left lobe (from the cranial bud) on exther suit: of the gut and rexnams xn the upper part at the abdomxnal cavlty

By the sxxth weelg as the gut rotatu to the nght sxde the central attachment to the duodenum xs stufted dorsally Wxth the atrophy of the 501}: sac, the vntellme xexns become the \n‘o~m-ans’: oi the ‘he-pan: pofial vein. Snsteaa Di hvm the yolk sac, the blood )5 now collected from the capxllarles of the mtestme and IS paged on to the lwer through the portal Lem From the smusolds of the hver the blood reaches the centre of each lobule and 1s dramed by the hepatxc veins ‘fine DEVELOPMENT OF THE AL!MF.N'l‘ARY SYSTEM 139

portal vein and the branches of the hepatic artery divide into smaller branches with the divmon of the lobules

The splanchnic extra embryonic mesoderm over the yolk sac is the site of haemopoietic activity Ln the early embryo Later, as the extra-embryonic mesoderm extends into the sap tum trarisversuni, this property is maintamed in the mesoderm of the liver after the atrophy of the yolk sac Stray collections of cells. known as large macrophages are found in the lining layers of the sinusoids, which In the adult form the Kupﬂer cells

In the early stages, the right and left lobes are of equal size Later, due to the anastoi-noses of the vitelline and the umbilical wems, the changes in the direction of the blood ﬂou cause the nght lobe to become larger than the left The cuudaie and quadrote lobes appear by the seventh week as sub—dii.isioris of the right lobe The liver by the seventh wee}. atttuns a large size, weighing about 1/10th of the body weight of the embryo Graduallv, the growth is comparatively dimi rushed and at birth the liver weighs only about l 20th of the body weight Sympathetic nerve fibres enter the organ bv the end of the seventh week

As the liver grows, the developing gut is pulled away from the septum transversum by the ventral mesentery Sub sequently, the liver also is separated from the septum transwersum, and is enclosed by the ventral mesenterv from which the fibrous capsule and the ligaments arise The part of the mesentery extending from the stomach and duodenum to the liver forms the lesser omeiitum The liver is attached to the \entral body wall by the falcijorm ligament (owing to the persistence of the left umbilical vein) and to the septum transvcrsum by the coronary ligament (owing to the persistence of part of the mesenchyme of the septum transversum) The lateral extensions of the coronary ligament are the tnangulcir ligaments

Tm: Pnncxms

The pancreas originates from two primoi-dia-—a dorsal and a neutral bud. as endodci-mal outgrowth: of the duo1 W HUMAN DER’: OLOGY

denum The dc-Isa! bud anses by the third week [mm the dorsal sxde of the duodenum at a level nearly OPPDSHG to 21:

ongm of the Inn bud The dorsal bud grows dorsal]; between the Iayerg of tha doxsal mesentery of the duodenum The xenn-al bud anses by the begmmng of the fourth nee}. from the ventral side of the duodenum slightly to the nghl of the rmdlme and grows downwards. As the Ilse: bud grows rapxdly at ﬂux s(age the ventral pancreatic bud I5 earned along vuth 1! for some distance

aTDH.n\5X

nnaounm or an; iunbzl

VDfiIAL IUD

Fm 97—Dmgmm showmg the dnelopment of the pam.-72-n dun-mg the fourth week

4'|’OMA€|'l HDAHC nucr

Gnu ILADDI3

DGBAI. nulcltu

VETIAL PAJIQTAS

1-‘Ia 93—Du:grrm shmnng the development of the pancreas duv-mg the fifth week DEVELOPMENT OF THE AUMI-ZNTARY SYSTEM 141

The duodenum now rotates about 90° to the right, so that the ventral part comes to the right side The Ventral pancreatic biid on the right side grows into the clurszil mesentery of the duodenum, where the ventral and the dorsal buds meet and lie side by side By the sixth week, they fuse together to form the pancreas The lower part of the head of the pancreas is derived from the ventral pancreas, and the upper part of the head, the body and the tail are derived from the dorsal pancreas When the two primordia (use together, their ducts anastomose with each other Eventually, lhv tentral duct becomes connected through the common bile duct to the duodenum The proximal part of the dorsal duct slowly atrophies In the adult pancreas therefore the distal portion of the duct is derived from the dorsal pancreas and the proximal part from the ventral pancreas

CALL luinvm

ATIDPHY or non DU

DUODEN boizslu. Mr:

FUSION or vmr A

court. IILE m.n:r nails rANci1i'Jis

VDt'l' DUCT

fin {)9 -—-Dmgmm showmg the deuelopment nf the pancrcns duruig the scunitlt neck

The histogcnesis of the pancreas is as of other secreting glands The original cords of cells break up into cell clusters ‘ind gradually form acini and ducts of the gland The islets 142 HUMAN EMBRYOLOGY

of Langerhans appear as cell groups from the ongmal cell cords

Tue Snmu. lm'r.s1-me

In the fourth week the gut extends from the pharynx to the clean: In front of the notachard The cephalu: portmn forms the foregut With the development of the oesophagus the stnmach and part of the duodenum uptu the entrance of the common bxle duct The part of the ahmentary canal vthxch develops from the roof of the yolk eat: is the mxdgut from whxch the small mtesune anses The gut tcwaxds the caudal end xs the hmclgut The enmmunxcatxon between the fox-egut. and the mxdgut as known as the antenor mtestmul portal and between the nndgut and lnndgut the postenor In testmal portal. The gut, from the stomach to the cloaca Ls attached to the pcstenor abdumxnal wall by the not-sal me sentery

PHICIIDIUH

cucvu

CDHAIJC NM! or nmenr

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UIIOIZCTAL smw

The 100 —5chemutzc sxde new 0] the almxenttn-y organ: of rm embryo of about the seventh week. Dl~.V'l..bOP\l'EN'l' OF THE ALILIENTARY SYSTEM 14.!

During the fifth week, the yolk stalk becomes very nar row as the vitello-intestinal duct The midgut increases in length stretching out the dorsal mesentety with IL Consequently a loop of the gut: extends ventrally into the body stalk. The limb of the loop from the stomach to the yolk stalk is called the cephalic limb, and from the yolk stalk to the cloaca the caudal limb The vitelline arteries which are the ventral branches of the descending aorta fuse together as the superior -mesentcnc artery which passes through the dorsal mesentery to the apex of the midgut loop and finally reaches the yolk an

A bulging or diverticulum appears in the caudal limb of the loop which later on develops as the caeeum and marks the point of mnction of small and large intestine Very soon the primary loop twists in an anti clockwise direction around the course of the superior mesenteric artery As a result the original cephalic limb moves to the right and remains below the caudal limb which is shifted upwards and to the left as the proximal 2/3 of the transverse colon

By the seventh week the gut lengthens rapidly and is pushed into the extra—eruhryonic coelom of the body stalk causing a temporary umbilical hernia At the same time the cephalic and caudal ends of the midgut are fixed to the dorsal body wall by thickened mesentery The cephalic end \\hlt’.‘l1 forms a part of the duodenum is fixed by the prorimal re immon band and the caudal end at its iunction with the hindgut by the distal retention band

The cephalic limb now lengthens rapidly into many can volutions. By the tenth week the abdominal cavity of the embryo has increased in size to accommodate the expanding gut. The promoting part of the coils of the intestine returns into the abdominal cavity Soon the extra embryonic coelom of the body stalk obliterate: As the return of the intestine is etlected rather quickly, the convoluted right loop slips down first, pushing the hindgut to the extreme left side The remaining distal portion of the loop enters the abdominal cavity Liter, and lies above and xentral to the proximal loop The caecum therefore remains in the right upper region of the abdomen below the right lobe of the lixer The transverse

colon lies acros the abdomen anterior to the duodenum 144 HUMAN zmsnyonoay

H2581 SPLIIN PBICARDWM SYQNAUI BDTUN TBANSVESVM U“; DANGER! DBCDIDDIG CD!-OI

CALI. ll-ADDB

vm:u.:> DRE. DUC1 UMIWUD5

Imus

Fm 101——Scl:emauc sketch of the altmemary organ: at about the tenth week (sxde mew)

Due to the shortemng of lhe proxu-nal xetenhon band the termmal part of the duodenum along wnh the adgacenl mu num is pulled upwards and stationed to the left of the head of the pancreas Slowly by the seventh rnnnlh the caecum descends Into the right xllac Iowa esfablxshmg the nsceudmg colon

The cap)-lalxc hmb of Ike ongxna] gut locp grous to about 20 feel. forrmng the terminal pan of the duodenum the new num and the upper pan of the ileum and the caudal lxmb DEVELOPMENT OF THE ALIMJZNTARY SYSTEM 145

grows to about 2 to 3 feet forming the lower ileum and the proximal 2/3 of the transverse colon

The circular muscle coat of the intestine appears by the 21 mm CR stage and the longitudinal muscle coat by the 30 mm CR stage

Tim CAi:cuivi AND APPENDIX

During the sixth week, the midgut takes the form of a U-shaped loop extending into the extra embryonic coelom of the body stalk the apex of the loop being continuous with the narrow yolk stalk In the caudal part of the loop a little below the attacliment of the yolk stalk there appears a dilata« tion of the gut called the caecum, indicating the Junction of the small intestine and the colon

By the eighth week, the cephalic part of the loop deve lops into it part of the duodenum, the much coiled Jeiunum and the upper part of the ileum Many changes in the position of the gut take place during the tenth week The small intestine twists and turns round the superior mesenteric artery on its dorsal mesentery and quits the extra embryonic cavity of the body stalk and occupies the abdominal cavity During all these complicated changes, the caecum at first hes at the upper region of the abdomen, Just below the right lobe of the liver and later, migrates to the right iliac fossa The colon (proximal 2/3 of the transverse) developing from the remaining part of the caudal loop at this stage passes obliquely upwards to the left side and assumes .1 positron at right angles to its gunction “Rh the tleum, where the caecril diverticuluni is being formed Gradually, the caecum grows into a big blind sac, the growth being more at its distal end By the thiid month, the distal end stops growmg and becomes narrow, appearing as if it is an extension of the dilated caecum This is the t,ermiforni oppeiidu: The appendix rs therefore the orlginal dastal pan of the caecum From the seventh niontli of foetal life, lymphocytes begin to appear in the \\alls of the appendix

10 146 HUMAN DIERYODOGY Tl-IE Lana: In-rrsrmn

The ascending and the right 2/3 of the transverse colon are derived from the caudal part of the original loop of the midgut, distal to the caecal diverticulum, in the early embryo The remaining part of the trans erse colon is continuous \Vlll't the descending colon, at the splemc ﬂexure This part of the transverse colon, the dscending and pelvic colon and the rectum are dented from the primitive hindgut

In an embryo of about the sixth week, the midgut grows into a loop supported hy the dorsal mesentery and herniatu late the body stalk. The retention bands at the proximal and the distal ends of the loop prev ent the herniation of the foregut and the hindgut. but allow a free play of the deseloping midgut The cephahc part of the loop yows as the much coiled snail intestine, which returns from the ext-a-emhrg onic coelom of the hody stalk tnto the abdommal cavity, thereh3 pushing the part of the hindgut. continuous wtth the caudal loop to the extreme left side This part of the hindgut later on forms the descending colon The caudal loop hem can the caecuxn and the distal portion grows rather slowly, forming the colon which re-enters the abdommal cavity aiteniards Hence the caecum and the colon assume a position Ventral and superior to the coils of the small intestine

The caecum lies below the liver in the upper region of the abdomen. The colon continuous with the descendxng colon at the splenie ﬂexure now stretches across the_ ahdonurial cavity, caudal to the stomach and tentral to the developing pancreas supported by the dorsal mesentel-5 As the colon lies tn a transverse dn-ectton, it is called the transverse colon

Because the dorsal ntesentery of the stomach fox-iris the omental bursa certain changes take place in the mesentery of the “’31'|5‘*‘-"5€ 0°10“ 4“-ERIE the sixth and seventh months. The oinental bursa extends to the left of the stomach, an .: caudal direction beyond the greater cunature The bursa comes in contact with the mesentery of the transterse co'on

and pages ventrally, spreading out as an apron cotenng the coils of the small intestine DEVELOPMENT OF THE ALIMI-ZNTARY SYSTEM 147

DISOPHAGDS

Doasu. MI3DGILsn11vM srowtcu

aruml

oumrm. auxsn

nonslu. Mnsmt "‘-“:S° °°"°“ or um:

SMALL M113’

Imus

Fm 102 —-Dmgrammnnc mew of the attachment of the me scmerxcs la the gut

The ptmumal part of the ovexhangmg pm-hon of the bursa fuse: wxth the lransverse mcsocolon and the cavity of the bursa xs obhlerated by the fusion of us layers Durmg all these changes, the duodenum becamcs fixed to the postenor H3 HUMAN EMBRYODOGY‘

abdominal wall by the proxunal retention band The caecum then migrates to the right iliac fossa, carrying a part of the colon along with it and establishes the ascending colon The dorsal mesentery of the caecum and the ascending colon luse with the peritoneum of the right abdominal wall

The left 1/3 of the transverse colon and the descending colon including the pelvic colon and the rectum are derived from the original hindgut Their dorsal mesentery is fused with the peritoneum of the left abdominal wall In the adult, the mesentery, except that of the pelvic colon, may degene rate The rectum is formed by the subdivision of the cloaca

The lumen of the large intestine is smaller than that of the small intestine in the early stages, but after the fifth month the lumen becomes characteristically large

Tiir: Rrci-im Ami Amii. CANAL

During the early somite stage oi the embryo, the caudal end of the hindgut (endodermal) into which the allantois open: is dilated to form the eloaca It lies in contact with the ectoderm (between the tail end and the body stalk) as the clonal membrane The early mesoderin cells from the priirutive streak migrate around the lateral walls of the cloacal membrane to reach the body stalk

During the fourth week, the inesonephni: ducts open on each side of the cloaca By the fifth week the in-arectul .l‘BEltI.1l‘l arises from the mesenchyme cells at the Junction of the allantois and the hindgut and grows towards the cloacal membrane and finally fuses with the endodermal side of the membrane Thus the cloaca is divided into the primitive in-a emtul sinus in front and the rectum behuid The region of the fusion cl the cloacal membrane and the urorectal septum is marked on the surface by the formation of the primitive perineum The eloacal membrane in contact with the urogenital sinus becomes the itmgeiiital memhzane, and that in contact with the rectum, the anal membrane Owing to excessive growths of the mesoder d membrane, there forms an ectoderrnal depression toward the gut called the proctcdaaum Gradually by the third month, the anal meniDEVELOPMENT OF THE ALIMENTARY S‘1Sl'I2?-l 149

brane ruptures and the rectum opens to the surface

In the early stages, the rectum and the anal canal are cnnunuuus and straxght, adaphng to the relamely straxght sacrum After birth, the pelvns enlarges to accommodate the VlSCE!‘d and the urogemtal organs Due to the curvature of the sacrum and the change of posxuon of the up of the coccyx, the rectum curves antem-postenorly and the anus move: forward The lateral curvature of the rectum xs probably due to the further mars.-use m the length of the gut. The anal canal grows II1 length due to the mcrease m tlnckness of the pelvxc floor The 1unctxon of the anal membrane and the rectum IS marked by the edges of the anal valves The small projection of the htndgut tuwanls the (311 end (taxlgut) gradually atrophxes The Internal and external sphlncters are formed slowly tn the fourth month

Flt: 103 —Schematu: secuan shawxng the attachment of the mese1I(e11es to the gut, durmg the second month 150 H'U\1AN ZMIBRYOLOGY A Review or me ATIACHMENT or 'n-ii: ltlzsxxrzairs

The endoderrnal gut is covered by the splanchnic mesoderm and is attached to the dorsal body wall by the dorsal mesentery and to the tentral body wall by the tentral me sentery These mesenteric attachments divide the coelom ii-ito a right and a left portion except at the caudal region where due to the early atrophy of the ientral l'hE5€ﬂlE]-y J12 X\\D coelon-iic cavities communicate with each other

The fate of the ientral nieseiiter) hear the septum trans versum as the liver cords arising from the foregut pass into the Ventral mesentery the part of the tentral mesenter) between the stomach and the liver is transformed into the gastrohepatic ligament or lesser omentum The mesentery between the liver and the ventral body wall forms the falci form lxgament of the liver due to the fold of inesentery around the left umbilical vein The mesentery Just caudal to the gastro-hepatic ligament is the duodenolzepauc ligavieiit

The remaining portions of the Ventral mesei-itery degenerate converting the two original coelornic cavities into a smﬂe peritoneal cavity

The dorsal ii-iesentery The dorsal niesenter) persists throughout fixing the gut in position and is the chief palhtta) of the blood vessels and nerves to the gut irorn the dorsal body wall The part of the dorsal mesentery attached to the stomach is called the dorsal mesogastmim and that attached to the colon the dorsal 1 iesocoloii

The «mental biusa The changes in the shape and position of the stomach and the shifting of the dorsal mesogastmim to its left side in a caudal direction he) and its greater ciina ture cause the formation of the onienznl bursa The bursa comes in contact and later fuses with the mesocolon and passes tentrally to die colon spreadu-ig out as an apron covering the coils of the small intestine. The walls of the apron like portion of the bursa unite and obliterate its catity

Along with the formation of the omenta] bursa the pleen which arises from the doisal mesogasti-iuin is pulled to I-ht’ left of it and has close against the body wall doi-solateral to the stomach. At the same time the pancreas de\2lOpt.D3 be DEVELOPMENT OF THE ALLMENTARY SYSTEM 151

xuzc-mu

fiG 104 —-Schematic section showing the attachment of the mesentenes to the gut dunng the fourth month

tween the two layers of the rnesocluodenum extends Into the dorsal mescgastnum \Vhll:l'l Is shxfted to the left side agamst Um dorsal Body wall Eventually the pancreas becomes at tached to the dorsal body wall and placed xetrcpentoncally The mesoduodenum fuses with the. penloneum of the dorsal body wall Soon, the fused perxtoneal layers atrophy and most of the duodenum comes to he agamst the dorsal wall retmpenloneally

Lesser peritoneal Sac, vcshllulc and cpnplolc foramcn The bay of aha pemoncal cavuy doxsa) in the stomach (omenlal bursa) bounded by the ventral and dorsal mast» 152 HUMAN EMBRYOLOGY

gastria is sometii-nes called the lesser peritoneal sac The uesttbule is the space in the lesser peritoneal sac between the ventral mesogastrium and the dorsal body wall A commu nication between the main peritoneal cavit) and the vestibule of the lesser peritoneal sac is known as the epiploic fornmev It is bounded ventrally by the free border of the gatro hepatic ligament, dorsally by the inferior vena ceta, cranially by the caudate lobe of the liver and caudally by the wall of the upper part of the duodenum

Dsrecrs or Dsvssomsnr or THE AXJMENTARY CANAL THE OEOPHAGUS

Atresia and Stenosis During the second month there is excessive growth of the lining epithelium of the digestive tube, practically obliterating the lumen Gradually, the lumen becomes wider and the lining cells rearrange in proper layers If the latter process does not take place, it may cause atresia of the lumen or stenosis, resulting in the follow ing conditions —

In Atresin The upper part of the oesophagus ends in a blind pouch The lower part reinau-is normal or may be transformed into a fibrous cord The remaining part also remains as a fibrous card In some instances, the lower part communicates with the trachea or bronchus

In Stenosis There may be fibrous narrowing at any re gion of the oesophagus (Both these conditions are ineurn patible with life)

Oesaphegeal Webs They are redundant folds of the lining epithelium Sometimes they are capable of causing obstrue tion

Tiie STOMACH

Transposition of the Stomach to the right side This conditien is due to failure of uorn-ial rotation of the foregut

associated with the general transposition (mirror image) of the whole viscera DEVELDPMXZNT OF THE A!.l\lENTARY SYSTEM 153

Cascade Stomach Owmg to change of shape of the stomach a grater pol-non of us vemcal hmh ls dxsplaced forwards

Volvulus of Stomach (total or partial) This occuxs owmg to excessne rotauon of the stomach (May be assocxated wxth rlnsplacement of the colon, pancreas and spleen)

Congenital Pylonc Stcnosls This condmon xs due to the thzckcmng of the musculature and nan-owmg of the pylorus

(symptoms of regurgxtauon and vommng occur wxthm a few days after birth)

Tm: SMALL lnnzsrma

Non Rotation of the Intestine This 15 a condmon of finlure of rotation of the mtestme The C0llS of the mtesunc remain In the nght side of the abdomen and the colon In the left

Non Rotauon of the Duodcnum The second part of the duodenum mstcad of tunung toward the left becomes con tmuous with the xcjunum on the right

Mobxle Duodcnum Due to the perszstence of the embryo

Inc duodenal mesentery allowxng free rnozzhty of the duo denum

Au-esxa ol the Small Intestine (Same as of atresna of thn oesophagus) The common sttes of atresla are m the second part of the duodenum, the ducdeno 1e]unal junction and the Ian. er part of the xleum

l\Icckcl‘s Dxvcrhculum A dlvertxculum of the ileum caused by the pexsxstence of the vltellme duct connectmg thv. pnnnuve mndgut with the yolk vesicle (fig 23) Normally

the vntellme duct )5 closed durmg the thxrd or the fourth week

Umlnlxcal Fxslula or Umbnlncal llcrmn Instead of a blmd pouch, the pcmstent part of the yolk sac may hemlate or cxtcnd as a cord to the umhtlncus, drschnrgmg faecal matter

lrrcgulnr Rotauon of the Loop of the Mnlgut or llcrmzh non of Gut Into the Umbilical Cord Thus 15 owmg to the defective twtstmg of the pnmary loop of the gut The loop IN HUMAN EMBRYOLOGY

Fm 10a ——Congemtnl umbslscnl hernm (From Pathology Mx scum ’1'.N M College)

of the gut extending mm the body stalk dux-mg the fifth week {ads to return by the tenth week

Tnmposlhon 0! the Vtscen (lmersus Alsdommahs) This usually occurs m usccmxon “nth deem-ocezdla

Tue Lane: Ixnsrmz

lhgh Cnccum Owxng tn the nnndescenl the caecum re mams Just below the nght lobe of the In er as In the embryanxc condmon

Low Cnecum The caecum descends mare than normal sometunes It descends below the nght that fans to the pelus DEVEIDPIHENT OF THE A.Ll‘l\l1-ZNTARY SYSTEM 155

Micro-Colon Owing to clumnuhon m the length and sxze of the large mtestmes (rare)

Mega-Colon (llnsclusprunys Disease) Thxs is a cun.'e— mtal malfonnatmn wlnch affects the colon resulhng Ln hypertrophy and chlatatxon The demonstrable lesmn 15 seen mnzroscopscally as an absence or agenesns of mtramural gang~ lmn cells In the walls of the colon, distal to the drlated area (Bod1an et al, 1951)‘

Redundant Colon Th): 15 owmg to excessne elongatmn of a part of the colon

Absence of the Colon The colon may be reduced to a fibrous cord (rare)

Duplication of the Colon This 15 awmg to double formatmn on the right or left half (rare)

Tm-: Lxvm

Ptosxs of the Lncr Owing‘ to defects In the formation of the suspcnsuzy hgament

Abnormal Lohulztuons Drvnsmn of the lwer mto many lobes owmg to abnormalmes m the direction of the blood ﬂow through the embn omc lu er

Tm: CALL Buumm

ﬂllohcd or Double Gall Bladder Abnormalmes owxng to the sphttmg of the prnnmve caudal portxon oi the bepauc bud

Absence of Gall Bladder Owmg to the arrest of the canamculahon of the caudal dxvertxculum of the gall bladder

Tm: Paxcnms

Annular Pancreas A hand of pancreatxc ussue encu-ales the secand part of the duodenum and compresses xt Thxs as probably due to the persxstence of the up of the Ventral pan ‘ Textbook of Pacdmtncs mined by W ‘R F Colhs Wm. Huncmann hffilltﬂl Books Ltd. London. 1952 page 679 196 HUMAN EMBRYOLOGY

1.-ream: bud m an amenor posmon Tins part falls to rotate wnh the duodenum while the remaxnmg pox-non of I11: bud grows min the dorsal mesenlery (very rare)

Accessory Pancreas These are rsolated masses of pan cream: nssue separated from the mam pancreas Probably these are detached from the mam mass dunng the early stages CHAPTER VIII DEVELOPMENT OF THE RESPIRATORY SYSTEM THE LARYN\

In the pharynx of an embryo of about the third ueek, a Ventral diverticulum is seen just caudal to the hypohranchial eminence This is the trackeo-bronchial groove, the primerdium of the respiratory organs (trachea, bronchi and lungs) The loregut Just caudal to this groove lengthens forming throesophagus The oesophagus and the tracheal growth are separated by deep grooves, except at the base of the ﬂoor of the pharynx, where a narrow slit, the primordiuni of the glottis comunicates with the tracheal groove

From the fifth to the sixth week, a swelling from the hypobranchial eminence oi the third and the fourth arches constitutes the cephalic boundary of the gluttis The glottis is bounded on the sides by elevations from the fourth and fifth arches lorining the prirnordia of the nryteiwids, which grow towards the base of the tongue Due to the sudden growth of these swellings, the lumen of the glottis may be obliterated temporarily But it is later reformed with its upper part avoid in shape

By the tenth week, due to further growth, the arytenoids of both sides of the glottis are jﬂlned to the epiglottis by the aru-cpiglottic folds The vocal cords begin to develop at this stage By the sixth month the larynx is definitely formed

The cartilage: of the larynx are developed by the second month At about the same time, the muscles are derned from the fourth and fifth arch mesoderm and therefore, the nene supply is from the superior laryngeal nerve from the fourth arch and the recurrent laryngeal mine from the filth arch In the adult, the recurrent laryngeal nerve is pulled noun \\ith the caudal migration of the heart 158 HUMAN EMBBYOLOGY

DXOLOHIS

ASYTENOID

Fm 106—DeueLop1nenc of the larynx, at ahaue eight weeks

FPIGLCTTLS

/ /3/W

‘—~AI‘ﬂ'ﬂ1DlD

nncnn Fm 107 —-Development of the larynx, at about nme weeks

Tn: Tmcuzn

ﬂue tracheal groove at the floor of the pharynx, an thn. early embryo, grows mm the trachea Along mth the formauon oi the larynx. the tracheal divs:-Llculum extends caudally ventral to the oesophagus Its up Ls rounded and shows an early branchmg me the two bronchxal dsvxsxons The muscles of the walls, the connechve (Issue and the camlagmous rmgs nx-zvaznopmxwr or THE RESPXRATORY svsraw 13;

rmLo'rns

Amt EPIGL. ram

‘coxmcuufc Tunmcu:

\ cunmom

\ nmmcu: mm: Amrrtnom

Norm

fiG 108--Dewlopmznt of the larynx, at about ten weeks

are denvetl from the surroundmg mesoderm at about I111.second month The mucosa] glands appear by the fourth month

Tm: Bnoncm AND Lumzs

The trachea In the early fourth week xs divided Into two prunary bronchi Dunng the fifth week. the left bronchial bud assumes a posmon more transxerse than the nght At the same ume, the nght bud gives out two lateral buds. The bud Iouatd the cephalic sxde, later grons as the apmal lobe of the lung and :5 known as the cpartenal lobe, due to lhe pulmonary artery lying beneath It The left bronchml bud gxves out only one laleral bud These buds and the lermmal ends of the mam dlvmons grow mm many brzmchu lormmg the bronchxal tree of the lungs. It 15 esumalcd that dunng the smlh month, about seventeen generahons of bran160 HU‘\lAN EMBRYOLOGY

ches are present The three buds on the nght grow mm the three right lobes and the two buds on the left mm the hw left lobes of the adult lung

11' uzaucxus L! uoucxus was

urns

F:-:::::

Fm 109—De1,e2opment of branch: and lungs, at about the fifth week

1.1 nomznus

ET ncucuus

I s 1 u I I n V . . I . yunmx I I . . -. u

4--..--.......... msofmuzus

Fm 110—Dex.e1opmznt of branch: and lungs, at about the szrth week DEVELOPMENT OF Tlfil RESPIRATORY SYSTEM mx

The lung-buds grow mto the surroundmg splanchmc (vxsceral) mesoderm from wluch then‘ stroma, the muscular and the connectxve txssues are deuved At the same tune, the growmg lung bmk, wxth thezr mesenchymal stroma push the layer of the splanchmc mesoderm towards the cuelom on both sxdes behmd the heart This part of the coelom forms the pleural cavxty The splanchmc mesoderm attached to the lung-buds forms the splanchmc (or visceral) layer of the pleura By the sixth month, the termxnal portion of the lunghud branches lrregularly In all dlxecnons endmg m small sacs of eplthelnal cells called the alveolz. These become thumed and the grmung capillaries of the lung Invade them Con<equenlly, the foetal lung looks 11k? a glandular structure CHAPTER IX DEVELOPMENT OF THE BODY CAVITIES T1-n: Pazucannxmu

In the second week, during the presoxmte stage, the embryo appears as a bxlammar oval plate of ectoderm and endoderm “rub the prunxme streak 1n the centre It has between the ammohc and the yolk sac cavmes and 15 attached to the cl-mnomc V85lClE by the body stalk Towards the end of the second week due to the actnmy of the pnmltne streak and the notochoxd mesoderm )5 formed between the two layers At the same tune, the emhryomc plate lengthens and proyects mm the ammonc cavity as the head and the L211 folds Soon, the neural folds of the head regxon grow large, the head becomes more pnomment and I5 bent acutely down wards By the begmmng of the {bud week, the mesoderm arranges mm defimle segments or somxtes, stattxng from the regmn near the cephaltc up of the uotochotd The lateral plate mesoderm splits min a parxetal layer hnmg the ecto— derm and a vxsceral layer (splanchmc) Iuamg the gut endo derm. The space between these layers :5 the coelom The cramal ends of the parietal layers of each sxde meet and fuse together as an amhed cephahc boundary of the coelom

About the same ume, the pnmary heart tubes ansmg from the endothehal cells wltlun the vxsceral rnesodemx, fuse mm a smgle tube “Inch projects mm the cephahc part of the coelom—now called the perzcurdzal cavtty 111:2 Visceral layer around the endathehal heart tube becomes the epxmyocardxal mantle-the pnmordxum of the canhac muscle, connective nssue and the VlSC€Ial pencaxdxum As the heart :5 formed the endoderxn conunuous wxth the yolk sac recedes away from the cephalxc part giving nse fo the blmd endodermal tube of the pharynx and the foregut The vxsceral perx.cardium 15 pmned dorsally \Vlll) the pencardxal “all as a double DEVELOPMENT OF TEE BODY CAVITIE 163

layered dorsal mesocarduim which seem disappears The pencax-dial cavity, now becomes a horse shoe-shaped cavity in from of the foregut The embryo grovis rapidly during the third week and more somites are formed Consequently, the coelom extends along the whole length of the embryo beginning from the most cephalic pax't—-the periczlrdial cavity The yolk sac diminishes in size and the narroiving yolk stalk becomes continuous with the midgut

fic 111 —Sngiltal section through the cephalic part of III embrya

The pei-icardiai cavity is separated {mm the rfiﬂldlnmg part of the coelom by the septum, mznsucrsum It is a transverse sheet of mcsuderm produced by the partial fusion of the somatic and usceral mesoderm In} em yust caudal to the heart region bctvi can the yolk sac and the ventral body \i.:iII lsl HUMAN EMBRYOLOGY

It does not form a complete partition but extends like a shell between the heart and the liver The liver grows into its caudal aspect The vitellme and umbilical vein: and the duct of Cuvier pass t.h.i-ougli it to enter the heart

‘I01.-I IAC

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fiG L12 ——Sche1natu: sketch of an embryo of the four-tn uev-I , showing the eornununicritwn between the perieorduil and the peritoneal cavities

After degeneration of the dorsal inesocardium the heart has free in the penoardial cavity suspended by the venous entrance at the cnudal end and the arterial exit at the cephalic end. The coelom dorsal to the septum transversum is in communication “Rh the pericardial cavity through the per-. curdui-zientonenl canal on each side At about this time the lung buds appear by the bifui-cation of the trachea in front of the developing oesophagus The lung buds with their covering of the visceral mesoderm push out laterally into the pericazdio-peritoneal canal: The pericarrlxal pleural and the peritoneal cauties are still in communication with each other DEVELOPl\ll'.NT OF THE BODY CAVITIIS 16.’ As the embryo grows, the heart nugrates more caudally The common cardinal vein of each side passing obliqueiy through the septum transversum to enter the sinus venosus, causes a bulging of the dorso-lateral portion of the body wall continuous with the septum transversuin producing a crescentic fold toward the prunitive pleural canal This fold is the pleuro-pcricm-dial membrane As the common cardinal veins pass more medially, the lateral free edges of the pleurapeiicardial membrane from both sides gradually meet and fuse with each other and with the medially located visceral mesoderm of the anterior part of the oesophagus The pericardial cavxty therefore, is separated from the pleural cavity

As the sinus Venosus is merged into the right atrium and the pulmonary veins are absorbed into the left atrium the subsequent changes tn the reflection of the pericardium cause the formation of the oblique sinus of pencarduun

THE: Puzuzuu. CAVITIES

The pet-icardial cavity, originally formed by the coelom oi the lateral plate mesoderm on the cephalic sxde now shrits to the ventral side of the embryo due to the developmental changes in that region The coeloni of the remaming portions of the body is continuous with the pencardial cavity But the septum transversum growing from the ventral body wall between the heart. and the yolk sac, acts as a partition of the coelom ventrally Therefore, the coelom of the caudal regions of the body known as the peritoneal cavity 15 in COmmuﬂlt'-'1tien with the pericardial cavity through paired channels called the pericardio pleural canals (or primitive pleural canals) on either side, dorsal to the septum transxersuin Thus, the three prinutive (.‘aVll.t&5 communicate with each other in the early stage

At about the fourth week, the lung-lauds from the tracheal dnerticuluin grow with a covering of the visceral mesoderm and push out laterally into the pnmitive pleural canals The common cardinal win of each side (duct of Cuvicr) entering the septum trunsuzrsuni, causes a bulging of the dorso-lateral 166 HUMAN EMBRYOLOGY

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Fm 113 —-Formatzmt of pencardxmpleacral and pleura pen toneal membranes

hndy wall resultmg m a crescentxc fold on each sxrle This fold grows m a cramal and a caudal dxrectmn The cramal fold forms the pencardxo pleural membrane whxch along wnh 115 fellow of the opposite sxde and wxth the medlastmal mesoderm separates the perxcardrum i-ram the pleural cavxty The caudal portmn of the {old which 15 also contmuous wrth the septum transversum forms the pleura pentoneal mem lmme przuectmg mm the pleural canal of each sxde

By the sxxth week the lungs enlarge m sxze and the pleural cavrty expands ventrally and caudally lateral to the pencardlum The pleura-permmeal membrane rs thus stretched out from the dorsn-lateral portion of the body wall and pushed caudally By the seventh week the liver grows more m bulk and muscular txssue mvades Into the pleura perxtoneal membrane As a consequence the conunumcatmn between the pleural and the perxtuneal cavmes becomes very narrow By the exghth week the free edge of the membrane fuses wxth the lower part of the mesentery of the oesophagus and closes the pleurmperxtoneal openmg Thus the pleura peritoneal membranes wzth the septum transversum cansutute the draphragrn wluch separates the pleural cavxtxes from the penloneal cavrly DEVEIDPMENT OF THE BODY CAVITIES 167

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Fm 114 —Fonmztum of pleural cavzty at about the Szzm week

THE Dmpiumcm

It is derived from the following 1 Septum transversum

2 Pleui-o peritoneal meinbranes

3 Adjacent body wall mclurlmg muscles from the car

vical and spinal segments and 4 Mesentery of the oesophagus

Cauclal lo the pericardium the somatic niesoderm of each side is partially fused with the visceral mesoderm forming a mas: of cells extending dorsally into the body wall as septurn. lrt11lS'L8fS1Anl Its dorso-lateral portions are in contact with the third fourth and the fifth cervical somites By the end of the third week the lner cords grow into us caudal side and by the fourth week the liver is covered by a capsule (Glissons capsule) and is pulled -’i\\ ay and separated from the septum transversum by the expanding peritoneum Part of the \enti-ail mescntery bow. can the In er and septum (ran.versuni become: the coronary lxgmvirmt At about the fifth xsa HUMAN EMBRYOLOGY

week, myohlasts from the thzrd, fuurlh and fifth cervxcal so mxtes enter the septum carrying them nerve supply—lhe phreruc nerve wxlh them

By the sixth week, owmg to gradual caudal zmgrauon of the heart, the septmn xs pushed to the level of the thxrd tlmtacxc segment Very soon, at me pomt of entrance of the common cardmal Vein. a ctescentu: mesodermal fold of the body wall appears The ceplzalzzz part of thxs fold forms tne pleura perxcardzal membrane whxah gradually fuses Vuth lhe mesentery of the oesophagus and separates the pleural from the perncardxal cavxly

Aonn noaslu. uasnrrmr amorunaus nmn Pam’ Mnan. INF van emu mom BODY WALL

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Fm 115-Schemauc sketch of the fammtron of the dwphragm

The septum by the sexenth week, came: in the level of the tenth thoracxc segment, where the muscles from the spmal regmn, mxgrate xnto xt The penphera] portxon of the 93?"-1171 us supplxed by the sensory fibres of the lower su: spmal nerves. Later the pleurmpentoneal membrane 15 formed by the canDEVELOPMENT OF THE BODY CAVITIES 153

dal portion of the original mesodermal fold of the body wall, zit the place where the cardinal veins enter the septum By the second month (115 membrane acquires muscle tissue from the adjacent regions and is stretched out by the enlarging lungs Consequently the communication between the pleural and the peritoneal cavities is narrowed and finally the mem hrane fuses with the mesentery of the oesophagus on either side and separates the pleural from the peritoneal cavity at the level of the lower thoracic and the first lumbar segments The Lluiphrasrm between the thoracic and the abdominal cavi ties is thus estahhshed

The ventral portion of the diaphragm is dern. ed from the septum transversum and represents the central tendon of adult anatomy The dorso lateral portions are derived from the pleuio peritoneal membranes and the adiacent spinal seg ments The third fourth and the fifth nerves are carried into the diaphragm hy the migration mto It of the mesodermal elements of the corresponding somites lying adjacent in the early stages The lower intercostal nerves send sensory branches The connective tissue of the walls of the inferior venzi cava and the aorta also shares in the complicate structure of the diaphragm

Dsrzcrs or DEVELOPl\‘lEN'l‘ or THE DIAPIIRAGM

Diaphragmatic congenital licniia is a condition in which some parts of the abdominal organs are protruded into the thoracic cavity through a developmental defect caused by im perfect closure of the diaphragm The pleuro peritoneal mem bnines may not approximate in the normal way There ma} he a failure of the fusion of the mesentery of the oesophagus with pleuro—pericardial membrane More commonly hernia occurs in the left side than on the right probably due to the protection of the liver CHAPTER X DEVELOPHENT OF THE UBOGENITAL SYSTEVX

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The mtra embryonic mesoderm conszsts of a medxal part (paraxxal) which becomes the somltes and a lateral plate wluch farms the lmmg of the coelom The mtermedmte par: of the mesoderm gums origin to the excretory system and 15 therefure called the -nephrogemc cord

m:u)un. wxu. or seuuu. caan

Fla. 116 —-Tran.-:1.zrse sectxon. of early somxte stage showing the dxfierzntwtum mi :1 sonnte DEVELOPl\IENT 0!‘ THE UROGKNITAL SYSTEM 111

All vertebrates develop three sets of excretory organs in their embryonic life——the pronephros, the mesonephros and the metanephros All these arise from the intermediate part of the mesoderm as a collection of tubules The pronephros is the functmnal excretory organ in the embryos of amphihta and fish In these vertebrates, the n-iesonephros develops as their permanentsexcretory organ In the embryos of reptiles, birds and mammals the pronephros is a transitory structure and is replaced by the mesonephi-as which forms the cluef functional excretory organ in their early stages The mesonsphros slowly degenerate: (except a little part which becomes modified into the genital gland), being replaced by the metamphros formed in part from the rnesonephms and in part as an independent organ In the early development of the human kidney. the pronephros, the mesonephras and metanephros appear in succession, indicating a brief summary of the evolutionary history

The Proneplims is seen in an embryo of about the third week, as solid buds in the intermediate part of the mesoderm ventro—lateraI to the somites, extending from the region of the seventh to the fourteenth somite Each solid bud grows dorso-laterally toward the body wall and subsequently becomes a hollow tube The proximal portion of each tube opens into the coelom. while the distal end meets the one below and forms a continuous channel called the proiieplmc duct This extends caudally and eventually opens into the clnaca by piercing its lateral wall All the pronephric tubules do not appear at the same time The cephalic ones elegant.» rate while the more caudal ones are being formed The tubules slowly degenerate in a cramo—cauda1 direction and become replaced by the mesonepbros that arises caudally from the nephrogenic cord The caudal portion of the pronephric duct persists as the mesonephnc duct

The Mcsuncphros At about the middle of the fourth neck, mesonephric vesicles appear in the nv:-phrogenie cord at the le\el of the fourteenth somits: and slowly by thc'ﬂftli neck extend caudally to about the twenty-sixth somite (from the sixth cervical to the second or third lumbar region). 172 HUMAN EMBBYOLOGY

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Fm 117 —D1agram: shnwmg pronephro: and «mesonephrtr

In the begxnmng, there xs only one xesxcle m each sannte but later two or three or even more are developed The mesnnephm vesucles become hollowed out and their lateral ends mm the ptonephne duct wlnch hes lateral to the nephrogemc cord and :5 nuw called the metonephne duct, opening mic the clean: The ednal t af la la b comes 0 m n’ a sale enclosmg a tuft of capxllanes called a glomemlus from the vento lateral branches of the dnrsal. aorta The efferent vessels leaving Bowman’: capsules agam break up ante capnllanes and form a plexus around the whole length of the tubule The bloc] then passes to the collecting vein: and finally through the postex-wt ca:-dmal and the subvcznimal vexns 13 returned to the cnrculauon DEVELOPMENT or THE UROGENITAL SYSTEM 17-. MLSONEPHRIC Duct

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F15. 118.~—-Daigram of the mesoncphro: of the M111 week

There occurs a progresive degeneration of the cephalic lubules :1: new ones are formed caudally The degenerated shnmken tissue with its enveloping splanchmc mesoderm becomes the diaphragmatic ligament of mesonepllros By the scventh week, only very few tubules persist caudally \\hlCh later are transformed into the genital duets

The Mclmicphrus or the adult kidney, ai-mes from the outgrowth of each mesonephnc duct and from the interme— diate mesodci-m situated below the leul of Ihe mesonephro At about the fifth week, a hollaw divertxculum arises from the niemncphric duct on each side, near its opening Into the cloaca (at. the level of the 28th somile) This is the uretenc bud, the blind end of which dilales Io form the future pelvu 174 HUMAN EMBRYGLOGY

P16 119 ——Photam¢crograph of a. sagztml sectum through the -mesonephros

of the kzdney The hollow dlvemculum at first gmus dorsally, then cramally dlfierenuanng znto a slender stalk uhxclx later elongates to be transformed min the ureter As the hollow outgrowth grows crarually n becomes capped by the mtermedzate mesoclerm from the nephrogemc cord below the level of the mesonephras. Thxs mesoderm later forms the secretory pnrtxon of the ludney and 15 called the metlmephrogeruc blastema. (or nephrogemc blastemal As the stalk of (he ureter elongates, the melanephrogemc blastema 15 named crarnally By about the fifth week, the whole blastema remams m the mesuderm dorsal to the caudal part of the mesonephros, opposne to the second lumbar region The cranxal end of the dnveruculum, evagmales wxthm xts sur

roundmg mass of metanephnc mesoderm m a caudal and cramal duecuon These prlma-TY evaglnatxons form the future mazar culyces of the renal pelvis Tvm more d.xvertlDEVHDPMENT OF THE UEOGENITAL SYSTEM 175

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Fm 120 —Diugram showing the cloata, the mesonephnc duz: and the ureteru: bud in an embryo of the fourth week

culae develop between them, at the future muddle portion

of the renal pelvis The distal and of each evagination enlar gns to form an amp!/.lZa, from each of \Vl1.lCl'1 two or four

tubules arise as a second generation of tubules These open

into the mzuor calyces and become the minor culyces Fur ther generations of tubules continue to appear in a similar

manner, till about the end of the fifth month when about

twelve or more generations have been formed As a result

of the enlargement‘. of the pelvis and the primary and the

secondary tubules, the third and me fourth generations are

absorbed in the walls of the minor cal) ccs The tubules of

, the filth generation open into the minor calyccs as P¢lPlllC|.TI_l

dams The further generations of tubules elongate and be?’ mine the straight collecting tubules of the kidney

The melanephric mesodorm from the caudal pornons of

the nephmgenic cord forms the secretory portion of the l>.id~

‘. ncy This mass of mesoderm is moved peripherally oiung to 1:5 HUMAN EMBRYOLOGY

the development of the collecting tubules Along with the ‘ormatioii of new generations of collecting tubules the metane phric mass increases in amount and progressnely becomes subdivided into many parts to envelop the ends of the succeeding generations of collecting tubules Thus the priniarv renal pyramids are formed Each primary pyramid divides into two or three pyramids Since they are closely arranged their broad bases on the periphery appear as a thick layer This metanephric layer is the cortex of the kidney Each mass of tissue covering a group of collecting tubules becomes marked ofi on the outer surface of the kidney by yooves This results in the appearance of lobulation on the surface of the foetal kidney

The metanephric mesoderm at about the seventh weel. condenses into small spheres of cells which later differentiate into small vesicles. The distal wall of each vesicle becomes elongated and form an ‘S shaped secvetory tubule, which unites with the ampulla of a collecting tubule forming a continuous lumen The proximal wall of the vesicle invaginames to form Bowman’: capsule The cun ed tubule hem een the proximal and the distal ends elongates to form the tortuous procnmal conwluted tubule, the long loop of Henle ex tending down through the pyramids and the dtstul camoluted tubule The end of the distal convoluted tubule bends slightly in the form of an arch and unites with the collecting tubule Many generations of these tubules aie produced increasing the bulk of the kidney The uriniferous tubules continue to be formed till a few days after birth

The kidneys from the beginning lie behind the perito neuin They gradually move upward from the original pelvic cavity, probably owing to the rapid growtbchanges in the caudal region of the embryo As the kidneys move upward they also rotate medially on their long axis, allowing their original convex edges which were directed doisally to assume a lateral position finally, they reach their definitive position at the region of the 1st lumbar or 12th thoracic vertebra Here they become covered by mases of sub-peritoneal fat and connective tissue which hold them in position DEVELOPMENT OF THE UROGENITAL SYSTEM 171

Di:n:c'rs or DE\'ElﬂP\H'.NT or THE K!.D‘H7YS Anoi\iAi.L:s IN Nl1‘\tEE.R

Absence of Kidneys (Bilateral Agciiesis) Failure of development probably owing to defect in the gen-u plasm affect iug tlu. rudiments of the kidneys or arrest of development owing to mechanical factors in early stages of development (Incompatible with life)

Absence of One Kidney (Unilateral Agncsis) Failure of development of the renal bud or metanephric hlastema or failure of Vascular supply

Stipcriiumerar) Kltlll¢)b The inesonephric blastema may split into tivo or more or there may be separate hlastemas into which partially or completely reduplicated uretenc buds enter to form separate kidneys The kidneys may remain separate or fuse together

Azsoninnrrs or SXZE

llyponlasuc Kidney (Infantile or Small Kidney) Owing to the arrest in the development of the metanephros the kidney becomes smaller in size

Rciial Aplasia (No Evidence of True Kidney) Owing to early failure of fusion of uretenc bud with the metanephric blastema and consequent atrophy or arrest in the development of the ureteric bud The iiietanephnc blastema may atrophy

Renal Hypertrophy (Kidney Increased in Size 8. Fun: tioiial Capacity) The kidney is increased Ln size owing to compensatory mechanism associated with agei-iesis or hypoplasia of the other kidney

1\.\0\l4\l-IE 0!‘ FOR“

lnbulatcil Kidney —- (Foetal Kxdiiey) The lohul.-itlons are formed as persistence of the foetal condition as 2: result of the inclanephnc mass C0'\eX'll"lg a group of collectirg tubules

12 178 HUMAN EMBRYOLOGY

Horseshoe Kidney Owing to fusion of the inetanephric blastemas across the midline probably as a result of the two masses coming in Contact with each other by the developing umbilical arteries Mostly the fusion takes place at the lower pole

Disc or Cake Kidney Both kidneys become fused into a solid organ

Polycystic Kidney The exact cause is unknown

It was thought that this condition is caused by the failure of union of the collecting tubules of the iireteric bud with the secreting tubules of the i-netanephros Fluid accumulating in the blind ends of the secreting tubules causes the formation of cysts. According to Hepler‘ (1940) any interference with the blood supply of the parenchyma in a particular area re sults in a cystic degeneration The cysts may compress and diplace the renal tissue Besson (1933): attempted to explain that the condition is due to the relative poverty of nephrogenic tisue compared with the more abundant tubular collecting and vascular elemenk Kainpmeier (1923)3 believes that cysts develop from the persistence of the primitne collecting tubules which normally should atrophy

In the development of the kidney it has been stated that the primary evaginations of the ureteric bud form the primitive major cal) ces of the renal pelvis The second generation of tubules degenerate and are absorbed into the major calyses to become minor calyses The third and the fourth generations also degenerate and are absorbed into the walls of the minor calyses The subsequent generations elongate as straight collecting tubules. Sometimes the tubules of the second, third and the fourth generations may not degenerate, but become detached from the collecting tubules to form cysts The cysts enlarge and compress the adgacent tubules thereby increasing in size

1 HEFLER. A. B (1940) Etiology or multilaciilar cysts of Lidiiey .1 Urol-. 44, 206

2 Bmsoiv H (1933) Polycystic kidney .1 vi-oi. so ass

3 KAM'PMEIEfi-. 0 F (1923) A hitherto unrecognized mode of origin of congenital renal cysts. Surg, Gynec. !- Obst. 36 2118 \ DEVEIXJPMENT OF THE UROGENITAL SYSTEM 179

Congenital polycyslic kidney *5 assocmed Wm‘ Cyst formation of other organs such as liver and pancreas and also with other congenital abnormalities (Ferguson 1949) ‘

ANOMALD35 IN Posmmv

Ectopic Kidney This is caused by interference with the upward movement of the kidney. PY°l>3l>lY 35 3 result Of anomalous vascular attachments or other anomalies of adiacent organs

Abnormal Rotation The kidney during its upward movement fails to rotate medially on its long axis If the rotation is more than normal, the pelvis and ureter will remain hohind or along the external border of the kidney

Tm: Buumizn

The luimg menibmne is denved from the endoderm of the cloaczi The zngoric of the bladder is derived by the inclusion

of the proximal ends of the mesonephric ducts and hence from the mesoderm

The wall is derived from the splanchnic mesoderin

The allantmentenc diverticulum from the pi-imitne hindgut proiects into the body stalk. In an embryo of 10-12 SOI‘K\ltES, the gut caudal to the origin of the allanto-enteric diverticuluin becomes dilated to form the clean The hindgut ends blindly An ectodermal depression is formed under the tail of the embryo at the root. Sinking in toward the gut as the procxodaaum The thin tissue separating the gut and the pructodaeum is the cloacal membrane At about the fourth week, the mesonephi-ic ducts open into the doiso-lateral wall the dance

During the filth neck, there appears a crescentic fold of mcsodermal tissue, in the angle between the allantois and

4 rmausow, J o um) Observations on faimilitil polycysug .1“. case oflhe kidne3.Proc ii Sod: Med 42,305 130 HUMAN EMBRYOLOGY

the gut This gradually grows toward the cloacal membrare as the umrectal septum Ultimately by the sexenth “eel. the urorectal septum {uses with the endoderm of the cloacu dlVldl!1g the cluaca into the pnmmue urogemtal sinus wen trally and the rectum dorsally The point of fusion of the urorectal septum with the cloacal membrane is marked on the surface by the fonnahon of the przmmte perineiun The cloacal membrane in contact with the Pﬂmlllve umgenilal sinus becomes the urogenital membmne and that in contact with the rectum the anal membrane

The primitive urogenital sinus becumes elongated and 1.5 subdivided into a dorsal ves-zca urethral pow-non receiving the allantois and the mesonephnz: ducts and a ventral portion the definitive urogenital sinus The vesico-urethral porhon deve lops mm the bladder and the posterior urethra in the male (and the entire urethra in the female) The definitne urogenital sinus extends into a little elevation, ui front of the cloaca called the genital tubercle The extension of the urn genital sinus xnto it forms a portion of the urethra

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Fm 121 —UrogeniLal System. of a. human embryo of zime weeks sketched from u nadel bJ Ziegler (After Kiebel) DEVIELOPMENT OF THE UROGENITAL SYSTEM 181

As growth advances, the mfra-umbrhcal abdommal wall gradually mcreases m 5128 and consequently the ves1courethral portxon forming the bladder also enlarges xn slzc At the same tune, the prcxunal ends of the mesonephm: ducts become absorbed by the wall of the vesxco-urethra] pornon and the mesonephnc ducts and the ureters open separately Into It Dunng the process of absorptmn, the opexungs of the ureters are sluftod lateral and cramal to the opemngs of the mesouephrrc duets whxch remam close together and open In the dorsal wall of the prurutlve postenor urethra The regmn between the openmg of the meters and that of the mesonephru: ducts forms the tnangular area of the prxn-utxvc trzgone of the bladder It xs formed from the walls of the mesonephnc ducts, the proxlmal parts of wluch are absorbed mm the bladder “all together wxth the proxlmal part of the uteters Therefore It IS mesodermal m ongm

In the early stages, the hmng epxthellum of the bladder IS a single layer of cylmdnczﬂ cells Further layers are added from about the sxxth week onwards By the beg1nrung of the tlurd month, the epxthehum consrsts of two layers but In the trngone there are four to five layers The longrtudmal muscle fibres appear m the splanchmc mesa derm of the bladder wall by the eighth week A httle later, the clrcular muscles appear and by the thxrd month the rnrer lungrtudmal muscles are established The muscles of the neck of the bladder are well formed by the fourth month By thns pcnod the muscle layers can be clearly drﬂerenuated from the surroundmg‘ connective txssue

In the foetal stages, the bladder lxes near the umbxlzcus

and gradually alter btrth, It begms to descend Into the pelvxc cavrt)

DSFECIS OF‘ THE DEVLI-OPAIKNT OF THE BLADDILR

Absence of the Bladder Thxs 5 the result of defectue development of the urogemtal smus \uth or vuthout atrophy of the allantors (V er) rare)

Rcdupllcatmn of the Bladder Separate bladders, each havmg a urethral onfice. opcmng mto a complete or untam182 IRIMAN EMBRYOLOGY

plete urethra caused by the sphttmg of the vesxco-urethral portton appear The cause Ls unknown

Exstrophy of the Bladder There IS absence of the an terror abdommal wall and the antenor wall of the bladder The mner surface of the postenor bladder wall becomes ev ert ed and protrudes on the abdomen. Thxs is caused by a defect tn the fonnatxon of the xnfra umbthoal regmn of the body wall

In an embryo of about the second week, at the caud'1l end of the prumtxve streak there 15 an area where the ectoderm and the endoderm are m apposmon Tlus forms the cloacal membrane Later thus membrane extends to the dorsal surface of the allanto-entenc dn. ertxculum Mesoderm cells from the prmutxve streak xmgmte along the stdes of the cloaoal membrane Dunng later stages of development along wxth the formatxon of the tax! fold the ectodermal surface of the membrane comes on to the ventral surface of the embryo and later 112 the dxrectxon of the ultra umbxlxcal regmn The mesoderm cells extendmg to the body stalk form the pnmordxa of the Infra umbthcal body wall and the gemtal tubercle Any defect m the nugrauon of the mesodenn cells and thexr meetmg In the mud lzne may lead to the absence of the Infra umbxhcal body wall and consequently In later stages of denelopment. may cause exstrophy of the bladder and epx spadtas

Patent Uracbus Urachus is probably denved from me allantms But the studxes of Begg 0930)’ xndxcate that n‘. )5 denved from the ventral cloaca. The upper part of the bladder gradually gets constricted and elongated After bxrth the bladder begins to descend unto the *pe1vIs from the on gmal poslhon at the region of the umbxhcus Consequently the urachus xs pulled down with rt along wlth the ends of the oblxterated umbxhcal artena At bu-th the lumen becomes obhterated by the growth of epnthehal cells.

Urachal Cysts These are {armed when the lumen of the allantons )5 not obliterated but the ends remaxn closed

- EEJGG R. c um) The unchus, us anatomy xuszoxogy and dcve— loprnent J Anal.“ 17:: DEVHDPRENT OF THE UROGENITAL SYSTEM 183

Darn:-rs or Unumm

/Unzlatcml Agencsns )5 caused by {allure of development of the uretenc bud Double Ureter 15 the result of enther the development of two buds or hy fission or sphtung of a sxngle bud Ectupy One or more ureters may open at some point other than m the normal posxtxon m the bladder Dunng the process of absorption of the pmxxmal ends of the mesonephnc ducts Into the vesxco urethral portmn the uretetal orlfice of each side shnfts and opens mto the lateral angle of the tn gone Any defect of tlus normal alzsorptxon and separauon causes ectopy Uretcrocuclc (cysnc dllauon of the lower end of the ureter) lt IS thought that as a result of a small meatal open mg nf the ureter the adjacent uretero veslcal mucosa becomes dxlated to form a cyst Retrocamal Uretcr The ureter instead of passmg lateral to the mfenor vena cava passes helund It and partxally encu‘ ales It Tlus IS owmg to the persxstence of the emhrycmc pustennr cardmal vem whxch hes zmtenor to the ureter Normally the Infenor vena cava is derived from an anasto mosls of the postenor cardmals In the caudal regmn of the embryo the supra cardmals yom the termmal parts of the postenar cardmals As a result he caudal part of the right supra cardmal enlarges to form the post renal part of the mferxnr vena cava Any defects In thrs normal development of the mfennr vena cava cause the ureter to encxrcle the vessel Dllahllun of thc Urctcr (Megalo ureter) May be due to the persistence of a large foetal ureter Will) a defectwc neuro muscular mechamsm

T:-n: Ttsns

In an embryo bclween the fourth and fifth week Ihe Locloxmc epnthelnum on the ventro-mcdnal part (abnul the mxddle two quarters) of each mesonephros forms a thlck layer of cells known as the gemtnl ndge A httle later the 18-1 H‘UTvlAN EMLBRYDLOGY

gerutal ndge became: dtfierentxatcd mto an umer mass ox eptthehum and an outer layer called the germmal epxthe hum At about the seventh week the xnner mass of epz lhehum begms to proliferate mm a net work of epxthelxal cords whmh push mto the underlymg mesenchyme The epnhehal cords become separated from the germinal epnll-e 1m.m by a layer of connective hssue fornnng the pnrnordxum

MEGNDHBXC bUCT

SB COLD!

GEEK!-NAL EPITEDJVH Fm 122—Tran:v.e'rse sectwn through the mdtfierent stage 01 the gonad of an embryo of about :1: weeks

of the tumca albugmea Up to about the seventh week (15 to 17 mm) I! Is not possxbh: to chfierenhate the male and DEVELOPME.N'l‘ OF Tl-E UROGENXTAL SYSTEM 185

female sex cells and therefore the gonad at this stage 1: 5a d to be mdufierem

The epntheltal cords extend mto the mesorchxum where they {arm the fete Lexus These cards are made up of two kinds of cells, the mdxfica-mu: cells and the pnmorduzl germ. cell: The pnmordxal germ Cells probably take ongm m the caudal region of the embryo from the endoderm of the yolk sac or its mesodermal eovenng near the allanto enternc thver tmulum. Dunng the thud week, these cells begm to migrate through the emloclerm of the gut and thmugh the mesentery mto the epnhehum at the gemtal ndge (wuscm 19-18) ‘

The eonnecuve txssue between the pnmordxum of the cords and gemunal epxthehum grows xnto the underlying mes euehyme between the cords to form the connectlu: tzssue septum By about the filth or stxth month the cards acquxre lununa, first m the penpheral ends and later umte wxth the lumma of the rete testxs

The dtstal parts of the tubules became dxﬂerentlated mm the pnmnnllum of the semmzfzrnus tubules and the proxunal parts the straight tubules connecting the rate tesns The walk of each tubule are made up of the ongmal mdxlferent cells wluch form the sustentacular or Sertolt cells and the larger pnmnniuxl germ cells The mtersutxal cells anse from the surmundmg mesenchyme

By the (uurth month the tests becomes round and compact The ongmal attachment to the mesonephrus degenerate: mm a mesentery called the suspensory lxgament whnch ultx mately atmplnes As the mesonephros degenerates, some of the mesonephnc tubules remzumng close to the testas form the eﬂemnt ductules By the end of the stxth month, the lumen of these tubules becomes contmuuus wxth the rete tests The mesonephnc duct at the 1e\el of the testxs gets much cunvoluatecl and forms the epumlynus The further amunuauan 0! the mesonephne duet becomes the was defrrens, opemng mm the urethral part. of the Dnmltne ux-o~

I WITSCHI. E (1943) Mlgnunn of the germ cells of human cmhr).»s from the yolk sac lo the pnmmve gunadul folds Conlr Dnbnol Canals lnsux 32. 57 186 HUMAN EAERYOLOGY

—~°“5?w"=

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arms or mu:-rm GlumNAcm.Uu

uu.1'1-.13

saonu

Fm 123 —Schemut1c plan of the development of the testes and thzxr rzlatum to the mesomzpbroz

gemtal smus Cephalzc to the mes-onephnc tubules “Inch are convened unto the efierent tubules a few tubules persxst as ductuh aberrantes and a few caudal ones persxst as pamdxdy 17113 The cephahc end of the mesonephnc duct remams as the appendx: of epxdxdymts DEVEDPMENT OF THE UROGENITAL SYSTEM 181

DJSCENT or 1111: Trsrzs

The mwonephros with the developing testis arises retropentnneally and projects as a big mass mto the cnelom The cephalic part of the pentcneal fold extends to the diaphx-agn_: as the duzplimgmatu: ligament of the mestmephrus The ciiu dal end extends to the low ermust part of the coelom, becomes fibrous and changes into the inguinal ligament of the mesa mzphros As the testis grows and the mesonephms degene rates the mguinal ligament of the mesonephros becomes the hgamentum testis An evagination of the coelom takes place on each side of the anterior abdominal wall in the mguma! region at the attachment of the ligamentum testis These evaginalmns winch are peritoneally lined extensions of the eoelom are called the processus Lagmalzs From the outer layer of the pi-ocessus vaginahs a fibrous cord called the scrotal hgament arises from the connective tissue and ex tends to the skin (of the future scrotum) Thus the scrotal ligament becomes continuous with the ligamentum testis and forms a fibm-inuscuiar band the g-ubernaculum holding the tats xii position The region around the processus vngmalis and the gubernaculum is the mgunux! canal Perhaps mvmr; to the relative growth of the embryo and the increase of intra abdominal presume the testis maves caudally beneath the peritoneum to the lower part of the abdomen By the end of the third month the testis hes in the mid abdomen During the fourth month probably as a result of the traction of the shortening gubemaculum, the testis moves caudally and mines to he in the abdominal end near the internal mgumal ring By the seventh month the testis pages through the nntennr abdominal wall (inguinal canal) reflecting the pen loneally lined extensions of the pmcessus vaginalis Just before birth or soon after birth the testis enters the strata] hag Since the lesus and the gubemziculum arise retrupen toneally, they lie beneath the posterior wall of the processus vaginalis The degenerating mesonephros the V35 dcferens “In! their blnod vessels and nencs, also enter the scrotum together with the testis. The lower part of the pmcessus vaginalis forms the tiuiicu uigmnh: of the testis. The upper npemng becomes closed \\ ithin :1 [cw weelu after birth Fad 183 HUMAN EAIBRYOLOGY

ure of the processus vagmahs to close results In the {ox-matxon of congemtal mgumal hex-ma

Gl75L1.KACl7L|7H nxocrssus Vnauuuxs

rxocxssus vAa.mu.ts Guamx xcuum

scxonm

PROCSSUS VAGKNAIJS

SCBOTUM

Fm 124—Duzgrzzms shawzng the descent of the it-‘S1!-9 (5139 mew) DEVELOPMENT OF THE UROGENITAL SYSTEM 189

The exact cause of the descunt of the testis is still not known It was thought that the atrophic shortening of the gubernaculuin resulted in the downward pull of the testis Wells & State (1947) in an article on the Misconccption of the gubernaculum testis mention that the gubernaculum plays only a very little part in the descent of the testis Lewis (15-1.8)‘ Ihinks that the downward pull of the cremaster causes I Experimental studies have, however shown that endo criiie factors probably play an important role in the descent of the testis The studies of Wnrnziek and Koch (1932)‘ have shown that cliorionic liornwnes present in the foetus in large

1mount are responsible for the descent of the testis

Du-izcrs or DI-2VELOPl\ll:N"l' or -mi: TESTIS

Aiioreliidisiii (nlisciice of both Testes) and Monorchidisni (alisciicc of one Testis) are caused by total or partial failure of development (ram)

P0l)0l'Cl’LIdISlII (Siipcriiuinerary Testes) This is the re sult of rediiplicatinn of the germinal epitheliuiii (rare)

Cryplardiidmii (Literally Means Hidden Testis) Uii dcscuided Tish: The arrest of one or both testes is due to developmental dcfects of any of the elements involved in the descent of the testis

bctupia Tlis The tests deviates from its normal course in the descent and remains in any of the following situations interstitial femoral penile or transverse

True Hennaphrodilisni The testis and the ovary are present in the same person The germinal epllllellum of the iiidiﬂerent sex stage gX'0\\5 as the testis on one side and the (nary nn tl'iL other Usually one gland grows more while the other atrophies. The external genitals show» rudimentary characters at both sexes (very rare)

1 ivi-:Li.s L .1 and STATE. D (inn) MlscuiiL:l‘l7linn at the gubcr nziruliun iisus Surgnn 22. 502

2 u-zwis L G no.5) Cfnilorchi.-sni J Urol so 315

4 ivoimcx. E B and xocu, F (L (1932) Undvsuzndcd testis End.-i m-min_y is 251-213. 190 HUMAN EMBRYOLOGY

Fm 123-11 case of undescended testes Both the testes are rammed at the abdonmml end. of the mgunwl canal (By the courtesy of Dr E J Ramdas)

False Hermaplu-od.1usm (Pscudohermaphmdntxsm) The gexuta] glands are under-developed and of the same sex, but the external genitals and the secondary sex cbaractexs resem ble those of the opposzte sex Thus there 15 a hlendmg of sexual characters and the sex depends on the presence of testes or «wants. False hermaphrodmsm may be of mtemal or external typ= dependmg upon the changes brought about ‘by the blendmg of the sexual characters affecting the mtemal or external structures.

In the mole aster-nu! type there 15 an anumalous deve lopment of the scrotum which remams splxt resemblmg lalna mayors The penis as underdeveloped w1t.h the arrest of closure DEVELOPMENT OF THE URDGENITAL SYSTEM 191

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Flt: 126-The ezlernal gemtal organs of a. case of pseudohen mapltradzlmn The zndzmdual had grown un as a girl, but on mtehml ezammatxon was found to be a mule -unth urvdescended testes, a mulfonned scrotum

and a pcrns wxth hypospndxas (By the courtesy of Dr J M D251 Scum)

of the urethral groove The testes temam m the abdominal cavity or an the mgumal canal

In the female external type the external sex organs xescmhle those of the male, whxle the ovancs are present In a consxdemhle number of cases, Lhxs condmon IS the result nf congenital hyperactivity of the cortex of the adrenal gland The external sex characters show an enlarged chtons and the presence of a um-gemlal smus openmg below the chtons dralmng both the urethra and vagma

Tm: Ovmur

The development of the mdxflerent stage of the ovary 1.5 as of the testis The gemtnl ndge becomes dxlferenhaiwd mla an Inner mas of epnthchum and an outer layer called Lhg gznmmal epntheluum Unlike m the case cf the testis ::.g 192 HUHAN EIBRYOLOGY

___.__..- _.. ..... .. ..... *...._:...‘L’...... ...__.‘L..

Fm 127 ~The enema! gemtnl organs of the cane den-rsb‘c7d x x Fng 126 (B_/ the courtesy of Dr J M DcSa SaxL.a)

connectne usuua: layer under the germmal epxthehum xs wt} much less than m the pnrnordlum of the tumca albugmea The epxthebal cords become artanged m goups of cells The pnmordxal ova are surrounded by the eptthehal cells to form the prunar) m.n.n.-m fallu.-les In a full term foetus these folhcles resemble those of the adult condttton probably, due to the maternal sumulatmg hormones m the cxrculatmg blood After btrth, these pnmary uvanan Iolheles atrophy pung place to fresh oxanan follicles, produced by the germmal cpxthehum throughout adult hie Some mvensgators hehexe that the pnmordxal germ celk take ongtn from the eudoderm of the yolk sac and finally reach the gernunal epxlhehum.

The mesonephrm and that ducts atroph) by the second. month The upper part of the mesuuephnc duct mm the rudxmentary rate mam may pexsxst as zpoophorcn. Some of the mesonephnc tubules of the lower part may persnst a; paroophorml T_he‘mL-sanephroxﬂand the oxanes attached to them, anse retropentoneally and bulge mto the coelom As the mesonephmt atrophy, the pentoneun holds only the dexe DEVELOPMENT OF THE UROGEWTAL SYSTEM 19.1

lmumn ‘ W "° s K r‘ i ....

fit“?

’ 90 ‘'“'¢.{;’{. urnun: m: I ‘'‘'‘'Uu‘I’ll ‘ \ m or u-ma ucxmu. CINKI umuu rum

LAIIUM HINDI

Pu: 123—Schemal:c plan. nf the development of the counts and their relatwn to the mesonephrai

lopmg wane: and the Mullex-Inn duels When the Mullurlan ducts fuse together as the uterus the pentoneum becomes attached to the uterus and forms the broad hgament ai the

H 154 HUMAN EMBRYOLOGY

uterus Between It and the dorsal body wall 15 a bay of pen toneum called the uterorectal pouch (pouch of Douglas)

The ovary 15 attached to the degeneratmg mesonephros by the suspensory lxgax Lent As xxx the tests, the lower end 01 the may )5 attached to the mgumal hgaxnent whxch becomes contmueus with the eonneehse txssue m the genital SWBHIHQ: (Lahxum majus) as the gubemaeulum. R5 the ox anes gradu ally descend mto the pelvxc cavxty stretching out the broad hgament, the guhemaculum becomes attached to the memxagmal canal. Thxs forms the Toxmd. Ligament of the awry The remamgxg porhon of the guhemaculum het__\vfin__the uterus and the labmm maps 15 the round hgmnent _oj the uterus The Mary remams postenur to the utex-me tube where xt rotates eausmg the hm er end dxrected mevhally

THE Unznus

In an embryo of about the sixth weelg an the outer sxde of each mesenephnc duct there appears an xm agmatxon of the coelomn: epxthehum. Thxs mvagmatmn contmues as a duct parallel to the mesonephnc duct and then crosses m from of xt reachmg xts medxal sxde and along wzth the duct of the opposxte sxde proyects unto the pcstenor part of the urogemtal smus These are the pnrmnesonephnc ur Mullennn ducts The prcuectxon mm the urogemtal smm forms the Mullermn tubercle

The Muilenzn duct Is dxvxded mtu

A cramal part, A horxzanml or mtermedxate part and A xemcal part

The cranial opezung of the lnvagmatmn of the Mullenan duct forms the abdummal osttum and the fimbnac of the utenne tube The crmxal and the mtez-med1ate parts form the uterme tube

The uertxcal part fuses wzth the duct of the opposite sxde formmg the utero-Lagmal canal In the begmmng, the uman ls parnal as there 15 a temporary partxnon between the twa ducts By the <econd month the septum dxsappears leaung DEVELOPMENT OF THE UROGENITAL SYSTEM 19:

1 single cavity of the utem-vaguial canal, the upper part of which is the uterus and the lower pan the magma

By the end of die third month the walls of the fuluru uterine portion become thicker Al: the same time vaginal Iornices appear near the future cervical region The ulerus grows rapidly during the last few months of pregnancy prob ably due to the maternal hormones reaching the foetus By the fourth mouth the muscles and connective tissue of the mags begin to be formed from the surrouudmg mesonrhyme

At birth the cervix is larger than the fundus Dllﬂng the post natal months the size of the uterus becomes smaller probably due to the withdrawal of the maternal hormones

/ KLLANYDXS

/ Misoimpi-in oucr iauuaimi /’

/ }; / /Lzuiinm arms or uiazrizii » Z / /—\_j —~ rum: MUl..LI'.R. nucrs

Muu.i:n— TIJBDL / 7"

OPENS or MESONLPI-IR DULT

fiG 129 —Dt-zgrxxm showing the fused Mullerum ducts project mg into the urogenital sums as Mullenan tubercle

THE Vmunn

In an embna at about the sixth week, the para ineso nephric or Mullerian ducts appear on lhe outer side of the mesonnphric duels originally as invagimilions of the coclonuc epithelium Each Mullenan duct runs parallel to the mcsonaphric duct then crosses in front of it and come; lo the medial side {using with the duel. of the opposite side 156 HUMAN EMBRYOLOGY

and projects Into the posienor part of the urogemtal smus The caudal up 15 packed wath sohd cells and the progechon unto the umgemtal sums IS seen as an elevahon called the Mulleraan tubercle Tlns hes between the lemma] endmgs of the mesonephnc ducts The crama] and mtemedlate parts of the Mullenan ducts develop into the ulenne tubs. The xerucal pan fusmg with the duct of the opposne sxde, becomes the u.tero-magma! canal The upper part is the fundus and body of the uterus and the lower part the vagma The up of the utero-vaginal canal grows Into a Sﬂlld long vagmal cord

JVTBINE TUIS

. _;V1tiUl

__ _,_ ucnu

SD10-VAGINAL mus

UIDGDCYAL SUV:

Fm 13o——Dex.e1opmem of the menu: tubes, uterus and .,ag.m Ventral new Modxﬂed from K05

Accordmg to Koﬂ (1933).° the upper 4«5 pm of the xagma is {armed tron: we lane: an! of 97? U’Pﬂ>‘v’3-3-‘-D35

mm; and \he xemammg 1/5 from the urosemlal sums 0 9»

.Kon.-_ ,,_ 3,, (ma) Development ol Lb: mm In 9'! hm” [ems Gomr In-nbrsol Caz-nos 1n5'-|- 1'» 5’ DEVEIDPLIENT OF‘ THE UROGENITAL SYSTEM 197

{mm exidoderin) The epithelial cells of the urogenital sinus

near the attachment of the mcsonephnc ducts proliferate rapidly causing endodermal evaginations into the Mulleriaii tubercle These are called the sine vaginal bulbs The simi

xagmal bulbs are transformed into solid cord of cells pushing in and replacing the epithelial cells of the Mullerian tubercle

The solid vaginal curd fuses with the solid cells of the vaginal bulbs as the vaginal plate By the fourth month the xaginal plate is convened into a canal by the extension of the urtero vaginal canal from above and by the breaking down of the sino-vaginal bulbs from below establishing the xaguial orifice The deep and narrow urogenital sinus becomes widei to form the vestibule Consequently the vaginal orifice come nearer to the surface

Just near the orifice between the dilated sino xaginal bulbs and the urogenital suius there is :i partition called the lumen It is therefore composed of an internal layer of vaginal epithelium and an external layer of urogenital sinu epithelium

Dzracrs or DI-1V'ELQPhll1N'l‘ or Tl-ll‘. Owiiiv Unznus AND Vmmn

Agcncsis cf Marian follicular cpillichuiii and rudimentary development of ovaries (may be associated with other can geiiital defects such as <plXlZ| hifida defects of the eye deaf ness mental retardation etc.) Due to defective germ plasiii rather than defect of the genital ridge alone

Dnuhle uterus iiul vagina occur when the mulleriaii ducts do not fuse

Double uterus with single \ﬂgI1l1l is owing to the paitial fusion of the inullei-izin ducts

Biparlite uterus occurs when the uterus is separated by -'l median partition due to the defective fusion of the mulleriziii ducts

Bicorniiatc uterus The upper ends of the uterus arcpnuclicd due to the defective fusion of Lhe mullerian ducts

Infantile uterus The foetal condition may be retained in the adult. 158 HUMAN EMBRYOLOGY

Absence of uterus and magma This is associated with non union of the mullerian ducts (Very rare)

Solid vagina and impertorate hyinmi The vaginal por tion may not acquire a lumen This condition is usually associated with hermaphroditism

Hydatitls near the fimlirizited end of the uteruie tube are caused by retention of extra mullerian inwaginations which become cystic

Recto vaginal fistula is the result of imperforate anus and the subsequent opening of the rectum into the vagina

Ti-ii: Pnosmrz

In the male at about the twelfth wee}. multiple solid outgrawths arise from the primitive posterior urethra and extend into the surrounding connective tissue Later these develop lamina and branches and form the tubules constxtutmg the nucleus of the prostate These are arranged in five groups —~anter1or median pasterzor and two lateral lobes corresponding to the lobes of the adult prostate

The Anterior Lube arises from the ventral or anterior vtall of the urethra The tubules at first grow and become branched but later atrophy At birth these appear as small

nlid outgrowths

The Median Lube arises from a gioup of nine or ten tubules on the ﬂoor of the urethra between the neck of the bladder and the openings of the ejaculatory ducts and the prostatie utricle These tubules grow and become branched

The Posterior Lobe arises from large branching tubules on the ﬂoor of the urethra below the openings of the eiacu latory ducts and the prostatic utricle They grow backward in the direction of the bladder The tubules increase in length and become branched and are separated from the lateral lobes by a lay er of connective tissue It is this part of the prostate that is palpated by rectal examinatirm

The Lateral Lohcs arise on either side of the urethra in the prostatic furrow The tubules are larger in size and more DEVH.0Pl\ﬂEV'l' OP TEE UROGENITAL SYSTEM 133

umum Loni:

auonzx

Iunnuoa Lon:

nos-r Irmlcuz

LATERAL Lou:

DGSTIZIIOH Lost: unzmzm

fiG 13! —Devzlapmcnt of the prostate clurmg the thmi month

In number than of the other lobes Many of \hem grow hackwaxd 1n the dl)‘ECllOn of the biadder but few ones at the apxca} region grow m a forward dxrecnan.

The Capsule of the Prostate LS denved from (he sur roundmg mcmnchyme Whlth xs modxfied mm muscle and can rcctue ussnc

Tu: Sam». AL Vasxcuzs

The caudal porhon of each mcsunephnc duct has become Che vas defcrnns. At about the Lbzrtecnhh week, each was defexens near us Junction wnlh the pnmxhve urogemtal sinus, dllalvs min a dxverhculum \\hzch exlcnds back“ nrd and lair.-< rally and ulnm.-me!) {arms the semmnl uszclc. The mam duct of the vsncle mums with the ampulla of the vas defercns at “S lcnnmal pan to {arm the ejaculatory duct 400 I-lU‘\l.AN BlBRYOlJ)G‘I

Tn: Ai=i=r.mmt Trans AND Pitosrnic Uriucni:

The Mullerian dues develop in both sexes by about the seventh “eel: In the male the ducts atrophy as soon as they are formed, except the upper and the lower ends The upper end persisu as appetldt: testis and is attached to the connective tissue covering the testis. The lower end persists as the prostate tttrtcle (masculine uterus)

THE Who: Uizrnuu AND nu: Exri:n.wiz. Ge\mu.s

In an embryo of about the filth \r\ eek the external genitals begin to develop in a similar manner in both sexes During the fifth and the sixth weels, the external sex genitals look -like in both sexes Therefore, this period is known as the sexless or mdtfierent stage From the seventh week (16 mm), the embryo shows indications of the fori-tiatioi-i of the male or female genitals

In embryo 11 mm long (about 38 da)s) there appears a round elevation in front of the eloacal membrane between it and the inf:-auinbilioal body wall This is the genital tubercle On either sirle of the cloacal membrane, a pair of eluations called the genital fold: appear On the outer side of the genital folds are the elevations of the genital swelling: The formation of the uroreetal septum is responsible for the division of the primitive cloaca, by the seventh week, into a ventral urogenital sinus and a dorsal rectum The septum extends to the cloacal membrane dividing it into a ventral urogenital membrane and a dorsal anal membrane In later development both these membranes break down, thereby establishing the urogenital and anal OPEDMIBS

The primitive urogenital sinus elongate: and divides into a dorsal iesica urethral portion and a ventral, the definitive urogenital sinus. The former develops into the bladder and the prostatic and membranous portion of the urethra, whlle the latter extends into the genital tubercle as a phallic portion to form the cavernous urethra. DEVELOPMENT OF THE UBOGENITAL S‘1S'.l'EJ\I 201

Gum‘ tom stun‘ SWDJ.

UEDGENITAL smus

About the sevewzth week

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Ptms

warm. GRDOV cwszn

GENXIAI. swtu. uscxorum

SCEOIVM

About the sntth munch

Fla 132 ——Slane3 I'll the dt.-Lalapnxe-nt of the crtcnlal genitals tn. the male 20" HUMAN EMBRYOLOGY

Bet“ een the stxth and seventh week (13 to 18 mm) the posterior part of the urogenital membrane breals down to farm the prumtue urogenital openmg This LS the urethral groove the elevated margms of wluch are contmuous wxth the genital folds wluch are new lcriown as uretlsml folds The uogemtal suns further extends xnto the under surface of the phallus toward Its up Soon the endodermal lmmg of the region of the urogenital sinus extending with the phallus proltfel-ates and ulttmately the lumen becomes obliterated into a solid urethral plate

By the end of the sexenth wee}. the genital tubercle further elongates into the cylindrical phallus the up of which ennsxsts of the urethral plate The tip becomes slighlty elevated and rounded to form the glans 1727113 Between the region of the glans and the body of the phallus Is the groove of the coronary sulc-us

With the further increase m length of the phaI'us as the pems the urethral groove also Increases 11) length By the twelfth week the urethral folds fuse together convertmg the urethral groove into the tubular urethra. The fusion begins from the basal part and extends toward the tip of the phallus below the urethral plate The fusion as marked on the base of the urethra as the urethral raphc By the fourth month the sohd urethral plate of the glans deepens mto a grome and subsequently into a tube to you: the urethral openmg at the tip o{ he phallus thereby estabhshmg 3 contmmty oi the urethra to the tip of the glans Meanwhile the gemtal swellzngs now called the lahtwscrotal svuellmgs grow more and approach together at the base of the phallus on either \lde of the urethral raphe to form the scrotum The line at fusion 15 marked on the surface as the scram! mphe

The corpora cavernosa are formed 3 paxrecl mesenchymal conderisatxons mth the shaft of the penis Unpaxred conden satxuns m the glans and the shaft canstrtute the cvrpus spangiosum The bulb» urethral glands arise as a pan‘ of dIVE!1lCl1lae from the wall of the urogenital sinus and become embedded in the corpora catemosa.

The preplice IS formed from a tag of ectoderm dxpping Dl-IVE'LOPl\IENT OF THE UROGENITAL SYSTEM 203

down at the root of the glans pems and growmg over the glans as a fold

Dzrzcrs or DEVELDPMENI‘ or ‘rm: Unnrmm ANY.) Pnms

Altscnce of the Pcms Fatlure of development of the gemtal tubercle The urethra opem on the permeum near or at the anal border

Double Pcms Owmg to lack of fusmn of double gemtal tuhemla there may be double glans or two separate organ: each havmg a urethra uxth or wxthout duplxcatmn of the bladder (assocuatcd with ether abnormality.-s)

Cangcmtal Phxmasls owxng to adhesxuns between the glans and the surface

Ilypospadxas The urethra Instead of opcmng at the up of the glam opens, at some point on the under surface of the pems Thxs 15 caused by an arrest In the closure of the urethral groove. which normally closes from behmd forward to the hp

Epupndtas (The urethra opens on the upper surhce of the pews Instead of at the up) T1115 15 caused by defect m the fnrnnatmn of the xnfra umbxhcal [E3101] of the body wall (see Exstrophy of the bladder)

Congcmlal C3515 and Frstulac Owmg to unpcrfect closurc of the urethral gram e. epxthelml rests mxght be enclosed These grow as cysts or futulae, or the cysts may be outgrowths from the fused urethral groove

Stenous ni Urethra The urethral canal becomes narrcmed as a result of the defectwe closure of the urethral groove

Congenital Valves ol the Postcnor Urethra (Lnfolds :: the poster-nor urethra) May be caused by enlargement of _e ulethral falxls or by the pexsxstence of the umgemtal memb'.::: or by defective development at the mesonephnc or ;:3~ maonephnc duct or by {BS1011 of the collxculus vulh the c;.:g. hum In the roof of the postenor urethra (Watson 1922) ‘

-\VA'lSON, 1-: M. (m2; The structural buns tar .=,~. \aIv:—lorm:|loII ol lhe posterior man. J um, 1, an.

n 204 HUMAN EMERYOLOGY

Cnngemtal Dnertxculum of the Urethra (Pouches open mto the urethra at any pomt usually on the ﬂoor) Tins may be caused by defective closure of the urethral foltk, or by the persxstence of embryomc epxthehal rests

Tm-: FEMALE Uxxmx-um AND -rm: Exrannu. Ganrmxs

It has already been stated 1n the descuptwn of the development of the male urethra that external gemtals begm to develop m a sxrmlar manner xn both sexes Dunng the fifth and sxxth weeks the prumtwe urogemtal suns elnngates and dnxdes mto a dorsal vestco-urethral portlon and a Ventnl defimtwe nrogemtal sinus

The veslco urethral portion m the female develops mic the bladder and the entxre urethra. The urethra becomes narrow owing to the dxﬂerenhal cellular gowth m the reglon The female urethra assumes xts defimte form after the seventh week. The gemtal tubercle growmg as the phallus does not elongate as m the development of the male urethra but rernams short to form the chtons The pm-tmn representmg the glans m the male becomes the glans clxtoruhs and prepuce The genxtal or urethral folds enlarge and remam open to form the labm mmoro. and the onfice of the vestzbtzle The genxtal swellmgs grow downward and mute in front of the anus to form the postenor comrmsszssure Laternlly, the genital swellmgs enlarge as the lubza. majm-n

cur ms nu wlms 9 Lu MAIDS

u'aa::}:m'r smvs

ANVS

About the tenth meek DEVELOPMENT OF THE UROGENITAL SYSTEM 205

CLHOIUS

um. Mums Lu. mun:

vnzrﬂax

VAGINA

unis

About the twelfth week

Fm 133 —Stages m the development of the external gmxztala m the female

Dantcrs or-‘ Dzvawutmw or nu: Fzxmxuz Exumnn Gam'r.u.:

The fnllowmg are a few defects of development of the hmale urethra.

Absence of the urethra Duphcatxan of urethza Congennal smclure

Congemlal dnvernculum Hypospadnas As m the male Epxspadms As 1:: the male CHAPTER XX DEVELOPWIENI‘ OF THE \!’-JRVOUS SYSTEM THE Seam; Conn

The nervous system nngmates from a duckened area of ectodem: called the neural p'ate, situated In the axxal regwn Just dorsal to the natochcrd at an embryo of the second week. The lateral walls of the plate gnaw In thxclnmen furmmg the neural groan: uh oh by the thud week Ls very conspncuous as an elongated area suth a broad and thick cephahc portxon gradually namnung and tex-mmatmg Md: a slight expans on

anumu. c.=u. | XITOHC DIVLIION

KEVILLGIOO

RUIDCEOID ‘VIVLIX. YLKTI @0531!)

fiG 13-1-Sectwn of early neural plate and neural graoue (The arrow palm: to a. magmfied uew) DEVEIDPAIENT OF‘ THE NERVOUS SYSTEM 207

towards the caudal end As the embryo grows more m length, the cepbalxc portxon bends down beyond the notochord By the end of the thud week (embryo of 7 sormtes) lhc margins of the lateral walls of the neural groove at. the region cf the 4th, 5th and the 6th sormtes meet. and fuse dorsally as the neural tube By the 10 soxmte stage, the formauon of the neural tube extends m the cephalxc and the caudal dxrcchons except at the extreme ends of the tube, where the closure of the walls at’ the neural groove is slightly delayed These temporary openmgs are known as the antenor and the pnstenar nzuropores By the end of the fourth week, [bow

won: or uA2I‘ru: LA'n:n

onmzx Lmrrma / Mmnun:

nzuuu. cans: ‘ utunowr or srwv Noun uzux-onusr °"“‘°""‘

Tn: 135 —SecIu:m of the neural groove (The arrows pom: to magnified mews ) 208 HUMAN BERYOLOGY

are closed The ceplnahc part of the neural tube, up to about the -lib samne, forms the bran: and the remaxmng pcrhon, “luck )5 of a umform (hamster forms the spmal card

When (he margms of the lateral walk. of the neural folds close to form the neural tube, at the mncuon of the lateral max-gm and the surface ectoderm, are seen pasted masses ni cells known as the neural crests As the neural tube xs fully developed, these crests mxgrate to enlaer side of the lube, constxtuung the pmnordxa of the dorsal spuml ganglia and the sensory ganglia of the cranial nenes and probably also the penphetal cells of the autonnrmc nervous 5.V&teIn.

own menu: wlzaan. 1-uuarsmnx. LAYE8

ECIDEBM

c.\m:.uD'1

Fm 136 ——Sectum of the neural tube (The arrau: pomts to a magmfied. new) DEVELOP‘\1EN"l' OF THE NERVOUS SYSTEM 209

During these early stages, important hxstological differenti.-itaons take place in the neural wall In the beginning, the cctoderm of the neural groove is made up of only one layer of cells. This layer proliferates rapidly and becomes thick, by the time the groove is closed to become the neural tube The nuclei of the cells approach nearer the lumen of the tube as the inner -nucleated layer with the mner limiting membrane The peripheral portion becomes the non-nucleated layer with the ertenial lmutmg membrane Some cells near the lumen of the tube show marked dlVlSXOnS of the nuclei and constitute the germmal cells The nuclear masses from these cells are crowded away from the inner liimtmg mem brane in an area in the spinal cord called the mantle zone The remaining cells near the inner limiting membrane are arranged radially as an ependymal layer Outside the mantle zone is the non-nucleated marguml :ona with the external liouting membrane

The cells of the mantle zone divide into two types—the neumblosts or parent nerve cells which are distinguished by the large size of their nuclei, and the spongzoblasts or sustcntacular cells with smaller nuclei The spongioblasts give origin to the neuroglza, wluch functions as the special connective tissue of the nervous system The spongioblasts have the potentiality to form three kinds of neuroglial cells

1 Ependymal cells 2 Astmcytes 3 Oligodendroglial cells

The epemlymal cells arise from the spongioblasts near the internal hn-utmg membrane The proloplasrmc processes of these cells fixst extend to the external limiting membrane, but later lose connection with it and remain as the lining cells of the central canal of the spinal cord and the brain Towards the lumen of the neural canal. these cells grow cilia, which later dlsappear

The onrocyte: arise from the spongioblasts of the mantle iaycr and are distlnguished by their peculiar protoplasmic prcuectinns all over There are two varieties of astrocyte:~,— protoplasmic and fibrous The protoplasmic astmcytes are developed in embryos of about three months and remain in

II 210 HUMAN EMBRYDLOGY

i:iai:im~na.u. csu yggfo mm '47 ASTBPDCYTEC

nnxaus Asmacrt:

oucoimrbaoculu. clzu.

fin 137 —Schemanc sketches of the deielapment of neuroglia from spongwblusts

the mantle layer The fibrous ast-rocytzs and the oligodendrv glzal cells arise later and migrate towards the peripheral region of the spinal cord in association with the nerve tracts There IS another variety of supporting cells known as microglzul cells which are bEl1BVEd to be of mesoderinal origin entering the cpmal cord through the adjacent connective tissues of the blood vessels etc. They are originally wandering amoeboid cells which become spindle-shaped with many branching pmtoplasnni: pmcesses

The nerve cells arise from the germinal cells The germinal cell divides into a daughter cell and an a.-polar neuroblast which acquires processes at the opposite ends forming a bi polar -neuroblast Later lhe process at one side atmpliies to be replaced by many slender protcplasmic pro1ec— tinns the dendntes The other process grows into the long axon This is the young nene cell. The dendrites come in contact with the dendrites of other nerve cells and the axon passes into the marginal zone and may associate VVlLll other neurones travelling in the marginal zone or may pass out piercing the external limiting membrane as a motor fibre As the nene cells become functwnally acuve their axons acquire protective sheaths called myelin and neunlemmal sheaths The axon of a nerve cell emerging from the spinal cord to form a peripheral nene acquires a neunlcmmul sheath by the activity of certain ectodermal cells carried away from the DEVELOPMENT OF THE NERVOUS SYSTEl\l 211

GDIMJNKL CELL

Flggfﬂ X Araum NEDIOILAII ﬂ 1 menu xmmosusr i -mouux “'m'N:v:ou.As1'

I 4N‘DlVI SEI bmbnxn E

DDUDMTI

AXON

Fm 138—Stage.s m the development of a motor rm-ve cell

card The neunlemma IS not formed mlhm the cord Itself The axon of a nerve cell also acqunres a muelm. sheath, probably due to the aotwxty of the allgudendmcytcs The myelm sheath whuzh is formed xnsade the cord extends the whole length of the axon of the peripheral nerve

ROG!‘ HJTI

MAIGINAL ZONE

ELL! LAKIEA IUIGUI I-IKHAIII

IAIN. LAXINA

Illlln I-ANNA

GIN I. WON;

noon rum: Fm 139 —-Sclmnnm: scene» of the early spxual cord 512 HUMAN EMBRYOLOC-Y

Dunng the aboxe hxstologlcal changes the mantle zone Lecomes LI-ucL A section of the spxnal cord <hows the followmg appearance —The central canal 1s m the form of a dorsuxentral cleft wxth thick walls on the sxdes composed of ependymal, mantle and xnargmal zones. The dorsal and the xenlral walls are reduced to a thm roof and a ﬂoor plate The neural crests are seen on the do:-sclateral sides of the neuxal tube By the end of the fourth u. eek, the axons of the nene cells lymg m the ventro-lateral portxon of the mantle zone develop mto the anterior roots of the spmal nenes uhxch are motor In functxan. Thxs Ventral portwn becomes Lhxcker than the rat and ptojez,-Ls more mto the central lumen as the basal lnmmu. The neuroblasts m the dorsalateral portmn farm the dorsal mots of the spmal nerves whxch are sensory and cmordmahne In functxon Thxs dorsal portion xs called the ular lamuuz The boundary between these laxmnae xs a furrow

called the sulcus lmutans, extending the whole length of the cord and the bran

n.oaan..r:

Fxr; 140 —Schemauc sectwn of the later stage of the dcceIop— men: of the spmal card

The roof and the floor plates do not dexelop nerve cells but consxst only of the ependymal and the neumghal cells Gradually, certam changes take place 11: the alar plates The fibres of the sp.naI gangha from the neural crests gran unto the alar lammae on the medial sxde fox-mxng the dorsal fibre DEVELOPMENT OF THE NERVOUS SYSTEM 213

tracts of the fumculus gmmlns and the I'u.1uculus cuneatus, which pass upwards to the bran: Due to the medzal duckenmg of the alar plates, the lumen of the central canal becomes narrow and then Ls completely closed on the dorsal sxde formmg the poslenor -medum septum The mantle zone now appears butter-ﬂy shaped m a transverse sermon and cons!» tots the gray matter of the spmal cord, showmg three dxstmct columns of cell groups—1.n:, the dorsal, the ventral and the lateral column: The dorsal and the ventral columns extend the whole length of the cord, whrle the lateral columns only extend wnthm the thoracxc and the first three lumbar segments

The margmal zone grows larger due to the growth of nerve fibres mm and through it The nerve fibres when myelmated appear wlute 1:: colour The margmal zone xs therefore known as the wlnte matter of the cord At the same nme, the basal lanunae from both sxdes push out\va.rds medxally and create an antemmedxan groove, whmh later becomes a narrow

POST Ntoum SEPTVM

ltunuu. cnmu.

Am’ Mn! cnoov:

Fm 1-l1—Sch¢matIc sectxon of the fully formed spuml cord

By this tune the spznal cord undergoes changes m shape, sue and posxhon At fust, the neural tube occupxes the whole length of the body mcluclmg the tau], curvmg along the general curvature of the embryo. The acutc bending Of lhe held regmn produces the COIVIC-7} ﬂcxuro, \vl1er1.- At Ls conlmuous 214 HUMAN EVERY OLOGY

wlth the brzun The mesoderm on exlher sxde of the cord develops as the vertebral column, enclosmg the cord ux HS neural canal Nerves emerge out through the mtervertebral spaces After the thud month, there is an unequal growth of the spmal cord and the vertebral column In the pastel-not regxons of the embryo The tax] 15 reduced m sue and the membranes covenng the termmal end of the card are drawn out mto a thread like filament called the filum te-rmmale In the full lerm foetus, the cord extends only down to the level of the (bud lumbar vertebra.

The Spmnl mclunges appear as condensatxons axound the developmg neural tube 111 the form of a sheath Thu.» later dxvxdes mto an mner layez (endornenmx) and an outer layer (eciomerunx) The mner layer forms the pzamater and th/’ arachnov./:1 The outer layer forms the dummater Extensums of the outer layer m the regxon of the bram become transfozmed mm the cartilage and bone of the neurocramum

xnnmmm MIDBRAIN tantalum ornc vsxcuz Mvmﬂnfi SPHAI. Conn

fiG 142 ——Bram of an embryo of four weeks

umumuun /

Mxmalux

omc cup rnsnum -— smuu. com:

fiG 143 —Bnzm. of an embryo of five weeks

Dwx-znapuizxrr 02‘ ‘ms. lVImuLLA

The dxlated antenor end of the neural tube whxch forms the bran: at about the fourth week, exhxbnts three regmns The lundbram or rhombertcephalon, the uudbram or mesence~ DEVELOPIHENT OF THE NERVOUS SYSTEM 215

phalnn and the forebram or pmsenceplmlon. The hmdbram IS dexnan-sated mlo two portmns, mz, the matcncephalon and

the myelencephalrm, The mylencephalon xs contmucus With the spxnal card and forms the medulla

tllnxum xmnauuu

ronnun HYPOPHYSIS

Fm l44.—Lateml mum of the bram of an embryo of the eighth week

From the hegmnmg, the cavtty of the myelencephalon Is dxlated wxth a hroazi and thm dorsal or root plate of ependyma and the lateral walls of the alar and basal laminae, separated by the sulcus humans The cavxty becomes the postermr part of the IV ventricle On the lateral surface of the myelencephalon a few elevatnons and constncuons suggest its segmental ongtn. Blond vessels develop over the thm roof plate and push towards the cavxty as the charoxd plexus of the IV \'e'ntnl:le Thzs plexus of blood vessels wnh the cpendymal layer us the min chormdea.

The lustologlcal structure of the myelencephalon Is 111:»: that of the spinal cord, rzonsxstmg of the ependymal, the mantle and the max-gmal zones, with the basal and the alar laminae separated by the sulcus lznntans. Due to the expansmn of the roof plate. the alar lammae come to he laterally to the basal lammae. As a result, the alferent centres of the alar Iammae are situated lateral to the motor centres of the basal lammae. Unlike m the spmal card, the gray and the whale matter are distnbutcd xrxegularly due to the secondary growth and zmgratxon of cnrtmn groups of nerve cells 2x6 HUMAN EMBRYOLOGY

xsnnaus 1100? or Mmxnluu

m vmmnn ul vmrmcu: MAI-M non? sur Mtnuu. vnlm >.c£m:x:z.l.uu Powrm: PLEXIIR xv vnrrmcu:

auvuav rruc!.ms—

cannon) nuns

300? or xv vnmx. cwmum: mycurusx

Fm 145 —SchemM:tc sketch of the czunty cf the hmdbrmn of an embryo of the seventh week

The cells from the lateral part of each alar lamzna grow and settle down m the marginal zone vemrolfiteral to the hasal lamma Th: 15 the bulb»-pontme extenslon formmg the olwary nuclear bovine: and the pontme uncle: (m metencephalon) The remaxmng portion of the alar lamma forms three groups of receptor nuclex, 1:12,

1 The somatnc afierent,

2 The special vzsceral afierent (branchzal),

3 The general vxsceral afierent The somahc afferent group 35 assocxated wnh the fibres of the 8th nerve and a few fibres of the 5th nerve Towards the lower pornon of the myelencepbalon, this group yoms Lbe group of the opposite sxde and forms the gramle and cuneale

nudes The specsal vxsceral afierent group recewes fibres from the branchlal region through the 9th and 10th nerves DEVEIDPMENT OF THE NERVOUS SYSTEM 217

FOREIEAIE (CV?)

svuuu. COED

Flt: 1-S6—HmIibrmn of an embryo of about the sulh nee! (sketched from a model by Ztegler)

and movs ventrally to form ﬂue nucleus of the sulttury tract The general vxsceral gmup ms the most medxal and forms the sensm-1] nuclet of the vagus nerve

The basal lamina also dzfferentxates mlo three groups viz the smnnlu: efierent specml -visceral efferent and the qzuerul vtsceral eﬂetent The sumauc eﬂerent forms the nucleus of the hypoglossal nerve The special vlsceral group sends fibre; In the muscles of the four poslenor pharyngeal arches through the 9th, 101}: and 11th nenes Later, the specxal vxsceral group mnves ventrally and forms the nucfeus ambtguus The general visceral gxoup gives onmn to the visceral fibres at’ the glnssopharyngeal and the vagus nerves

Dn'£.LoP.su:N'.r or rm: Czmzaumm

The hmdbram or rhumbenccphalon xs dwnied unto —

&I3 and Melencepbalan Hie ms elencephalon Ls conhnuous wnh the spma) cord and forms the mcdulla. The melencephalon :5 made up of three regxons —

1 The anal portion, which 15 conunuous with the medulla. forms the leymcntum of the pans 213 HUMAN EMERYOLOGY

2 The dorsalateral poruon of the alat lamma becomes tluck formmg the cerebellum

3 The basal or fibrous portion of the pans

The cavxty of the metencephalon 1s a part of the IV ventncle

The ma} plate of the metencephalon IS thm and wxdened out, but towards xts apex (on the cephahc side) It nartnws mto an angular regmn where the alar lammae of bath sndes meet together The dorsolateral portnuns of the alar lammae are tluckened as the pnmordxum of the cerebzllmn Due to this sudden thxckemng, small outward projections are produced on exfllex‘ sxde, known as the rhombu: lzps By the sixth \V eek, the thxck alar plates at the angular regxon of the apex of the IV ventricle meet together and form the cerebellum In the begmmng zt proyects mu: the cavtty of the W ventricle, and later on to the suriace These prcuecttuns meet. at the apex, where the medial portzon forms the vennls On either sxde of the ven-ms, the cerebellar regmns grow as lateral lobes By the fourth and fifth months, the outer surface grows quicker than the rest, producmg many lohules and fissures

v):x:1L.s

czxzxnmu

B1-IOMIIZJ LIP

Fm 147 -——Dorsal men: of the Iundbratn cf an embryo of about the etghth week DEVEIDPMENT OF THE NERVOUS SYSTEM 213

Du:-mg the second month, a ﬂexure occurs at the regmn of the pans, squeezmg the rhcmhrc lxps whlch become bulged out on both srdes The outwaml exterlsxons of the cavxty of the IV venlncle mto the rhombxc hps are the lateral recesseg cf the IV vent.m:le The part of the rhomhlc hp in front of the lateral recess grows as the jloculo nodular lobe cf the cerebellum and the part hehu-Ad, as the nuclex of the ’DESIIl)|Lll11' apparatus

Hlstuloglcally the prutrutrve structure of the cerebellum consrsts of the ependymal, the mantle and the margmal layers Later, the cells from the mantle layer rmgrate through the rhombrc hps towards the outer part of the margmal layer {unnmg the superficial cerebellar cortex, dxfierennatmg as the granular and the Gnlgz cells At the same txme, more cells from the mantle layer xmgrate toward the surface as the Purkmje cells From the mantle layer, the dentate nuclm are developed The axons of the Purkmje cells reach the clentate nuclei, which xn turn are connected to the mxdbram as the supenor cerebellar pezluncles

VERMIS !-A'ﬂ‘.l'lAl. Lon:

"’°$-'é'?m.:.u mu: \\ //

I-‘xx: 1-l8—Cerebel.lum at about the fourth month (Dorsal tmuu)

‘Du: !\1mnnAnv (Mr.sz.-zcrr-nIu.o:«)

The mt-sencephalon 15 marked all from the hmdbrzun by .1 namzw nsthmus. The walls are made up of the alar and 220 HUMAN EMBRYOLOGY

the basal lannnae, with the sulcus lmutans between them Its cavxty xs continuous xuth the IV xentncle caudally and with the III Ventncle cranxally Where the early roof plate 101115 the cerebellar regwn of the myelencephalon ms the supenor medullary rzlum. The alar and the basal lammae grow 1:: tluckness towards the nudbne, chhteratmg the roof and the ﬂoor plates. The cavity becomes gradually narrowed mto a thm cerebral aqueduct The dzfierent components of the mid» bram are dem ed from the neural walls From the alar Ian-unae form the corpora quadngemma

From the basal lannnae form the nuclei of the Ill and the IV nerves, the red nude: and the suhstantla mgra.

306? Pun’:

cAVm

uslu. umlzu

Fla 149 —Schemat:c szctwn. of the mxdbram, at an early Stage

The Corpora Quadngemma are pmnunently seen durmg the fourth month as two pans of elevahons on the lateral part. of the alar Iammae These are the super-u:rr and the mfenor quadrtgemmal bOdlE5 functmmng as the. visual and audntnry reﬂex centres The supenor quadrxgemmal bodxes anse fmm the neurohlasts of the mantle zone and mxgx-ate to the surface as stratxfied layers of ganglxon celk Nene fibres run bet“ een these layers establzshxng the co-relahon of vxsual nnpulses “nth pam and temperature. The mfenor qxmdn~ gemmal badzes anse as thxckemng of the gray matter of the mantle zone DEVKDPMENT OF THE NERVOUS SYSTEM El

QuIu:iuL:}'.M. Bonn:

fin 150—Schematu: section of the midlirnin at a later stage

During the second month, changes occur in the mantle zone of the basal laminae The neuroblasts form the mate'iiiiclei of the third (oculo motor) and the fourth (trochlear) nerves By the third month, at the medial portions of the basal laminae goups of nerve cells give origin to the red iiiurlm, one on each side’ Each red nucleus consists of a lurye—czlled and a sma.lLcelled portion In the marginal mm: (of the basal laminae) there are many fibre tracts. On their doisal side are masses of gray matter called the siLl)sla1Itm mg-m, containing ’Il'H?l1l1lHl pigment These are developed in the late foetal stages and continue to gmw rapidly from about the sixth year to adolescence

THE DXENCEPHALDN

By the sixth week, the iorebrain or the prosencephalon is divided into two parts—

The dlencephalon, continuous with the midbrziin and The tclencephalon, anteroqniericrly

The neural wall of the diencephalon is originally made up of the same structural plan as the spinal cord, but it is disprutahle whether the basal laminae and the floor plates extend mto this region or nut A section of this region shows

‘Same imaligatais belicic Lhat the red nucleus and the subslanun nlzn originate by the migration of the cells at the mantle znnc at Ihi ah: lamina luwarda the marginal zone of the basal lariiinz. 222 HU'l\!AN EMBRYOIDGY

(hat the lateral walls on each slde are separated min a dorsal and a ventral portion by a narrow sulcus, the hypothalarnw sulcus Some hold the vnew that thxs IS the ccntmuahan of the sulcus lmnlans and the lateral wall :5 made up of the alar and the basal laxmnae Others thmk that the sulcus humans has terrmnated at the antenar end of the mxdbram and the basal lammae are not present 11: thxs regxon and that the wall Is made up enurely of the alar lammae The latter vxew seems to he more reasonable, as the cell groups of the ventral regmn are mere concerned with sensory and co-nrdla natmg funchons than motor The alar lannnae are full of gray matter The cavnty of the dlencephalon, known as the III vemrxcle 15 at first broad, but due to the tluckenlng of the wall, rs reduced to a narrow opemng

Dunng the second month, the roof plate becomes a tlnn ependymal layer and a plexus of blood Vessels (the chomxd plexus) grows on 11. By the thxrd month, the choroid plexus pushes towards the cavxty of the III ventncle as finger-lxke pmjechons The thm and folded ependymal layer wzth the charcud plexus xs known as the tela chnro1deaL The rest of the dxencephalon 1s rnodxfied to form —

Dorsa1ly——the epxthalan-ms Latera1ly——lhe thalamus Venn-ally—\he hypothalamus

Roar run’: . / munms

nYo'nuu.AM1c smcus

HYro'rHIu.AMvs

nxumlunr nxcxss

F11: 151 ——Tmnscc1-se sectzon of the dzencephalan, at about the st.-nth week DEVELOPMENT OF THE NERVOUS SYSTEM 223

The Epnthnlamus 15 developed from the roof plate and the adyacent dorsal pcrtxons of the alar lammae By the second month, the roof plate 15 modxfied unto the tela. chnroulea A part of the roof plate Just postenor to 1t pnuects as an evagxnatwn medxally Tins evagmatxon gmws as a thxck and comcal mass known as the eplphysls or the pmeal body From the alar lax-mnae, adgacent to the roof plate, are developed a pair of nuclear masses called the habenulae, whxch are Interconnected by commzssures In from of the eplphysts IS the hubenular commzssure Behmd the eplphysxs, m the area between the dneucephalan and the mesencephalon, are developed the fibrtx of the posterior commtssurz

The thxck neural Wall IS dmded mto two pumons by the hypothalamxc sulcus

The dorsal part, the thalamus, The ventral part, the hypothalamus

cI::u:anJu. m:Mzs.

mmum

fit; 152—Sagutal secuan a_f the Llsencepimlon 0] the second month (Skctched [ram 41. model by Zlcglcr) 224 HUMAN EMBRYOLOGY

The Thalamus In the regmn of the thalamus many gruups of nuclen are dxfierenhated from the gray matter They are 1. The ventral nucleus (ventro-laterally), 2 The lateral nucleus (dorso-laterally), 3 The lateral geruculate body, 4 The medzal genxculate bmiy, 5 The mednal nucleus (dorsamedxally), 6 The antenar nucleus, 7 The central nucleus (between ventral and medial)

By the thud month, all the nuclen are developed and the thalamus grows 1n thuzkness and reduces the SIZE of the central lumen, The thalmm of both sides may meet together as the massa. mtermeduz (or the mterthalarmc connezus) The ventral portmn of the thalamus contams many fibre tracts mergmg mm the tegmentmn of the mxdbram

The Hypothalamus Ls developed fram the ventral portion and forms the centre of the sympathetuc nervous system, contmllmg the vegetatxve luncuons of the body The followmg are denved from tlus regxon 1 The manullary bodies 2 The Lnfundxhulum

The cells of the mantle zone zhfierentxate mto many groups of nuclex One group IS the pan-ed -nwmzllary bodxes pm Jectmg out on the ventral surface The other IS the tuber cmereum

The Infumllhulum 1s developed 111 the ﬂoor as a local evagxnauon, farmmg the stalk and the neural luhe of the hypophysts

The Optxc Chmsma Durmg the early stages, a pan: of 01:11:: vesxcles are formed at the rosttal end of the dnencephalon as outgrowths from the vent;-o-lateral walls. The eavxty of the optxc stalk xs ohhterated and xts neural wall becomes the pathway of the optxc nerve Part of the cptxe nerve fibres decussate 1n the regum of the lauuna terminahs to form the optac clues-mu

TH: Tzhmcerxuwu Thu xs the must amermr part of the bram lts neural

walls are made up of only the alar lammae and the roof plate the basal lammae and the ﬂoor plates hemg absent It can DEVELOPMENT OF THE NERVOUS SYSTEM 2:

sists of a median portion continuous with the diencephalon and tvm lateral hemispherical evagmations of the cerebral vesucles The rostral part of the median portion forms the thin wall of the lamina tcnmnalzs The cavity of the cerebral vesicles, called the lateral. ventrurles, is continuous with the III ventricle through the paired I1tt2r1ientncular fomnieii In the early stage this foramen is wide but gradually it becomes very narrow

By the sixth week the cerebral Vesicles extend forwards upwards and backwards as the cerebral hemispheres CO\EX'lI)g the median portion in front and the diencephalon and the mesencephalon behind The extensions of the tv.o cerebral hemispheres come in apposition where the adjacent connective tissue forms the fol: cerebri Histologically the neural wall is made up of the characteristic ependymal mantle and the marginal zones in the early stages During the second and third months the neuroblasts of the mantle zone migrate to the marginal zone as the superficial gray matter the cortex By the fourth month the nerve fibres from the cortex and from the adxacent thalamus constitute a layer under the cortex These fibres appear white in colour alter myelination and are known as the l.LlltlL' matter The white matter towards the basal portion of each hemisphere comes into relation with the nuclear masses of the basal, ganglia The basal ganglia of both sides constitute the corpu stnatum

The Corpus Strintuin is formed as a thickening in the mantle zrine of the ventnrlateral walls of the telericephalon By the sixth week, it prmects into the cavity of the lateral Ventncles It is continuous with the thalamus of the dience phalon l\er\e llhres passing between the cerebral corte\ and the thalamus enter through the corpus striatum These nerve fibres form a white band called Lhe mteriial capsule dividing the corpus striatum into a medial cizudatc nucleus and a latent lcntiforni nucleus

In the second month, the caudale nucleus lies in the [loot of the lntenentncular forainen slightly proiecting into the lateral Ventncles By the third month, the caudate nucleus increases in length along isith the rapid growth of the cerebral hemispheres. lts caudal portion extends into the cerebral

15 225 HUMAN EMBRYDLOGY

Part around the mfenor horn of the lateral ventrxcle as a tall hke body

The Lenulorrn Nucleus IS dwtded min a lateral part, the putameu and a medial part, the globus pallxdus, contamxng many nerve fibres

BXPPOCMOUS

cnoaoxb PLZXVB

urmuuu. CAPSIVI-I7 PUTAMDI

TAIL Bl‘ cnuwu-n N

ED’POCAN?17s

)u:GIo'1 or msuu

¢A|7DA'|'2 N

t-man-trsxs ou nxcr °?\’ EH1-ASMR

AN‘! PEN‘ SlllS‘|’A.HCl2

Fm 153 —Sagxtta1 sectxon of the telencephalon of the thml month shouung the formatmn of the corpus stmzxum

The Cerebral Cortex ls developed {ram the porhons away [rum the corpus stnatum The rnedzal wall of the cortex near Its boundary to the dxencephalon rs Lhtn, bemg formed of the root plate A plexus of blood vessels pushes m from each Sld9, towards the cavxty of the lateral xentncles through an mvagmatmn called the chormclal fissure, sztuated at the le\el of the mterventncular [oramen The protrudmg thm neural wall becomes the tab: chorondea of the lateral ventncles. The cerebral cortex rs dxvrded Law an olfactory or rhunwwephalon and a non-ollactary or neupallmm

TH: Rruxmczvn.-man

The tollowmg are dem ed from tl-us regnon — The olfactory bulb and stalk The olfactory tracts DEVI-IXJPMENT OF THE NERVOUS SYSTEM 31

F14: 1.54—Phutmmcrogmph of a. sectum of the cerebral hem:

sphere of an embryo showmg the clmrmd plexus of the lateral ventricle

The antenor perforated substance The lnppocampus

Dunng the sxxth week .1 pmjectmn appearmg on the ventral surface of each cerebral hemxsphere becomes the olfactory bulb It grows mm the olfactory stalk wxlh the olfactory bulb at the extreme end. Fastener to the olfactory bulb certam areas of the medial part of the cerebral hem; spheres function as the olfactory centres One of these LS (hr. untermr perforated substance The olfactory regxon also Includes the part of the ndjotnmg cortex known as the arcinpalhum which forms the arched luppocampus projecung Into the cavity of the lateral xcntrxclns an each suin228 HUMAN EMBRYOLOGY

Tn: NEOPALLXUM

In the early stages, the cortex is Smooth but as the surface of the gray matter increases m size, many convolution: (gyri) and fissures between them are produced. Each hemi sphere expands into four lohes viz. the frontal, the parietal, the occzpttal and the temporal lobes The lateral sentncles also extend into these lobes

During the tlurd month, In the region of the cortex between the frontal and the temporal lobes of each hemisphere a marked depression called the Sylvmn fossn is seen The cortical area of the floor of this fossa 15 known as the msula where the growth is dinumshed compared \vith the rest of the cortex. The adiacent areas avergmw as folds or opercula to cover the insula The opercula growing from the frontal, the parietal and the temporal lobes meet together and form the Sylwan or lateral. cerebral fissure

From the fifth month onwards, the follosi mg fissures dei elop —-—

1) The central fissure, between the lrontal and the parietal lobes

2) The panetc-«occipital fissure, between the parietal

and the occipital lobes m) The calcarme fissure demarcating the wuual area of the cerehrum iv) The collateral fissure on the ventral surface of the temporal lobe

Along with the formation of these fissures many shallow depressions called suit: and their characteristic d.|VlSlOnS marl. the peculiarxtxes of the high es olution in the human cei-ebmm

The cavzty of the lateral umtncles expands on both sxdes mth the bulging of the cerebral vesxcles

The tivitcnor horn is the anterior extension into the frontal lobe

The pastenor horn corresponds to the growth of the

occipital lobe The mferwr ham follows the temporal lobe and extends laterally, downwards and form ards DEVELOPMENT OF THE NERVOUS SYSTEM 229

cl:N'mA1. nssrnu:

“C

SYLVIAN nssum: -rmromu. Lon:

SYLVIAII mssn

Fm. 155—The cerebmm at abmu: the eughth month (Suit: mew)

nolrmx. Lou:

PAnl!.‘I.Iu. Lou:

occlrmu. Lou:

1'-‘JG l56——The cerebrum at about the mm]: month

Tm: Cmzmul. Conmssvxas

The mslral part of Lhe medxan pamon ox’ lhe !elencephalun forms the lamma termluahs After the cerebral hemisphera am developed, the lamma termmalxs becomes thxck and IS converted Into the camnussural plate thmugh whxch the nerve fibres cross from one hemxsphere to the oLher The cmsmg of the nene fibres through its dorsal portion con emula the cutenar conzarmsurc. During the tlurd month, another crossing of nene fibre:

occurs :11 the N.-gran of the commasural plate near the wall 30 HUMAN EMBRYOIJOGY

of the III xentnele dorsal tn the ante:-tor cornnussure Tlus as the luppocampal commuaurc The nerve fibres from the cortex of the neopalhum JDXB the dorsal part of the lilppa czunpal comnussure to form the corpus cullasum “nth further growth of the corpus callosum, the htppocampus IS muted backwards mto the temporal regxun of the cerebrurn placmg the hxppocampal comnussure caudal to the corpus callosum.

The Corpus Callosum 15 developed In the tl-urd month as a bundle of fibres tn the roof of the lamuul tezmmahs dorsal to the luppocampal eomnussure Soon it grows more an sxze and extends anternorly and postenorly tn the upper part of the commusural plate The comnussural plate becomes tery much stretched out as the septum. luctdum acquu-mg a narrow cavxtg lmown as the cthmm sepu lucult.

Tm: Cnaxmu. Neat as

I The Olfactory ‘N,-rte Is the sxmplest type of seruory nene lt xs pecuhar an that at has no ganglmn and the fibres are non medullated The nerve cells sntuated m the surface of the olfactory ptls send theu- axens mm the olfactors bulbs where they are connected by other neurones mth the centres tn the rhmencephalon. The axons on thezr way to

the olfactory bulbs pass through the detelopmg cnbnforn: plate of the ethmnxd hone

ll The Optic horse ls not a real nene but a nene tract tn the bram The two lateral outgrowth: from the proseucephalan form the opuc ves cles uhxch later become the vague cups “fill! the tnner sensxtne retxna and the outer p gment lave: The nerve layer of the retma grows as neural fibres whxch pass out through the choroxclal fissure Into ‘lhe granted optxc stalk and finally enter the bxam tn the ﬂoor of the dzencephalon. At tlus region some of the fibres from each eye cross over to the opposite sde to farm the optu. elutumn From the cluasma the nene fibres contxnue as the

optxc tracts ten.-runaung In the vtsual centrs of the thalamus and the nudbratn

Ill’ The Oculnmotor hone artses from the basal lanuna of the rmdbt-am on each stde and pages out as a nerve trunl. DEVELOPMENT OF THE NERVOUS SYSTEM 231

to supply all the €)J.l'lX1SlC muscles of the developing eyeball, except the superior oblique and the lateral rectus muscles

IV The Tmchlcar Nerve arises from the basal lamina of the mulhrain. caudal to the origin of the oculomctor nerve The nerve fibres of each side pass through the neural wall dorsally and decussate with each other under the roof plate and emerge between the mid and the hindbraui to supply the primnrdia of the superior oblique muscles of the eye

V The Tngeminal Nerve is a mixed nerve mostly con sisting of sensory fibres and 1S concerned with the supply of the dermatwes of the first pharyngeal arch lts semtlunar ganglion is situated near the roslral end of the hmdbram The sensory fibres are derived from the neural crest at this region The sensory root from Lhe ganglion enters the wall of the myelexicephalon at the pens and pins the sensory nuclei where some fibres pass downwards as the spinal tract of the nerve The ganghon divides into three branches —

The ophlhalnuc the maxillary and the mandibular

The ophthalmic nene supplies the eye and the fronts)nasal process

The rna\illary and the mandibular nerves supply the surface of the face

The motor fibres arise from the basal lamina of Lhe pans and follmv the mandibular none to supply the muscles of mastication

VI‘TlIe Abrluccsit Nerve arises from the hasal lamina of the metericephalon The nerve fibres pass out ventrally caudal to the pens to supply the lateral recius muscle of the eye

VII The Facial Nerve is a mixed nerve, mostly composed of motor fibres

The motor fibres arise from the facial nucleus in the basal lamina of the inyelencephalon The nucleus shzits its position, bending around the nucleus of the abduoent nerve to form the gem: The nerve is distributed to the second pharyngeal arch to supply the muscles of facial expression A few fibres also supply the sublingual and submandibular glands

The sensory fibi-:5 arise from the gcniculate ganglion denied from the neural crest, close to the acoustic ganglion. 232 HUMAN EMBRYOLDGY

KYPOGIDSSJI CJVICAI. NS.

sup GANGL or _.l,.._R\ vncas K ‘ ‘- _

vxcus Iv

0 4

uaxnxuvx

Fm 15'! -—-Schematic sketch of the ongm of the cramnl nerves, m an embryo of the sntth week. ' l

A few fibres from the gemwulate ganglmn enter the alar plaie to form part of the sohtaxy tract A few fibres pass out through the motnr fibres of the chorda tympam branch and mm the lmgual branch of the mandibular nenve ta termxnate m the taste buds of the tongue and submamilbular and sublmgual glands

VIII. The Audﬂnry Nene IS entxrely sensary and )5 made up of nene fibres {mm the acoustu: ganglum denved from the neural crest at the level cf the faunh neummere of 1113 hmdbmm. The acousuc ganglwn and the genxculate ganglxan of the facxal nerve are m close Contact wxth each other In DEVELOPMENT OF THE NERVOUS SYSTEM &

the early stages, but slowly they separate The acoustic ganglion is made up of bipolar nerve cells the proximal proIISES of which reach the rhombic lip of the brain and the distal processes pass to the otic vesicle growing in the vicinity Meanwhile the ganglion is split into Vestibular and cochlear ganglia along with the division of the otic vesicle into the cochlea and the semicircular canals Nerves from the vesti bular ganglion are distributed to the samcxrcular canals and from the spiral ganglion to the derivatives of the cochlea

IX The Glossopliaryiigcal None is a mixed nerve mostly composed of sensory fibres It arises from the region of the liindhrain below the one vesicle The motor fibres take origin from the basal lamina near the arigm of the vagus nerve at the nucleus ambiguus The motor fibres supply the muscl°s derived from the III pharyngeal arches and also the parotid glands

The sensory fibres originate from two ganglia a superior one, close to the brain and an inferior one on the nerve trunk From these ganglia fibres enter the alar plates of the myelencephalon to ]0m the solitary tract (along with the lib: LS of the facial nerve) The fibres growing outwards pas towards the III pharyngeal arch as the tympanic nerve and nerve fibres to the tongue

X The Vagus None is a mixed nerve The neural crests at the level of the IV V and VI pharyngeal arches constitute the jugular or superior ganglion close to the hindbram and the ruidosiil ganglion on the nerve trunk. The sensory fibres from these ganglia enter the alar laminae of the myelei-ice phalon and ]DIl’l the sensory fibres of the facial and the hypo glossal nerves to form the solitary tract The fibres which are penpherally distributed from these ganglia form the chief branches of the vagus nerve supplying the pharynx larynx traelica oesophagus the thoracic and abdominal viscera

The motor fibres arts»: from the nucleus ambiguus in the myelenizephalnn and also from the dorsal motor nucleus These fibres pass out and for a time 1011! the motor fibres of the accessory nerve and later supply the (1€71Lt1lI1,¢S of the IV and V pharyngeal arches, such as the muscles of the pharynx. larynx, etc From the dorsal nucleus parasympathe\

Z4 HUHAN EMERYOLOGY

he pre-ganglxomc fibres also arise supplymg the thoracxe and abdoxrunal viscera

XI The Accessory Nene Is composed of a nramal and a spmal portton The ctarual portmn areas as an extensmn of the \ agus nerve The spxnal pomon ans:-s from the antenor columns of the first fise eervtcal. segments of the spmal cord Thane fihzes ascend through the foramen magnum and pass out at the level of the ongtn of the cramal portton The sternnmastozd and the trapezxus muscles are supphed by thxs nene

XII The Hypoglossal None 1s a motor nene supplyxng the muscles of the tongue It anses from the columns of gray matter tn the lower pnrhnn of the medulla and constsu of three or four pre-cervical tennal roots. The sensory roots eltllef atrophy or dxsappear tn the human en-Abryo

DEVEXDPNIINT OF THE SPINAL NERVES

In the fourth week. the motor nenes grew from the basal laxmna of the spmal cord and emerge out through the external lumttng membrane as the central nr the antenvr roots A few cells mtgrate from the spmal cord among the fibres oi the antenna: roots to form the neunlemrnal sheaths of the nene fibre and the gangha of the autononuc nenous system.

The dorsal or the postenor root xs dexelnped about a week later from the neural crest The neural cxests form defixute masses aecordzng to the znetamertc segnenn of the myotomes of the embryo The neureblasts of the neural crests are of the bxpolar VETIBI)‘ (1 e wtth two proceses). one process growmg outwards and the other entenng the spma! cord The processes from one neural crest-mass collect as a nerve bundle to enter the gray matter of the postermr horn ef the sptnal cord The nerve bundle growmg outwards Jmhs the antenor root in form the spmal net-Le Some of the undxfierentxated neuroblasts from the neural crests may mag:-ate \Vﬂh the nerve roots to contnbute tn the neunlemmal sheath: of the pertpheral nerves and the gangha of the sympathetic nervous system Gradually, the bipolar sartety of the neurnbluu of DEVIIOPMENT OF TED-Z NERVOUS SYSTEM 35

the dorsal yiiglia becomes unipolar as the two iiene processes become continuous

The spinal nerve trunk laterally, divides into a dorsal or posterior and a Lent-ral or anterior branch The dorsal branch supplies the dorsal muscles and the skin The anterior branch after giving all a small branch medially to connect with the sympathetic ganglia divides into a lateral and a ientrdl portion and supplies the media lateral muscles and the corresponding skin Connecting loops are established from one anterior branch to the other These are the 7lCl'DE plea.-iises the cermcal plexus the bracriial plexus Supplying the upper limb and the lu-mbo sac-ral plexus supplying the lower limb

DEVEl.X)Pl\XI2t’l‘ or 'riia AU'i'o'voMic NERVOUS SYSTEM

The autonomic nervous system is divided into sympathetic or thuraco lumbar system

and

Paiasympathetic or cranio sacral $5 stem

The sympathetic system consists of a chain of ganglia and peripheral nerves. During the fifth week a few cells migrate along the fibres of the antenor roots of the spinal nervu Some investigators believe that a few cells migrate Ix-om neural crests and pass along the posterior roots of the nerve trunks These celLs accumulate in the region dorso lateral to t.he aorta as paired ganglionic masses linked together by a longitudinal nerve card This is the sympathetic nervous system Gradually, the chain extends to the cervical and to the caudal portions of the embryo Nerve fibres from the spinal cord pass to these chains as the preganglwmc mm: or white ram: cominumcmites and the fibres are returned to the spinal nei ve as the gray ranu camnmuicantes or postgavigliomc TIIHII

Some of the sympathetic neru. cells migrate peripherally as the collateral gmigliu, i c , the cardiac, the COBlh1C and the liypoguxtne iiene plexuses Others migrate towards the 236 HUMAN EMERYOLOGY

developxng heart and vxscera as the p'reu,ortu: and the msceral sympathetzc ganglza

The P417“-Yympathetxc system has the preganghomc cell bodxes m the bx-am and In the second thxrd and fourth sacral segments of the spmal cord. The postganghoruc nerve fibres are distributed to the sahvary glands cxlxary muscles of the eye and to the vxscera

Darzcrs or Dz-zvenoemerrr or -me Srrmu. Conn AND Baum

Rhadusclusls (Cleft Spme) The neural arches of the vet febrae throughout the length of the vertebral column do not meet together to enclose the spmal card This condman Ls transnutted by recessnve gene

Memngocnele The membranes around the neural tube protrude through the cleft sp ne The sxte of Involvement may be m the vertebral column or skull

Splna Blfida Occulta The neural arches of the vertebrae do not meet together to enclose the spmal card at the lumbar or sacral regmn The space As filled wxth connective tlssue to whxch the membranes of the cord hecome attached

Ancncephaly The furebrzun and the outer bony cavenng are not formed. Tlus cnndntxon is probably due to arrest of development as a result of defechve germ plasm

Porencephaly Cavmes are formed m the lzram substance These may commumcate With the ventncles and the sub arachnmd space

Vxcrocephalus Very small bram Is developed

Hydrocephalus The ventrxcles of the bram are expanded nuth a large amount of cerebro-spmal ﬂwd dunng or after bnrth The skull bones also become enlarged to accommodate

the lug hram The cause at this condmon may be due to excesswe formatxon of the C S F or due to the obstruetmn of us How nr due to defectwe absorptxon of the ﬂmd m the veins It xs probably transmxlted by IECESSIVE gene DEVHDPMENT OF Tl-I12 NERVOUS SYSTEM 231

It :5 probable that hereduy Is the most unportam factor xn many of the congenital defects of the nervous sys|em

Fm 158-A newborn mlant um): auenccphalus (By the courtcxu of the Pnncnpol, G M College) 238 HUMAN EJXBRYOLOGY

—.,....o<m.»-p...«~.n—,.,... ‘’ 1‘ _ ‘ w x ’ 4

._ -.....>._.4

fiG 1:29 ~—A1I uxfimt unth hydrocephalus (By the courtesy of the Pnncqml, G M College)

THE Annsamx. Guws (SVPRAREVAL)

The adrenal gland develops from mo prunord1a~—tl1e cortex, derned from the mesoderm and the medulla from ectodennal chromafiin txssue

In an embryo of about the sixth week, the cells from the splanchmc mesoderm between the root of ihe dorsal mesentex-3, and the upper bender of the mesonephros pmhferate on each sxde By the seventh week. these cell masses detach from their source of ongm and accumulate on either sxde as the cortex Soon these cells pralzferate and became dxﬂerenuated mm large aczdoplultc cells By the exghth week the cells become arranged 111 curds wxth smusmdal Vascular spaces between them and become xmested by a ccnnecum hssue

capsule DEVELOPMENT OF THE NERVOUS SYSTEM 239

VHO’ H 3001' MOTOR N “"‘”‘ szvuv GANGL Sn” )7 CD15 1.15 IEO“ CKRD V 0051.. nix. MESONDH

Gm. Ermn Gin‘

Fn‘. 160-Schenwitic plan showing the ortgm of the suprareual gland, m an embrjo of the strth uzek

Certain cells from neural crests migrate \enti-ally along with the sympathetic ganglia and pass through the coehac pli-xmses They are later separated from the sympathetic ganglia and are accumulated on the medio~dorsal side of each cortical mass By the seventh week, they begin to invade the cortex to be transformed into the substance of the medulla The£e cells are known as clxromazfin cells. due to their peculzar staining reactions wzth chromic acid salts The cumplnte pene tration of the izlirumaffixi cells and the subsequent EX1CélpS|.IlZ|ll()X‘ of the cortex progress slowly, from the third month onwards

During this stage, the curtex possesses two regions The inner region or the part in contact “llll the medulla consists of mature cell cords indicating the early production of seem (ion: This part 15 the Ioetal at prausumal cortex This thick zone contributes to the great bulk of the foetal adrenal gland The miter layer shmis smaller cells forming the penmtnent or the defiriiliie cortex Atter birth the foetal curler slowly ’£0 HU\!AN EMBRYOLOGY

Mama I! Aoxm C£lJaiANl‘g‘D_M sum. conncu.

rnmosnnzx

\ HBDNDI-lacs

fiG 1fiI..—SchzIm1tu: plan shazcmg the ongm af the suprareual gland m an embrya of the sete-nth wee}.

degeuemtes and the gland becomes smaller m 5128 The outer regmn gradually grow: larger as the permanant cortex

rotnu. coma

crus man 5710 GANGL (u|:nu'u.\)

Pu: 113:2 —-Sc}ne'mauc sec-tum of the supranwml gland durmg the stud foetal pznod m:vm.onu-:m- or -ma mzavous svs-mu an

Fm 163-S1.-humane secnon of the supmrenal gland dm-mg the first post natal year

Tu: Hvpomvsxs

The hypophysls anses from two pnmcrdxa In an embryo of about the second week, ‘a small ectodermal dwex-uculum known as RaI.hke’s pouch appeaxs on the roof of the stamodaeal depressmn, gust m {rent of the buccophax-yngeal membrane It grows m the dxrecuon of the dxencephalon of the bx-am

Hnrnnnmu

IATKKES POUCH TOVGUI rm 16-Isfixgntlal sectwn of the head. of an embryo of the strth week IND?“

unru-5 voucx ll(Al.IO\V'l‘lIG) . I-‘xc 165-Dutal dxlatauan of Rntlnkeis pouch

IS 242 HUMAN EMBRYOLOGY

am: cxusmx

/ 1m-mm nzmss com: 'H$S.’.['\ / cnvmr (AHOPHY)/I

PAM mrzn.

Fm 166——Fm-{her fate of Ruthke’s pouch

By the sxxth week, the mfundnbular process growmg as a tubular outgrowth from the ﬂoor of the dxencephalon meets the dxstal end of Rathkes pouch. This part of the mfundl hulum xs transformed mm a Sﬂlld cord of nerve cells to form the pars nenosc and the stalk. oi the hypophysxs Graxiually Rathke's pouch elongales and becomes naxmw, probably due to the rapxd growth of the maxxllary and mandnbular regions m its nexghboux-hood By the seventh week, xts ongmal connecnnn with the stomodaeum becomes cut off The distal porixon of the narrow pouch ddates 11110 a double walled expansxon enclosmg the pars nervcsa of the mfundxhulum The mner wall of the dtlatatmn remams closely Ln contact wxth the pars nervosa and ultxmately fuses wxth It to constitute the pars mtermzdw of the hypophysxs The 1-ostral part of the outer wall grows rapidly Ln sxze as a glandular structure to become the anterzar lobe or the pars Llzstalxs From the tlurd month onwanis, xts cells dxfierentxate Into the chaxac tensnc chmmophohes The narrow lumen of Rathke’s pouch exzstmg between the pars mtermedla (Inner wall) and the pars dutalvs (outer wall) IS known as the rescdual lumen whleh becomes reduced to a narrow sht In some mammals, the resxdual lumen 15 promxnfiml)’ 53911

A further outgrowth from the dﬂated expansxon of Rathke's pouch forms a pan‘ of lateral buds whxch enclose the stalk of the mfundzbulum to constitute the pars tuheralzs of

the hypophysxs mzvznopmzm or 11-:3 NERVOUS system 24;

B51-D Luna. PARS INTER

Pu: 167 —For-mutton of hypophysu

Fm l68—Schcme oi the adult hypophysu CHAPTER XII DEVELOPMENT OF THE BAR NOSE AND THE EYE Tm: EAR THE: INTERNAL En

In an embryo as early as the thud week, a small area of ectodermal thxckemng called the one pkmode, an the null open cephalxc part 0f the neural plate on exther side lndlC31E5 the pnmordxum of the Internal ear By the fifth week, due to the excesxve growth of the sux-mundmg tissues, the one placode smks m on either sxde of the myelencephalon as the one Leswle and IS separated from the surface ectodexm Just at this region of separatmn, a narrow tubular dnveruculuxn known as the endolymphatnc duct anses from the medial part of the one xesxcle Gradually, the one vesxcle elongates Into a bmad dorsal Lzstxbular portum and a narrow and ﬂat cochlear portwn The denvahves of the one vesxcle are known as the membranous labynnth The endolymphauc duct 101115 the vestibular pomon

one YLACODE

Nmuu. WALL

e——/Narocacxn \\

ytuurna POUCH

fiG 169-—Part of a sectxon of an embryo of the thzrd week, through the one plecode

Dunng the sxxfh week, due to dxfferenual gruwth, three ndge—hke pmyectmns are seen at right angles to each other DEVELOPMENT OF THE EAR NOSE AND THE EYE 245

In the vestzbular pamon Slowly, the nuddle parts of these pr-oyectnons atrophy gxvmg ongm to the tubular senuczrcular funds The vestxbule dxﬂerentxates mto a dorsal utrxcle where the sermurcular canals open and a ventral saccule contmuous Wxth the cochlea. At the same tune the tip of the cochlear pox-tron grows out as a spmzl cochlea The passage between the saccule and the cochlea becomes narrow formmg the ducttu 'reume1|.s The connectxon between the utrxcle and the saccule also hecomes narrow

zlmolnuu. nut:-r n'nc vnsxc - @ \

\

PEAR. -rmr mu:

Pu: 170 —Purt of :1. sectzon of an embryo of about the fourth week through the one ues-xcle

Dunng these developmental changes the smgle lmmg ectodennal layer of the ode veslcle undergoes many changes as a result of the fibres of the audxtory nerve reachmg them The hnmg cells are transformed mto the charactenstxc sense organs. Each semxcu-cular canal whxle opemng mto the utncle produces a slxght enlargement called the ampullu at one end The hrung cells of the ampulla are modxfied mto an elevauon called the crzsta, conlammg specxal cxllated cells of the sense organs concerned \Vl|.l’l the dxrecuon of movements at the body In the utncle and m the saccule are dcwelnped areas known as macuhze whxch are sense organs concerned with the eqmhbnum of the body Durmg the third month the hmng cells of the floor of the celled cochlea trans~ form into .1 spccml tlnckcmng known as the spmzl organ (organ of Corn) wxth the charactcnstu: tectanal membrane 2€6 HUMAN EMBRYOLOGY

and the neuroepnthelxal haxr cells and guitar cells constntuung the organ of heanng

lama SAC VESHIU1-I KEV! WALL VETIIVLD-COCK GAIIGL \}

F16 111-—Membranous labyrmth at about the fourth week

moo SM:

SEMJCIL CANAL

\BfﬂUl-I

comm Ext: 172 -—Memb1-anous laburmth at about the uzth «Leek

ENDOLYMFK DUE!

Fm 173-Membranous labyrinth at about the eighth week

Along wnth the development of the sensory mechanism In the membranous Iabynnth, the mesenchyme cells su: roundxng It ax-gamse mm a compact lay, at of camlage Later, the loose mesenchytne cells, between the membranous labyDEVELOPMENT 0!‘ Tim MR NOSE AND THE EYE 241

math and the camlage hquefy as the peruotu: or pez-nymph tssue cunslshng of a duck ﬂuxd In the regun of the cochlea

the pen-one nsue extends the whole length above and below Ihe spzral organ The upper extension xs known as the scale. uembuh and the lower the scale tympam In the thud month

these two spaces meet at the apex of the cochlea to form the helzcotrema Dunne the fifth month the outer cartilage Is transformed xnta bony hssue

DCDOLYNPIL IIC

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Fm 174-Schematu‘ sketch of the pans of the mternal ear

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Fm l1’5~SecIxou ol the cochlea, of the tlurd r uanlh 248 HUMAN EMBRYDLOGY

THE Miiwu: Em:

The middle ear consists of the pharyngo-tympaiizc tube and the tympanic cavity

The phnryrigo-tympani: Tube is developed from the first pharyngeal (endodermal) pouch with which the second pouch also may (use together During the eighth week, the proxi mal and vi the pouch narrows and forms the pharyngetympumc tube The distal end (which is separated from the brarichial cleft by a thin membrane of ectaderm and endo derin) dxlates to form the tymparuc cavity

NB ACEVNVL

Fm 176—Schematic section of the middle ear nf an early embryo

The T):-npanie Cavity The distal end of the fiist pharyngeal pouch recedes from the pharyngeal cleft and expands in size At the same time, the mesennhyme cells in its neighbourhood, between it and the one vesicle accumulate rnigrating trom the first and second pharyngeal arches These mesmchymal accumulations condense mtu three carulagmous masses forming the rudiments of the auditory ossicles The aims and malleus are formed as mesenchymal condensations from the dorsal end of Meckel's cartilage in the 1st arch. The stapes arises from similar condensatxons of Reichervls cartilage in the DEVELOPMENT or THE BAR NOSE AND THE EYE 249

313 331111 L313’: ‘hey 0551f? Into distinct bony structures

In 918 earl)‘ Stages these ossieles he abuve the primordial tympani: cavity embedded in loose mesenchyme cells During the seventh and the eighth months, the tympani: cavity expands and engulfs the auditory ossicles including their muscular attachments and the chorda tympam nerve The tensor tympam muscle is derived from the mesenchymal con

deusations of the 1st arch and becomes attached to the malleus. Hence its nerve supply is from the mandibular division of the 5th nerve, the nerve of the 151. pharyngeal arch The stapedius is derived fmm the condensation: of the 2nd arch and is attached to the stapes its nerve supply bezzig the 7th nerve of the bid pharyngeal arch Further expansion of the lympanic cavity in a backward direction is responsible for the fomialion of the tympanic imtrum and the mastoid air cells

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fin 1'l1—Sc7iemaiu: seciian of the middle ear, showing the auditory ossicles 250 HUZXIAN EMBRYOLOGY

TH: EXTERNAL But

The External Dr consxsts oi the uzmcle, the external meant: and the tympnmc membrane

The Auncle of the external ear anses from the mesen nhyme of the pharyngeal arches around the first pharyngeal groove Dunng the second month, three tubercles anse {mm the fu-st arch and three from the second arch, along the outer border of the groexe These tubercles JDLD together to ion-n the aunele

The External “oatus rs derxxed from the first pharyngeal goove In the early enahrg e, the ectoderm of the pharyngeal groove and the endoderm of the pharyngeal pouch are :21 contact. Later, the distal end of the pouch recedes from the surface to be rnodtfied as the tympanum. By the exghth week, the pharyngeal grooxe deepens as a result of excessne growth of the ectodernzal layer Exeessne prohleratzon of cells at the deeper portxon of the groot e, forms a loose plug of ectoderm near the endodermal “all of the rympamc cavity By the seventh month, the plug breaks oﬂ centrally to form the deep parhon of the external audttory meatus

The Tympanzc Membrane As the deep end of the external meatus comes near the tympamc cavnty, the ongmal meso— dermal condensatxons amund the cavity become thm In bnet’, the tynapamc membrane Is made up of the endoderrnal lmmg of the tympanum and the ectodermal extensxon of the external meatus, mth a thin sheet of mesoderm hekween them to form the membrane propna. The ty-mpamc mernhrane became: surrounded by a nng hke tympanxc hone ansmg mtra memhranously

Dzrecrs or Dr:vnI.oPM.L\r or -rm: EAR

tlal/onnauom of the erternal ear are due tn the defects tn the fusxon of the tubercles of the hyomandlbular cleft

Congemtal deafness )5 due to taulty nerve connecuons or defecuve formauon of the audttory osxcles

Tax Nos:

The formatzon of the external nose has already been descnbed tn the chapter on the formation of the face and nzvanopiiizm or THE ma, NOSE AND 1111-: an 251

palate (chapter Vll) Along with the {on-nation of the palate, the mouth cavity and the nasal cavities are separated by the roof of the mouth Sunultaneously, the inner openings of the nostrils (posterior mares) are shifted back. dorsal to the palatine process.

About seven elevations of the adjacent meseuchyme grow out from the lateral walls of the nasal cavities during the sesenth week These acquire cai-tilages and form the nosetiti-buuzls, iiza.:-Lilo-turbinal: and the ethino-turliiiials The nasa-turbinals develop along the wall of the nasal pits and finally are reduced to the ogger nasi. 'I‘he maxilla-turbinals grow in relation to the palate and constitute the inferior conchae The ethmo-turbinzils develop above the maxillatui-binals as five scroll-like elevations The first or the lowest elevation forms the middle conchae, the second forms the oupanor canchae The third may {use wzth the second elevation The fourth and the fifth are transitory elevations which may persist as supreme conchae

The eotodermal lining cells of the upper part of the nasal pits are modified as the olfactory epithelium These cells clifierentiate into nerve cells the axons of which gnaw into the olfactory bulb of the brain The nerve fibres on their way pas through many small foramma in the dEV0l0p|X|E cnhriform plate of the ethrnoid bone The renizuning cells of the nasal epithelium lsmng the conchae and the septum become stratified and ciliated This layer contains the opening of many mucous glands

The nasal cavities are continuous with many chambers in the cartilaginous and bony portion around the olfactory pits. The hning of the nasal epithelium extends into these chambers as paronasal sinuses During the fourth or fifth month the lining cells extend to the maxilla as the rna.z-illary sums, to the ethmoid as the etlimoid sinus and to the frontal bone as the frontal. sinus All the sinuses are wry i.riconspicuous at birth, but de\ elop fully by about the 25th year

‘his Era

The Optic Vesicle and the Optic Cup In an embryo of about three weeks. at the anterior part of the still open neural Z52 HUMAN EMERYOLOGY

plates are seen slight thickenings By the time the neural plates are closed to form the forehrain, these Lhickeniiigs become grooved and pro)ect outwards as round optzc vesicles By the fourth week, the optic vesicles are seen 1JI‘OJECllXlg out as outgrowth: of the forehrain with broad optic stalks By the fifth week, the part of the surface ecloderm of the embryo in contact with the optic vesicle becomes thickened to be modified as the pnmordium of the lens At the same time, the surface of the optic vesicle is ﬂattened and gets invaginated into a two layered optzc cup The uivagmation is efiected in an eccentric manner in that a groove 15 produced on the ventral margin of the optic cup This is the charoidal fissure In continuation with this fissure there is a slight. groove on the ventral surface of the optic stalk through which blood vessels pass into the optic cup As a result of cellular differentiation, the outer layer is reduced to a thin pzgment layer and the inner layer becomes the sensitive layer of the retina The cpace between these two layers remains as a potential cavity Lhroughout life

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CHOROIDAL nssuai:

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fit: 178—Opt:c vesicle of an Fm 179-Fm-matxmx of Dpllc embryo of four cup in. an embryo weeks of _fi1.e weeks

The Retina By the fifth week, the cells of the inner layer arrange themselves into an outer nuclear and an inner nonnuclear (or marginal) portion. About a week later, cells from the outer nuclear portion grow inwards forrning a distinct layer of neuroblasts This is the inner nuclear layer By the third month, a few large cells of this nuclear layer proliferate to farm specialised ganglion cells constituting another layer DEVELOPMENT OF Tim EAR NOSE AND THE EYE 253 MLIKGXNAL OUTER NUCLEAR

PIGMDCT LAYER

Fm 18{I——HIstogenesLs of retma during the fifth week

nums LAYER

Fm 181—Htxt0gen.es1s of retma during the swath neck

INNER. NIJCL GANGL 551.15 \ ovrm M151.

Pmunn‘ un'|'.n

Pu: 132 —Hulogem:su of retma durmg the Hun! month 251 HUMAN EHBRYOLOGY

lime: to the xnner nuclear layer The nerve fibres ansmg from the ganghen cells form the 1181129 fibre lower The nerve fibres pass through the optzc stalk as the optic none '11:; cells of the outer nuclear portxun are later converted Into rod and cone cells

Fm 183 ——Hutogeneats of retma durmg 6-7 months

The Lens The prxmordxum of the lens is the thxclcemng of the surface ectoderm 1:: contact with the optxc vesicle m an embryo of the filth week Due to the mvagznatnon of the outer wall of the aptxc vesxcle to farm the optxc cup the (luck lens pruncrdlum smks mto the cup as a lens zeszcle, bemg detached from the surface ectoderm Gradually xt grows as a round hollow layer of cells By the sxxth week, the cells of the deeper or the postenor part of the wall elongate more than those of the antenor wall. The cells of the antennr wa‘l grow as a thm layer of cuboxdal cells By the setenth week, the cells of the pastenur wall have elongated to such an ettent that the cavxty of the lens vesxcle as gradually ohhte rated to n potentxal space The elongated cells are modlfitd Lnto the lens fibres by the degeneration of thexr nuclex New fibres are added to the enamel lens fibres tn an equatonal regxon by the prnhierahon of cells lymg arhacent to the layer at cells of the ante:-nor wall. DEVELOPMEN'l‘0}.-‘Tl-EEAR NOSEANDTEEEEYE 255 H \ (

an-onnu

W-IILDFDIH V5

OPTIC VEICI-E

I-‘It: 184——Sectum of optzc vesicle at about the fourth week

Fa: 1SG——Seclum of optic cup at about 1"!“ "mi "' ""If ‘-""*‘ 256 HUMAN ENEERYOLOGY

In the thu-d month these new lens fibres wlnle extendmg to the outer pole cf the lens In a mendwnal dxrecnon grow more In length overlapping the ends of the ougmal fibres Thus the lammated arranganent of the adult condxtmn xs estabhshed The formatxan of the new fibres 1s contmued m a slow and methodxcal manner throughout hfe

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ECTODERM

Mfifs H

Fm 187——Sectwn. of lens fiG 188 ——Sect1.or of lens ueszcle at about uexzcle at about the fifth week the stzth week

ANTWALL

fiG 189 ——Sectwn of lens at about the seventh week

Low: or Pnouru.

The capsule of the lens probably anses from the sur roundmg mesenchyme cells The poslenor porhon Ls supphed by the branches of the hyalond artery and the antenor partxon by the thin annular vessel Dur ng the last months of preg nancy the vascular supply slowly disappears

The Chm-oxd and Salem After the formahon of the lens vesxcle and nts separation from the surface ectoderm the apuc xesxcle xs surrounded on all 5 lies by ihe adyacent mesenchyme DEVF_LOPMEN'.l‘ or me me NOSE AND -rm: mm 271

ix:/“‘= Ease ICIODEEM era‘ “"11. 4' JV .-*'X , /

fiG 190 —Le1is fibres in about the third month

cells These cells organise into two compact layers The inner layer (endomeninx) lies in association with the pigment layer of the retina and is known as the choroid The outer layer (ectomeninx) becomes tough and surrounds the choroid layer pasteriorly as the sclera and is contmued anteriorly between the surface eeoderm and the lens as the pi-in-iordiuni of the substantia propria of the cornea

The choroid layer in contact with the pigment layer giuws over all sides of the margin of the optic cup to form the ciliory bod / It is to be noted that the ectomenim. (outer layer) near the developing brain becomes fibrous and forms the duramater of the brain and the optic stall: (after the closure of the choroidal figure) Theretore it is C0nl4nu0uS with the sclera

The Cornea The cornea is derived from three sources 1 The outer or anterior lay er is darned from the surface ectodei-m from which the lens vesicle has been already formed and detached away from the surface The surface ectodei-m closes up over this region and is transformed into the outerlayer of the cornea

2 The maior portion of the cornea (subslantia propria) arises from the anterior continuation of the sclera between the surface ectoderin of the embryo and the lens The cells are modified into transparent fibres to constitute the nilistmuia prapmz oi the cornea

l7 258 HUMAN EMBRYOLOGY

3 The posterior epithelium is derived from the condensed inesenchyme cell layer of the anterior chamber

The Anterior Chamber and his The anterior chamber is the region between the cornea and the lens This space is filled with loose mesenohyme cells which condense into a thin layer covering the back of the cornea as the posterior corneal epithelium and the from of the lens as the papillary membrane At the same time, the neural extension of the margin of the optic cup grows forward in front of the lens and behind the pupillary membrane The pupillary membrane and the neural extension fuse to form the in: The muscles of the ins are developed from the neural outgrowtbs, being ectodcrmal in origin The neural outgmwths do not reach the central region, where there ls only the pupillary membrane During the late foetal stages, this central part of the membrane ruptures to form the pupil

The Ciliary Muscle This is derived from a mass of mesenchyme cells between the outer surface of the margin of the optic cup and the sclera The longitudinal striations of the muscle are recosiused by the third month and the meridional and the circular portion: become differentiated later

The Posterior Chamber The region between the inner layer of the optic cup and the lens is the posterior chamber In the early stages, this cavity is filled with loose irregular cells, possibly derived from the neural epxthehiini of the inner layer of the optic cup and from the surface ectoderm of the lens ‘There is also an invasion of the mesenchyme cells through the choroidal fissure constituting the pnmordiuoi of the hyaloid blood 'v'E§ElS This area, at fiist known as the primary mtrecus, becomes the uecoiu'lo.1-u vitreous after the formation of the lens capsule and the atrophy of the hysloid artery Later. by the fourth month, the ciliary body and the iris also "secrete vitreous fibrils" Gradually, the loose cells traxufonn into a ielly like material called the ternary utreaua or the vitreous

The Blood Vessels In the early embryo, a branch of the ophthalmic artery known as the hynloxd artery, enters the optic cup through the choroidal fissure continuous with the groove on the mantra! surface of the optic stall: and supplzes DEVEUJPMENT OF THE EAR. NOSE AND Tl-E EYE 25:)

blood to the vm-eous body and the lens The hyalmd artery IS dxamed antenorly mm the annular vessel’ at the nm of the apt): cup The ophlhalmxc artery also sends a few branches to the choroid and sclera Some of these branches break up mm a vascular plexus m the clwroxd. the amenor most part of lhe plexus hemg the annual vessel Durmg the fourth month, the hyalmd artery atmplues after glvxng out a few branches to the renna

In later development, the vems fmm the 1:15 and the czlxary body temunate m a venous channel known as the canal of Schlemm, through which excess of aqueous humour VS drained nut to mamtaxn normal mtra—ccular pressure

PIGMDH’ LAYER

I-‘xx: 191-Secmm of optu: cup at about the strth week

The E11.-luls Folds of the surface ectoderm Will! “)9 uncle:-lying maoderm, grow over the cornea to form the pram. urdnum of the eyelxds The upper lid anses from the frontanasal process as a medial and a lateral prunordlum. The 10“ at lad rs an extenswn of Ihe maxxllary process These fold; appear by the seventh week as an outer la) er of skm and nﬂ mner layer, folded back over the anterior part of the scle.-3 ant‘! the cornea as the ccvnynnvhmz. The nnderl) mg mcsudg-,3 develops as the tarsal plate, the connectzve and the mu.:4~_[;, lxssues. Soon, lhese fnlds elangale and co» or the eye B_v:m_.

‘II. is : Whl of dxspulu whether the armular K-ssel is an ray 9, a nun Fuchs hn\-mvu bchewd I: to be an artery 260 HUMAN EMERYOLDGY

tenth week, they meet and fuse together Durmg the fifth and the sixth months, the fusmn breaks through separatmg the hds The eyelashes and the associated glands appear by the fourth month.

The Lacrlmal Apparatus The lacrzmal gland develops durmg the rum}: week as a collectxon of a dozen epnthelxal outgroxvths from the outer part of the upper ltd, where the epithelmm turns back over the sclera The sohd uutgmwths acqulre a lumen and become the secretmg glands The secretxons enter the nasal chambers through the nasn-Iacrunal ducts

These ducts arxse as sclxd epxthehal oulgrowths from the upper and the lower eyehds at the medxal angle of the eye The two outgrowth: meet together and mm the narrow groove between the maxxllary process and the lateral nasal process The angmal outgrowth: become canahsed to form the masohzcnma! duct whxch xs placed medal to the naso-maxnllary

groove CANAL O1‘ SCHLDGI

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P16 192 —-Schevrumc seccum of the eye, at about the fiffil month DEVELOPMENT or Ti-in i-mt noss AND ma EYE 251 DEFECIS or DEVEUDPMENT or run Err.

Cyclopia is a condition in which the elements of the eyes or orbits of the two sides become united The degree of union varies In the least marked forms the two orbits are separated by a single small nasal aperture In the most marked forms, the two orbits fuse together \1Jl() a single one with a single eyeball and an optic nerve Owing to this fusion the frontanasal process from which the elements of the nose arise Ls pushed forward The nose, therefore, become: situated above the orbit in the form of a proboscis (figure 193) The condition is due to a disturbance in the organization of the anterior part of the neural tube

Annphtliiilmlii (Absence of one or both Eyes) This is caused by the failuie of the formation of the optic vesicles from the forebrain (very raie)

Optic Cyst This is due to failure in the involution of the primary optic vesicle

Cololmiiin of the Retina and the Optic None is owing to some disturbance in the normal closure of the choroid fissure of the optic cup

Eetopia of the U1. I.-al Pigment Abnormal extension of the margins of the optic cup (Common)

Opaque Nerve Flbrcs of the Retina are due to formation of msdullary sheaths around the axis cylinders of the nerve fibres of the retuia

Congenital Colour Blimlncss Defect in the development of the percipient end-organs in the retina

Congenital Cataract The many different forms of congenital cataract are due to some disturbance in the development of the lens.

Persistent llyaloiil Artery The hyaloid artery disappears during the late {oetal stages Some portion of this may persist abnorinally as patent blood vessels or ubliteratr.-d strands of t sue

E Ectopia Lcntis and Cololiomzi of the Lens may be due to .i persistence of the fibro-vascular sheath of the lens or due to a maldevelopmcnt of the rim of the optic cup

Pcrsislciit Pupillzir) Membrane ln the fourth month, the HUMAN EBRYDLOGY

Pm 193-14 human Cyclops (By the com-xesy oi the Prmcapal G 3! College)

pupxlla:-y membrane °:der.ds along the from of the lens ..nd may persm throughout Me

Congenital Glaucoma The canal of Schlemm (cm-u.lar <m\xs) does not dexelop and hence than xs no dramage from ll’): ‘manor chamber

An lndsa Absence of the dex elopment of the ms

Cnlobnma ml the Ins An-es: of gram}; of the ms may be due to persxstznce of fibm vascular sheath of the lens.

Blue Sclen The sdera becomes mm th:-ougl: \\hlch the cbomnd ms seas.

Rafi-sruw Enos (‘U3 opsa and Hypermelropla) The epuc ans of the eyeball L5 longer or shorter DEVH.OPMENT OF THE EAR, NOSE AND THE EYE 263

Astigmatism Dxfierence m the curvature of the mendxa of lhe cornea or of the lens

Albinism xs charactensed by the congemta] absence of pxgment m the eyes T1115 15 due to the absence of a “chromo» genie factor“ raultmg m the faxlure of (he pngment (on-nauon

Colnbnma of the Eyelid, Absence of the Lacrimal Canal Faﬂure of the fusmn of the maxillary process wxth the frontenzsal process.

Ankylolsleplaax-on (Pm-hal Closure of the two Eyelids) The margms of the two eyehds remam umted

Plosis Defect m the development of the levator palpalnae aupenons muscle

Slrabismns Defect in the development of (he extrmslc muscles of the eye. DEVELOPMENT OF THE SKEl.!i‘l‘AL SYSTEH CHAPTER XIX] Tue Siizisi-an

The mesenchyme cells have the potentiality of iormuig the supporting tissues of the body, such as the fibrous tissue, cartilage and bone It is customary to classify bones according to their origin from the meseneliyixie cells Bones arising directly from mesenchyinal condensations are lmoivn as intramembranou: and those which are modified from cartilage are known eiidocliondral bones

lntraiiiemlirai-ious ossification In those regions where bone formation occurs, the inesenchynie cells multiply and form a condensation of cells which become elongated and ﬂattened in shape later, the mesenehyme cells of the condensation secrete a strand of collagaiious fibres Around these fibrous strands the meseiichyme cells became arranged in close layers of cells. These cells increase in size and difierentiate into osteobliists The fibrous strands increase in thickness probably due to their fusion with ii collagenous material called osseomucoid, formed iron-i the inter-cellular spaces This fibrous material is known as ossem or osteoid. 0\\l.ng to the activity of the osteoblasts, calcium salk are deposited in the adiacent collageiious osteoid material, which now becomes the bone malrcr The original bone matrix. after being fully ossified is called 5 trcbecula. Due to fiirther gmwtti. more osteotilasts are forrned in the xiiesencliyine cells and consequently more layers of bone matrix are deposited These term the lnmellae Many irregular bony tnlieculae thus formed, unite with one another enclosing areas of vascula.i- connective tissue to form a. cancallous bone While new laniellae are being added to the bone matrix, the layer of osteahlms is moved outward At the same time, a few asteoblasts become buried in the bone matrix and faim bane DEVELOPMENT OF THE SKELETAL SYSTEM 255

OSTEOBLAST

OSTEQCYTE

fiG l.‘l~l—Sectzon shoumg the formation of mtmmembranaus bone

cells or ostencytes Each osteocyle becomes enclosed In a space In the matnx called the lacuna The surmundmg mesencllymo cells transform mm a closely adhexeht penosteum

Elndoclwndral ossification Endochondral bones are formed by the replacement of n cm-nlagmous model Canllage cells when they age change then‘ character leadmg to a calcxficauon Wnh Incoming blood vessels, llus dxsmlcgratea and :5 replaced by bone forming cells and the dcposmon of bone In an cmbryu of about the seventh week. camlagmous masses of cells are seen at the regions where the future long bone: anse Gradually, the camlage cells first become enlarged In 256 HUl\li\N EMBRYOLOGY size at the site of the ossification centre. Later, these cells

destroy the matrix, creating irregular vacant spaces containing calcium salts. Just at that time the perichondral tissue lying

EON! GU-L

IDNI Nlfl-IX

Fm. lB5.—Ding'rom of endncliond-ml hone fomiotion.

adjacent to the region of degeneration proliferate: and invades into it with the accompanying blood capillaries. The perichondral tissue is made up of xnesenchynie cells and functions in the same manner as in the case of the formation of the membranous bone. The cells of the inner layer become osteoblasts and many layeis of bone are laid down on the degenerating cartilaginous strands by ixitxavmembranous ossification. The periehondriuxn is now called per-iosteum, on account of the ‘bone formation. These layers of bone unite together and form a typical cnncelloua bone, while the outer layer forms the compact bone. Later, secondary ossification centres appear in each end of the epiphysis of a long bone. In post~natal life, the growing hone consists of a shaft with the epiphyseal end separated by a plate of cartilage. Growth in length of the bone depends on this epiphyseal cartilage.

Ti-ie Vzerzeiuuz

In a transverse section of the early sci-nite stage, the medial portions of the masses uE inesoderm fanning the somites an both sides of the nutochni-d become condensed and are modified as the scleratomes. The sclerotonies of opposite sides DEVELOPMENT OF THE SK!-ZLETAL SYSTEM 267

Fm 196—Ca1m.*ra. lucuia draumig of the deuelapmg Lertebra of an embryo

fuse together as a thick mass enlrapping the notochord This is the pi-imordium of the inter vertebral disc and the body iii the vertebra Paired rnesndermal extensions from the Lentrum of the vertebra in a dorsal and a lateral direction form the neural arches and the costal elements

By the seventh week the original mescrichyme cells are modified into cartilage, first in each centrum with two centres of diundrification which fuse together to form the body of the \crtehra Later, ossification appears in each neural and each costal extension The neural extensions of each vertebra fuse together to farm the neural arch and the spinous process while the lateral extensions form the transverse processes Meanwhile, rib czirtilages are formed in the costal areas and chundrification occurs in each rib Gradually the rib becomes 353 HUMAN EMBRYOLDGY

NTUIM. WALL 01' S7. CORD

run. or NEURAL RICH

1.;-;L““.,"y°;,, new or var

Fxc I57 ——Tmnwe-rse scene“ of tfwrnctc vertebra, at about the second -month

separated from the vertebra and the tertebra ts fused tn one earttlagtnous mass The xntervertebral masses of cells form the tntenertebral dusts and the dorsal and ventral ligaments. By the tenth week, all the vertebrae have become osstfied, havmg a centre tn the body and one Ln each lateral outgrowth. Even after btrth, new secondary centres develop and finally coalesce wtth the adult vertebra

In the first three or four vertehrae of the neck regxon. t.here forms a centre of chnndnficatlon on the anterxor part of the body of each vertebra known as the hypachardal bar The atlas rs formed by the umon of the neural arch and the hypochor-dal bar of the first vertebra. The remanung part of the body at the vertebra gets dtsconnected from the atlas and fuses wnth the body of tne axns as the adontoad process

The sacral vertebrae fuse together durmg the fifteenth to the txt enty-filth year

'msRms

The nhs are developed from the lateral projechons of the vertebrae In the thnraexc region They extend m a curved nevmomvrem or 11-12; sxeuruu. system 259

Sun mocrss

nurocaonn ion‘: In viz!

Fm. 1925 ——-Transverse set.-non oi thorax.-u: vertebra at about the tlurd month

duecuon along the body wall, acquxrmg a centre of s)lG!Idl'|firatnon for each rub Secondary ossnfis-‘anon centres appear m the head of the nb and m lhe tubercle The ongmal can

nectxans to the vertebrae become separated and znslead. a mm: is formed between the 111: and the transverse procew of the vertebra Gradually, the camlage IS tnnsiormed into

bone

Tm: Sramvvn

The prunordmm of the sternum ns seen In an embryo ol the slxlh week as use long “'35595 “f mesehfihime N115 In the arm.-rzor body wall In from of the hearl, on enhr.-.r sale of the nudlme By the nmlh weal», the two masses meet logelher mednally as the sternum. lo “hid: the rubs become attached The fusion at first begins In the cephalic part and gradually progusses caudally BY the mnth neck the swrnum is fully formed Ossnficamm from eight or man: cnnms starts

from about the filth month. 270 HUMAN EIVIERYOLOGY

CLAVIGLE

Fm 199 —Dei:elopment of the sternum, at about the ninth week

THE SKULL

The skull consists of a ’ﬂ2‘lll’0ETl1.1l-lllfﬂ which fori-ns a covering for the brain and the sense organs and a 1Jl.50e1'0eramum derived from the modifications of the pharyngeal arches

The Orlﬂn of the neuz-ocranxurn is from a mass of mesen chyme cells in front of the riotochord. at the level of the base of the hindbrairi. The mesodermal condensation: become cartilaginous and form the floor of the brain as the choi1dracranmm This consists of a composite structure extending to the nasal capsule, Ln front, the orbital cage‘-ile laterally and to the one capsule on the sides near caudal part The mesodcrmal masses from the three cervical somites (the first disappears early) advance toward the caudal part and fuse with it Some mesencliymal conclerisatioris extend laterally and instead of becomuig cartilaginous masses acquire the characteristics of membranous ossification (e g frontal, parietal) Gradually, in this composite mass of chondrocranium separate ossificatzon centres appear and by the fusion of some and by the separation of others, the bones of the skull are formed

The ocmpztol. bone is mostly endnchomlral xn origin but a few parts also anse intramembranously Four endochondral ossification centres form, two distributed medially and two laterally Meiiially, they form the supra occipital in front, by DEVELOPMENT OF THE SKELETAL SYSTEM 271

one CAPS

Inca. CAM’ Sm “cc

THY can

Pu: 200-—The chondracrunmm. of an embryo at about the ezghth week (Modzfied from Lewzs)

the ninth week and the baskoccxpxtal belund by the tenth week. Laterally, on each side of the foramen magnum forms the ocmpntal arch The mtramembranuus osslficalxon centres form the mterpanetal portmn of the bone All the centres fuse together after bmh

The aphennui anses both endochondrally and mtramembnuxausly There are five pairs of endochondral centres distributed as follows —

one pair in the small wmgs (orbno-sphenmds) ane ,. ,, the temporal wmgs (a1x~sphenoxds) one ,, ,, In front (presphenoxd)

one ,, caudally (basx~5plxenozd)

one ,, m the hngulae.

There are also two pants 0! mlramembranous centres forming the ox-bxto-temporal regxon and the medial pletygmd plate on enhcr side Some of the centres appear by the ninth week.

The zlbmmd )5 a czu-ulagmous bone The ogxficnnon oi

Ils med».-:1 poruon forms the ethmmd plate of the nasal seplum and the crush: gall: Dux-mg ihe ossnficalmn of us lateral 272 HUMAN EMBRYOLOGY

tin‘ nun.-nu. on fun l Au-nu.

I-l\GUL H, 9:5,, us DH Fm 201 —Oss1,ficutiorn in fiG 202 —-Ossificutwn in occipital bane sphenoui bone

cartilages the trabeculae remain thin with wide spaces between them Before the union of the medial and the lateral portions the fibres at the olfactory nerve pass through it in the middle Later these spaces become closed by mases of cartilage ossified to farm the c-ribnfann plate

The temporal hone has tnree sources of origin. The main portion originates from the mic capsule The squamous part arises from the membranous layer of the ventrolateral exten sums (page 214) The styloid process arises from the second pharyngeal arch. The centres of ossification in the capsule of the internal ear start from the fifth month at four regions to form the bony labyrinth and the petrous portion at the temporal bone

’11:e frontal bone and the parietal bone are membranous u-i origin arising from the membranous extensions

Ti-ie Vxscrimciumxum

The origin of the visceral part of the skull or the visceranranium is from the cartilaginous bars of the pharyngeal arches These cartilaginous bars may persist or may become ossified or may be convened into ligaments From the 1st arch the maxillae and the mandible are formed These became ossified mtramembranously The cartilaginous con densations of the 2nd arch (Reicherts Cartilage) form the upper part of the body and the lesser horn of the DEV!-ZLOI’I\lENT OF THE SKELETAL SYSTEM 213

ligoid The posterior condeiisations at the posterior ends of the 1st and 2nd arch cartilages come into contact with the one region of the neuro cranium to form the auditory osszcles The malleus and incus arise from the 1st arch cartilage and stapes from the 2nd arch cartilage The part of the 2nd arch cartilage below the stapes is ossified as the stylotil process and the part between it and the lesser horn remains as thy style hyoid ligament The cartilage: of the 3rd 4th 5th and 6th arches are formed only on the ventral aspect and there« fore do not constitute the viscera-cranium The 3rd arch cartilage fuses with that of the 2nd arch to form the greater horn and the lower part of Lhe body of the hyoid bone The cartilage: of the 4th, 5th and 6th arches fuse together ven trally to be modified into the laryngeal cartilage:

Tue MANDIBLE AND MAxiu.A

The mesoderin of the first pharyngeal arch forms the maxillary and mandibular processes The rnesadermal core of the mandibular process is converted into a cartilage known as Meckels cartilage Around the cartilage mesodermal condensations take place constituting the primordlum of the mandible These condensations develop as intramembrai-mus bones Those on the outer side of Meckel's cartilage form the beginning of two membrane bones. one anleriorly ai d the other posteriorly The anterior condensation: appear lateral to the anterior part of the cartilage, where they grow and partially surround the cartilage to form the mandible The posterior condensation: at the posterior end of the cartilage near the region of the chi: vesicle constitute the priinoniium of the mo auditory ossicles, the 'InC1lx and malleu: Poslcrinrly. llie mandible grows upward as the ramus which comes in close association with the squamous part of the temporal bone pmducing the teniparo-imindibuliir Joint

Gradually, Me<:ki:l's cartilage disappears except zi small part of it in front at a little distance from the midline, where cndochondrn] ossification takes place This region gnes rise to the part of the mandible between the mental foramcn and the symphysls. The $)mpll)SlS is not derived from the carti IR 274 HUMAN EMBRYOLOGY

Snunmous van‘! or 'n:m=muu. EON}:

zrcowu-rc ' I ~ _

STAPB

Fla 203 —D:agrum of the -mandtble of a, foetus, showmg «ts relation unth the audztory osswles (Madzfied from Kollmann)

lage, but from a condensanun uf cells m the mxdlme Thxs develops mto fihro—cart1lage wluch fuses wxth the mandible m the mnilme at the end of the first year As the symphyseal fihx-o—carulage becomes osslfied, the mandxble grows us height length and thxckness

Growth 1n hexght of the mandxble xs brought about by the growth of ICS lower border and of the alveolar processes ansmg m connectxon wlth the development of the teeth The longxtudmal mcreme m length xs due to the development of the ramus svnh the coronmd and condyluxd processes The growth 15 more 1:: the condyle As the mandxble 15 relauvely fixed to the mandnbular fossa of the temporal bone, the growth progresses II‘ a downward and forward dxrectxon In the post natal penod, the surface becomes pmperly moulded by the alveolar processes and the tooth germs

The details of the early formation of the maxilla have already been stated an Chapter VII The two maxnllary DEVELOPMENT OF THE SKELl:Tl'AL SYSTEM 275

processs begm to fuse at the end of the second month There are four ossrficatxozx centres m the maxilla on each side, the prexnaxxlla (contammg the pnmordxum of the mmssor teeth, the aulenur part of the palatmc process and part of the frontal

process), the maxilla, the zygomatxc and the squamous temporal bones

Tn: Bangs or me Urran AND Lnwm Exmznuurs

The lunb-buds appear m an embryo of the slxth week as cundensatmns of mesenchyme cells. By the seventh “Eek the pnmordxa of the bones of the upper and lower extremmes are marked out In the mesenchyme By the eaghth wee}. the carhlags are well formed and the pru-nary ossxficatxon cfinlres bepn tn appear The clavxcle 15 the first home In the body to be osslfied It Is a membranous bone and begms to usslfy as early as the sxxth week The patella Is a sesamoxd bone, developmg from the tendon of the quadnceps muscle

(For details of ossnficauon refer tu 110013 on Anatomy)

urn cur uum 5°’“"""‘

Fm 205-—D¢t:¢lopIncu1 of the bane: oI17'¢' 1”“‘" 1”"b‘b“d 276 HUMAN EMERYOLOGY TH! Joins

The Joints between bones can be classified into two main varieties —

1 Syiuzrtliroree, where there are no free movements

between the bones and

2 Diarthroses, where the ioints are freely movable

with the development of the synovial membrane

In synarthroses the condensations of the mesenchyme cells between the cartilage or bone are transformed into a layer of connective tissue to be later modified into fibrocartilage or bone These tissues hold the bones in apposition as seen in the sutures of the cranium the symphysis pubis the epiphyseal union of bones, etc

The diarthroses develop a synovial membrane and a capsule During the fourth month, the cavity of the Joint is iornied by the liquefaction of the original mesenchyme cells between the bones The outer portions of the meserichymal mass become a thick capsule which is continuous with the periesteiim of the bone The inner layer of the capsule modifies into a thin layer called the smiouuil menibriuie Sometimes a cartilaginous plate may be interposed in the Joint cavity as an intra articular meniscus

Derzcrs or Devewruznr or THE Siteurrov

Rhachischisis (Cleft Spine) (See under defects of deve lapment of the Brain and Spinal Cord.)

Cleido-Cranial Dysostosis This condition is caused by the failure of development of one or both clavicles associated with delay m the closure of the fontaiielles

Achondropliisie There are disturbances in endochondral ossification resulting in short and thick bones Most of the cartilaginous bones become affected The cause is hereditary being transmitted by a dominant gene

Osteogenesis Imperfects The bones become porous and fragile and are liable to multiple fractures In late embryonic and prenatal stages There may be associated defects of other inesodermal structures oi the embryo The cause is hereditary being probably transmitted by a dominant gene

Aplcsin There may be failure of development of certain DEVELOPMENT OF THE SKELETAL SYSTEM 277

bones, especially the bones of the upper and lower lunbs

Amelus Absence of hmbs or presence of simple stubs as due to non-development of me or more hmb buds

Phocomelns The nands or feet may become attached lo the trunk due to the defectxve formatxon of the bones of the arm or tlngh

Irregularilia in Number of Digits at the Foot and Iland axe due to local dxsturbances of growth CHAPTER XIV DEVELDPHENT OF THE ‘UUSCULAR SYSTEM

The muscle fibres are modifications of the naesenchyme cells I-listologcally, three types of cells are recognized, in: the smooth cardiac and the skeletal. The smooth muscles constitute the walls of the viscera, blood vessels urogenital and other organs and they arse from the mesenchynae cells in their vicinity The only exceptions are the muscles of the iris and the nayoepithelial cells of the svq eat glands which are of ectodei-mal origin. In the formation of the primime heart tube, it has already been considered that the splanchnic mesoderin on each side contnbutes to the epimyocardial mantle

The skeletal muscles are derived from the myotomes of the body somits and from the pharyngeal arches Three distinct histological difiereiitiations of each sornite are seen tn an early embryo The medial portion is transformed into the sclei-otoine which fuses with that of the opposite soinite to form the pi-imordiuin of the intervertebral disc and body of the vertebra the lateral portion is modified into the derma tome to form the dermis and the subcutaneous layer under the skin, and the intermediate mass of cells {arms the inyotome In human embryos of four and fise weeks there are about forty two to forty four pairs of somites (4 occipital 3 cervical 12 thoracic a lumbar, 5 sacral and 8 to 10 coecygeal) The first occipital and the last seven or eight somites disappear early Each myotoine receives a spinal ner-.2 The skeletal muscles are formed from the myotome by a process of fusion by longitudinal or tangential splitting or by changes in the direction of the muscle fibres. The muscles of the head and neck arise from the inesenchyme of the pharyngeal archs The cells of the early niyotome are spindle shaped cells with central nuclei Later, two types of cells are distinguished the primitive connective tissue cells and the cells with man) granular nuclei which {arm the muscle cells or myvblasts DEVELOPMENT or -n-us MUSCULAR srsmr 219

\
,

Fm 206~—Mynl:lu.st stage of skeletal -muscle and fonnanon of myofib-nls

In an embryo from the seventh to the math week the myablasts become elcmgated wnh the centrally located nuclex By the tenth week, small cytoplasxmc granules appear an the penpheral part of the cells to form myofilmls By the eleventh week, the myofihnls mcrease m number and cum piss the nuclel and appear stnated By the thzrteenth and fourteenth weeks the nude: of the myucytes shrft to one <xde and the myofibnls conttnue to Increase In number and the muscle fibres mcrease lougxtudtnally

From the founh month onward the muscle fibres enlarge In SIZE and stnahons become well marked and all the myo tomes are tmnsformed into dzsnnctrve groups of muscles mnervated by the parucular segmental nerves The development of the muscles progresses from the cephahz: to the caudal regxons and tn the ltmbs from the proxxmal to the dzslal. The muscles of the neck and trunk anse at first followed by the muscles of the tongue and face and the prownal and mstal musculature of the upper and lower luubs

Mcmumchs 0923)‘ observauons on muscle formatxon can be summansed as follows —

1 The muscle fibres which are ungmally arranged m the long ans of the body later change in thexr dxrectnon

2 Same muscles migrate away from then‘ ongm. The hlzssmzus doxsa ansmg from the cervxcal myotomes becomes attached to the lower thomcm and lumbar vertebrae and to the crest of the Ilium. The muscles of facial cxpressmn mxgrate from the second pharyngeal 3"-hes

-nu-mmmcn .1 P The Dc\clc5ImL'ut of the Human Body TI: Blacklﬂa Co 230 HUMAN EMBRYOLOGY

CAVDAL mm )

sncmu.

Fm 20'] —Schenmt1c plan of the eatte'ns2an of -myotames, m an embryo of about five weeks

3 The successive myutomes may fuse mm a single muscle as m the teams abdommus and the sacrospmahs

4 The myotomes may be longxtudmally split min sub— dwzsxons of muscles, eg , the sternohymd and omohymd and the trapezzus and stemomastmd

5 The myotomes may be tangentxally spht mto two or three layers, eg, the oblxque and transverse muscles oi the abdomen

6 The myolumes may degenerate mm fascxae, lxgamenfs or aponeuroses The aponeurosls of the occlpxto-irontalls muscle 15 modxfied from the ougmal muscle txssue DEVELOPMENT OF THE MUSCULAR SYSTEM 281

T1-ii-: Onicm or THE MUSCLFS or rm: TRUNK

In an embryo between the fifth and the Sl‘(ll1 week eacli myatame becomes dwxded mta a dorsal portxon and a ventralateral portion By this time, the spinal nerves divide into dnisal and ventral branches supplying the different regions of the rnyotomes The dorsal portion of the myotome splits into a medial and a lateral group uf muscles forming the extaisois of the vertebral column The medial graup fuses with the suluacent ones and by a process of tangential splitting givts rise to the short oblique muscles, viz. the semtrpumlzs the multifidus and the rotatnres and the spmalss The lateral group fuses with the subjacent ones and by a tangentxal splitting gives rise to the longer muscles namely the zlxa-costal lung-isszmus and the splemus

The ventro lateral portions of the myotomes of all the lharamc and the first lumbar somites extend mm the hody wall as the Ventral and lateral muscles forming the fiexors of the vertebral column In the thoracic region they form the mtar carlal muscles which become split Lnto three layers the external, the mtenuzl and the t1'a1Lsvers-us tharacis muscles In the abdominal region, they form a muscle sheet ventrally composed of the rectus abdmntms and laterally a broad portion which becomes tangentially split into three layers, the atleriuil obhque, the internal. nhhque and the minsizersns abdommzs

In the lumbar region, the ventro lateral extensions of the first lumbar mynlume has already taken part in the furmation of the transverse and oblique muscles of the abdomen The remaining myotomes give origin to the quadrazus lumborum

In the sacral and in the coccygcal somites, the extcnsor muscles become replaced by the dorsal sacral lzgament

The third and fourth sacral myolomes extend ventralatex-.,il.ty to farm the muscles at the perineum during the second month These, in the beginning, surround the cluaca. Later, due to the dwisiori of the cloaca into the anus and the umgemtal sinus, the muscles separately surround the openmgs During the third month, the lctalor am and the urogemlal tphlnclel‘ muscles are differentiated The arrangement of the 282 HUMAN EMBRYOLOGY

cmncla

slum. M101...

Fla 208 ~—-Schematxc duxgram of the cloned dzmng the main! ferent sex stage

CUIOEIS

lscx-L cxvm

_j sum. CLVEE. VAGXNA j

mm Q

PEBDV )uu>1-u:

\

Imus GLU3‘ mx

Fm 209 —Schematxc I:lu1g'ra1n of the development of the pen nenl muscles m the female

uxmx. GROOVE

H.113. CAVEB.

P16 210 ——Schenmhc duzgram of the development of the pen neal muscles m the male DEVELOPMENT OF TEE MUSCULAE SYSTEM 283

fibres of the urogemtal sphincter muscles is different in the twin sexes In the male, the fibres form the bulbacavernosus muscle which meet from either side at the median raphe, during the fourth incirith. The ischzoctwe-rmsus is formed in the fifth month In the female, the urogenital sphincter muscles diﬂerentiate into the bulhocauemosus muscle around the vagina and a few inner fibres to form the urethral sphincter The lateial fibres become the ISCll10CdU£“l'Vl0S‘llS muscles of the clitoris

THE Oincm or 'i-ma Muscuzs or -1115: NECK

During the sixth week, there are 8 cervical somites The head is broadly attached to the trunk with no demarcation of a neck The dorsal portions of the myotoines give rise to the exlensor muscles of the back of the neck The ventralateral portions of the myotomes give origin to the scaleni, pi-evertehral muscles, gemohyoul and mfrahyznd muscles The musculature of the diaphragm takes origin from the ventro lateral extensions of the third, fourth and the fifth myotomes

THE Muscuas or THE HEAD

During the sixth week, there are four occipital inyotonies The first one disappears early The ventro-lateral extensions of the remaining three myotomes probably farm the muscles of the tongue

The fonnntwn of the extrinsic muscles of the eye, as studied in the embryos of lower vertebrates, is from the myotomes The first myotome is situated at the level of the auditory vesicle and the subsequent ones are situated caudally It is assumed that about three myotomes are transiturily formed, rustral to the auditory vesicle and they at once difiuse themselves and spread towards the optic region and the d._-tclopmg pharyngeal arches During the second month, the condensatinns of these muscles give rise to the Tecti and the oblique muscle: of the eye, being innervated by the Ill. IV and the VI ei’-ininl nerves 284 HUMAN DIBRYOLOGY

MI7sr:Lr.s Dmuvrn mom ms Pnmvncun Ancx-ms

The Fu-st (lllandxbular) Arch :5 supplxed by the mandx~ hula: dwxsxon of the 5th (tngemmal) nerve and @\es ongm to the muscles of mastwatzon (temporal), masseter, meduzl and lateral pterygouis, the mylohymd, the untenor belly of the dxgastnc, the tensor palm: and the tensor tympani

The Second (Hyaid) Arch gnes use to the muscls of fat.-ml expressum and all the muscles supphed by the 7!}: nerve Dunng the sixth and seventh vweels both the superficlal and deep muscle masses migrate min the regmn of the face The superficnal layer of muscles dnfierentxates tnto the muscles of the scalp (occxpxto-lrontalts), the exlnnsu: muscles of the external ear (the auruculares tmtenor, superwr and posterior) and platysma. The deep layer dlllerenuates mta the postenor belly of the dxgostnc, stylohyoxd and srepedms

The Third Arch gwes orxgtn to the style-plmryngeus muscle and part of the constructor muscles of the pharynx All these are supphed by the 9th nerve

The Fourth, fifth and the Sixth Arches probably gwe ongm to the muscles of the lar1_/n:c and the temammg part of the cmtstncbor -muscles of the pharynx, bemg supplxed by the 10th nerve

Muscus or me Lmss

By the end of the fourth week, the upper lxmb-buds project opposrte the lower stx cervxcal and the first and second tlzoracxc segments Dunng tl-us tune, the lower lunb-buds become located opposxte the second to the filth lumbar and the upper three saual segments By the sitxlh week, the mesadermal condensauons arrange mto gmups dorsal and ventral to the developmg bones.‘ The dorsal muscles dewelop as the extensors, and the Ventral ones as the ﬂexoxs Durmg the seventh week, the hand and the foot muscles are dxllerenixated. DEVELOPMENT OF THE MUSCULAR SYSTEM 235 CONGENITAL Dzzrzc-rs or Sxzmzmz. Muscnzs

Absence of Certain Muscles or their Variauons in Size, Origin and 1115915011 The Pectoralxs muscles especlally tho: sterno—castal portions are more frequently deficxent than other groups of muscles The fol-lowmg are a few syndromes in Wl11Ch the 001189111131 118551315 of muscles are associated wxlh other abnorrnallhes MOSC 05 these defects are attnbuted to heredltaxy factors mlerfermg wxtlz the proper dlflerentxatxoxx of the mdxvzdual muscles from the myotomes

Congemtal defect of mammary glands and pectorelxs muscle

Congemtal defect of the abdominal wall wxlh associated defects of the urlnary and gemtal organs Congemtal ptosxs

Congemlal facxal dxplegla

Congenital Shortemng of Muscles

Club foot (Talxpes)

Wry neck (Torhcollls)

Elevatxon of shoulder (Sp;-angel's deformity) Comractures of the lxmbs (Amyloplasla congemta) CHAPTER XV

DEVELOPMENT OF THE INTEGUREENTARY SYSTEM

Tm: Sxm

The skm is composed cf the epxdermxs developed from the ectoderm and the connectxve nssue or denms from the dermatame of the somnes The epxdermxs dunng the first month consxsts of a layer of cuboxdal cells By the and of the second month, a superficial layer called the penderm (epztnchml layer) xs added, whxch later an 15 cast elf Soon, an Intermednate layer 15 mterposed between the perlderm and the layer m contact me}: the dermis Duung the thud and fourth

MEENCH

Fm 212 ——Hx.stagenes;s of slcm at the end of the second manila DEVELOPMENT 02‘ THE mmmmurmy sysnjm 237

Fla 213——H:stogenes:s of skm, du-mtg the fourth month

months, the eplthehum gruws more m tluckness by the promeratwn of the layer in contact wxth the dezmzs and consulates the stratum gerrnmatnmm stratum gmnulosum, strutu-m laundum and stratum cm-neum The scales of the stratum oorneum nnxed wxth sebaceous secrehons and the perxdezm make up the uenux caseosa. The mesodenn fmm the damntome and probably from the waudenng mesenchyme cells m the vnumty, gne orxgm to the dermis

THE NAHS

By the mnldle of the Llnrd month, the stratum lucxdum at the ups of the dxgns becomes tlnckened and grows mm the underlymg dermts to form a natl bed Tlns Is bounded by a fold of epxdermns By the fifth month, the hall substance from the deep layer grows as the mall At first, the naxls are covered by the outer layer of the stratum comeum and the peuderm, but slowly they dtsappear The nails grow to the ups of the dtgzts about a month before but}:

Tm: H/an

Dunng the tlurd month, the eplderxms grows inward Into the demns as cords of cells, the lower paruon of wlnclz n mvagmalzd In accommodate the mescnchymal paptlla ,1; about five and a half months, the centml cells of the mgma;-, become cylmdncal and undergo keraumznuon to form 1’, shaft of the hanr The basal cells grow as the vmll uf ;_—. 288 HUMAN E\IBR'i0LOGY

halr follxcle The hall‘ grows by the pralzferatlon of the basal epxdermal cells near the papilla The bars are seen on. the mfiace by the seventh mtmth The first formed hairs are soft and are known as the lanugo Usually, Ihese are last before bzrth, to be replaced by new follicles and new growths of haxr The pxgment develaps later

mum

fiG 21~1—Scctmn of the develcpmg hmr, durmg the third munth

I-‘Ia 215 —Sect1on cf the denelapmg Imxr, at abunt flu: and At half months

T212 Szmcmus mm Swen Gmuns

The sebaceous glands anse as buds of cells from the sxdes of ham fcllwles The buds grow as glandular structures wxth narrow ducts dxschargmg the sebum mto the haxr folhcles Some sebaceous glands are also formed In regxons devaxd of DEVELOPMENT OF THE INTEGUMENTARY SYSTEM 28'!

hair follicles such as m the anal canal, the nostnls and the vulva

The sweat glands amse from the eplthelxurn as sohd Ln growths an the fingers, palms and sales dux-mg the fourth mnnth These mgrowths become coded and the Iumma are formed by the seventh month The myoepxthehal cells whnch an of ectndennal orxgm are seen durmg the seventh month.

Tm: MAMMARY GLANDS

The mammary glands anse as a par of longitudinal lhlckemngs of ectoderm on each sxde of the ventral body wall and extend hum the axllla to the lnguxnal regmn These tlnckemngs are called the mammary ridges or ‘the mxlk lmes‘, which can be seen m an embryo of about the sixth week

Fm 216—“Mnlk Lmes” c] an embrya of the second month

It IS possible, that mammary glands may arise all along ‘the xmlk hues‘ as In the lower mammals In the human embryo by the second month, the pnmurdxa of the mammary glands are (glabhshed as two local thxckemngs of ‘the milk l:nes' m the peﬂgfal region, one on each sxde The remmmng pm-um-.5 of ‘(he null». lmes’ atrophy The thickened mass of gglodeﬂn us pushed Iﬂ\\Ell’dS mlu the underlymg mesoderm By the fourth munlh, the ectodcrm cells pmhleralc mare and ,1,‘-g,-,d 1;; all dxrecnons mlo a cluster of cells dcslmed to become mg -mammary ducts The canalnzahan xnto the ducts

I9 290 HUMAN EMBRYOLDGY

begins during the later months of pregnancy. Further extensions of the ducts progress slowly. The original site of the ingrowlh of ectoden-n forms a pit into which all the ducts open. A local proliferation of the mesoderm just under the pit causes it to he raised up as the nipple.

In the male, the growth of the mammary glands stops after birth, but in the female the growth is progressive. After puberty, there are more extensive prolxferations of cells with more extensions of the ducts and the" deposition of fat in the surrounding connective tissues. The secretion of milk begins only a few months after the onset of pregnancy. The new born infant may show signs of milk secretion as a result of the maternal hormones entering in blood stream.

Dnrmrs or Devtweum-r or 111:: Iurmunmuranv Sysrm

Albinism Absence of the melanin pigment.

Melanism Formation of more pigment.

Hypertrichosis Extra growth of hair in any port of the body.

ﬂypotriehosis Diminished growth of hair.

Naevi (birth marks) Appearance of more vascular plexuses in the dermis.

Dermoid Cyst Inclusion of the epidermis along the regions of fusion of the ectoderm, especially in the midvdorsal line.

Sehaceous Cyst The ducts of the sebaceous glands may be occluded.

Supernumerary Breasts (Polymastia) These occur mostly along ‘the milk lines’.

Fm. 217.-—Supemu.merary breasts in an adult. DEVELOPMENT OF THE INTEGUMENTARY SYSTEM 291

Gynaecomastm The male develops breasts probably due to defects in the sex hormones May be associated wxth her maphrodmsm

A-Masha Breasts do not develop at all APPENDIX SEX CHROMATIN

Barr and Bertram 0949)‘ on a study of the inorphologiral distinction between the netii-ones ni the male and female cats, noticed a small ehromatsn body called the ' nuclear satellite ' present in the female cells end absent in males. They named it ‘Sex Chromatin" Further observetinns of Graham and Barr 0952)” showed that in the cat, must of the cells of the body could he sexed by the presence of sex diromatin Later, Moore and Barr 0954)“ demonstrated the presence 0! sex chromatin in human tissues as well.

The sex chromatin is s mnspicuous mass of inlmnuclenr structure usually lying in con!-set with the nuclear membrane of inter n-utatie nuclei of meet is! the tissues of fennlee lt ll belmed that the male nuclei which contain X Y-chromosomes do not show this distinctive mass of chromatin. According to Lindsay and Barr (w55)” the heterochromatie regions at the twin X-Chromosomes in the female nuclei adhere to each other to tom the sex chromatin Since this mass at chromatin constitutes many genes at the same type. it is evident that a uiufoi-in behaviour exists in the metabolism of its nucleic acid The main nucleic add winponent of the sex chromatin is DNA (Deoxyribonucleic acid) The not chromatin is approximately 1 xnicxan tn diameter and plane convex in shape. with its ﬂattened poi-tinn tn contact with the nuclear membrane This can be distinguished from the rest of the chromatin granules of the nucleus by its larger size lt stains with any good nuclear stain and shows a positive resetinn to the Eeulgm and methyl green stains. as these two stains are selectne for DNA The nucleolus is Feulgen negative, as it contains RNA (Ribonucleic acid) and therefore stains with pyrenin. (Lindsay and Barr 1955) 1' lennox (tS5£\'-'1 has desaibed 9. techiuque \\lt.h rihnniiclease-gallecysnin stain.

It is important to note that the identification of sex chromatin is possible unly hy its churaeeristie use and shape compared with other chramahn material ol the nucleus. lt an be easily deteeted In the following tssues « 1 The nuclei of the skin below the stratum panulosuin by the

skin biopsy test" After fixation, pamfin embedded sections are suitably siaineé (lfiwse et IL, $53‘-4|. Barr, 9541, Ind \$S9, ?ulam et ii‘i., l‘354iV‘. Emery and Me Millan. 1954)‘ The nuclei of the females show the mass of sex chromatin which is inconspicuous in the male

2 Oral inuuosal scraping from the inside of the cheek Moore and Ban (l.§a5)“ have described a teehnique for the detection of sex chromatin in smears of squsriious cells of the oral mucosa This APPENDIX 29;.

method is very simple and is advantageous over skin I: ops) as repeated "‘3mﬂ3“°l'|S 3|‘! Pfissible without much inconvenience to the subiect

3 Nerve cells in the brain and sympathetic ganglion cells (Barr and ggrstfiislli. 1949' ms! Crouch and Barr 19540 Lindsay and Barr

4 White blood cells especially neiitrophils Davidson and Smith. (1994) reported seating of the leucocytes in ordinary Roinanovakv —eIained hlood films. They observed that m a few nuclei during the process of segnientat on the sex chromatin segregate: as a well defined chromatin mas: about 1 5 microns in diameter Joined by a fine chromatin filament about 3 microns long to one of the segments ot the nuclei This segregated chromatin mas has the appearance o! A dnin-istink and is seen in approximately 1 7 per cent of the nuclei of the female but in less than 1 per cent of the male Rim of D¢.l\|n3lL 0951)“ made a study of sex chromatin in the lymphocytes 1:! peripheral blood He evolved a method for the demonstration of the we oliroriiatin pattern in the female compared with that ol the male nuclei.

5 Amniotic ﬂuid Alvarez and Caldeyro (1950)! showed that small Volumes of amniotic ﬂuid could he aspirated (row. the piegiant utei-us alter the ninth week The amniotic lliiid may tvntaln a few cells probably derived from the iiniiary tract of the foetus. These cells could he exaniined for sex chromatin (Rosa and I-‘ananl 1951 0 Scriet al 1855) ' ’l'he presence of sex chromatin in the cells of the amnion may prote a positive method of detecting the sex of the iiinborn child.

Cytological tests of sex chromatin have aroused valuable clinical and academic interest in the study of individual: with anomalous sexual development and in the study of tuinouis specially ieratomaia occurring in various parts of the body In cases ol female external type of pseudo hermaphroditism diagnosed as arising liviii congenital hyper activity of the adrenal gland the early l’¢c00'||l|°l\ 0‘ "*9 90* ¢’"°|'M|-|ﬂ certainly assist: in diooelng the line of treati-nuit In all cases of Inter sexes early recognition ol sex d'i.|'oI'nal-In pattern II o deciding factor in the line ol treatment.

On an investigation of the sex chromatin of diono-carcinoma (Oosterhuis et al.. 1957)‘ in 5 “omen it was found that in 2 case: chono aircinoina developed alter the birth of male foetuses In these women the sex dironiatiii ol the tumouu was ldentified as that ol the male type nose investigators liinlier point out no lnterorting ohscrva tion that in studies on teratomat.-i ol the testis about 50"‘. of cases showed female sex chromatin Pattern

ltlsoflnteresttoxiotethatin-fewlnstancos inonvi '~l|axD0$P'°"'“3 (complete absence oi 5‘P“'“' °°“'It) and extreme ol.lz°sl>¢l'mI= (5P¢I'I'| count below 1 million per ml.) showed a sex ehromalin pattcn-i nscm

hling females These men appeared to he genetic females. Lennon

and his co viorlters at Western liifii-mar)’ CHEW (70Pt'|-I-‘Oh SD10? Lcniiox Mule and Stewart. 195:)’ on their study o! testicular hiopsiu 294 HUMAN EMBRYOLOGY

on patients vull: mbferule sperm count observed that appruxnnafely 40% of men wxth Klmefelt£r’s syndrome (ymaecomasua, mcrease in urunry gonadotmphms and hyaluusauon of the senumferous tubules) and 3% of all subfertxle cases showed the presence of sex chromatm cf the females. In these sex chmmaun posxtne oases, testxcular bzopsy showed that large areas were demand of sexxumleraus tubules and the mterstxtzal cells were found aggregated mm massxve discrete well wsculanzed clumps In all these cases, physxcal exammauon showed the pnsence of small firm testes. It IS important therefore, that bays having unusually small firm testes or havmg cryplurcludxsm or gynaeco— masha should be exammed for the cytolagmal determluatxon of sex.

A few pracucal suggestxons on the treatment of zntelsexes have been emphasxsed by Lennnx 0956)” m an arucle on “Nuclear Sexmg’ Among other matter: he mentlnns “Lhe sex must he deuded as sown as posuble after bu-th. lnspectwn of the gemtaha by an expenenccd paedxamunn. cumbmed wxlh oral mucou smear sexmg wxll probably glVE an answer sufiicxent for nnmedlate Purposes In most cases. Lap.arommy should nut, however, be long delayed xi 1! 1.! necessary In later hfe the sex of the uphnngmg should be mamtamr.-d at all costs, xn spite of anatomy, chromosomes or hormone slates suz-gzcal nr hurmaual thenpy necssary to rnamtam the Individual 1:: the sex of uphnngmg causes less upset than at attempt at alterauan"

REFERENCES

1 ALVAREZ. H. and CALDEYRO. R. (19.70) Conrtactxlny of the human uterus recorded by new methods Surg. G)-nec. Obsteh. 91, 1

2 BARR. M L. (1951) An Intenm report on the appllcatmn of the slur: bxopsy test of chromosome sex to hex-maphrodltes Surg Gynec. Olstet . 99. 184.

3. BARR. M. L. (1955) The skin hxopsy test of chramasomal sex an Annual practice Anat. Rec, 121. 337

4. BARR. M. L, and BERTRAM, E. G (1942) A mnrpholugual dxstmctlan between neurons of the male and female and the behaviour of the nuclear satellttes dunng accelerated nucleoprutezn synthesss Nature, IS. 676

5 BARR. M L and BERTRAM E. G (1951) The behanuur of nuclear structures durmg depletxon and reswx-anon cl Ntssl matenal 111 motor neumns J Anal. Inndan, 85, 171

a CROUCH. Y 1' and BARR, M. L. (1954) The hehavwur of sex chmmatm durmg axon reactmn J Neuropath, 13, xx

7 DAVIDSON, W M. and SMITH, D R. (1954) A morphological sex dxfierence tn the palymorphonuclear leucocytes Bnt. Med. 1., 2, 6

8. EMERY, I L. and MCMILLAN. MARY (1954) Obsenatmns on the female sex ahromaun In human epxdenms and an the 10

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APPENDIX 29;

value of dun biopsy in delenruning sex J Path, Bacl, 63. 11 FERGUSON-SMITH, M A, unmox. 3, max w 5, mg STEWART, J S S (1951) Klinefellers syndrome Frequenq and testicular morphology in relation to Nuclear aex lance! 2. 157. (July 27

GRAHAM, M A and mum, M. 1. (1952) A sex dilference in the morpholoy of metabolic nuclei in somatic cell: of the ca! Anat. Rec, 112, 709

LENNOX, B (1956) Nuclear sexing a review lncorponung some personal observation: Scat. Med. J, 1, 51

LEWNOX. E (1956) A riboniiclease-gallocynnu-I nan: to; sum; skin biopsies Shin Technol., :1, 4 (July 1956)

LINDSAY H. A and BARR, M L. (1955) fi.u'I.her observations on the behavxour of nuclear structure: durmg depleuon end restoration of Nissl rnnierul J‘ Amt, S9, (1

MOORE. K. L and BARR, M L. (X95!) The rnorphc‘ogy ol the nerve cell nucleus In mammals. wilh special relerence to sex dii-omaun J Comp Neur-0L 98, 213

MOORE K. L and BARE, M. L (1.95!) Nudmr molphnlomv according in sex in human tissue: Acu. Anal. 11, 131 MOORE K. L. and BARR. M. L (1955) Smear: {rain the oral Mucus: in the detection of chromosomal sex. I.-meet. 2, ST OO5'.l'ERHUl5, W W and LEVJI. L S (1957) Sex chromatin in Chonn-ui-emorna (Chnno-eyinhelioma) and Carcinoma of the Skin Nerlei-l ugdschr genack. ior. 901

POLANL P Eu HUNTER. W F and LENNOX. B (1954) Chromosomal as in '.l'ui-tier‘: syndrome with aouciauon of non: Lem-et. 2, 120

ans, P (1951) A re: diﬂefence In the ehi-omaun structure ol the peripheral blood lymphocytes Nature, 121 No (553 (April 15)

ROSA, P A. Ind FANARD. A. E. (1951) A new method ol p,,.u1a1 dugoosu ol :9: Int. J‘ Sexol, L 160

SE33, 1) M . SACHS L. and DANON, M (1955) The dilshﬂsu o! in before binh using cell: born the uomloue ﬂuid Bull Res Count. Israel 5 B 131