Book - A Text-book of Embryology 2
Chapter II. The Segmentation of the Ovum and formation of the Blastodermic Vesicle
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.
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While the fertilized ovum is passing along the Fallopian tube to the uterus — a journey believed to require seven or eight days in man[1] — it undergoes repeated segmentation, or cleavage, becoming a more or less globular mass of cells or blastomeres. This mass is the mulberry-mass or morula.
The details of the process of division correspond closely to those of ordinary indirect cell-division, or karyokinesis. The first indication of approaching cleavage is seen in the segmentation -nucleus, just as, in other cells, the sequence of changes leading to cell-division is inaugurated in the nucleus.
The achroraatin-substance of the segmentation-nucleus forms a nuclear spindle in the ordinary manner, with a centrosome or pole-corpuscle at each apex. The centrosome is surrounded by the polar striation or attraction-sphere. After the usual preliminary changes, the chromatin-substance assumes the form of V-shaped loops arranged around the equator of the spindle in such a manner as to produce the wreath or aster. Each chromatin loop splits longitudinally, and the resulting halves of each move to opposite poles of the spindle, where they become grouped about the pole-corpuscle to constitute the daughter-wreaths of the new nuclei. The vitellus now begins to divide, the first step being the formation of an encircling groove on its surface ; this groove deepens more and more until finally division of the cell is complete. In like manner, each daughter-cell divides into
two, and each of these two into other two, the cell-division continuing until there results the mass of cells, or morula, already mentioned (Plate I., Fig. 1). The two cells or blastomeres resulting from the division of the segmentationnucleus do not always divide simultaneously ; if the division of one cell precedes that of the other there will be a stage when
three blastomeres are present. Further irregularity in division nr«ijlt?i in the production of five-celled and six-celled stages.
The processes have been followed the most accurately in the egg of the sea-urchin ; in reptilian eggs, as well as in ovum of the rabbit and other mammals, they have been
ni%u\m\ ai?^) and have been found to agree with the former \u all essential respects. Certain modifications dependent
nyntu the relations and proportions of formative-yolk and (tttA-yiAk will Ik* pointed out hereafter.
Whihr no one has seen the segmentation of the human tynm, th^fre in no reason to suppose that it differs materially from that of other mammals.
\u ifiterenting and probably significant modification of the Utt*^\iiA of eh-jivage as just described has been observed by Var» \U*uMi'U in the ova of the maw-worm of the horse. In tl|ij« #'a«M' male and female pronuclei do not fuse but merely \U* t'\tt*4* t^rj^ethrr. At the beginning of segmentation, the of #»jjeh pronucleus assumes the form of a ex)nvolul^'d lliri;aii, which divides transversely into two sisteriUrt'iuU, In thin manner are produced four loops of chromtiUUf whi^'h \u'(U}U\o ^roup<»d around the equator of the UWi'U'iw t-pindh* juht formed, and each one of which then tephlu loh^ifudififilly inio two threads. In the migration of thn tM'{iuu*u\n tliiit now ensues, each pair of sister- threads fee|mnih'h, on*^ flin'nd ^oin^ to one pole of the spindle, one lo \\n' oiIm-I', lliMMM', at each pole, and taking i>art, therefore, in iIm' (oniiiifion of each new nucleus, are two male and fvvo (f'hiiilc liircndH of chromatin. Thus the male and fcni/ilc pronuclei contribute e(jual shares of chromatin to each dan^litcr-niiclcuH.
Hincc tlic H-j^MicMtiition-ini<^lciis of the ovum gives rise to all the ccllh of the ImmIv, every cell of the adult orgjinism niMrtt conhi^l of cijual amounts of material from each parent.
Cleavage-planes.— The direction of the planes of cleavage is determined by certain laws. The ilirection of the plains of the first cleavage bears a definite relation to the long axis of the nuclear spindle, whose j>osition, in turn, depends upon the manner of distribution of the egg's protoplasm, lis (lirpction <;»iiicii1in}r with tlic limgest diamoter of an oval e^, but lying in any diameter of a spherical one. Tbe first cleavage-plane always cuts the axis of the nuclear spindle perpendicularly at its center; thp second bisects the tiret, also perpendicularly ; and the tliird is perjxtudicular to the two others and passes through tiie middle of their uxifi of intersection.
Kinds of Cleavage. — the mode of cleavage of the ovum is infinenced by the relation of the protoplasm and the deutoplasm to each other, and by their relative proportions. The classification of ova according to their method of cleavage is as follows ;
1. Holoblastic ova are those in which segmentation is total — that is, the entire ovum undergoes division. If the resulting cells are of equal size, there is said to be
(a) Total equal cleavage ; to this class belong the alccithal ova of uiammals (Fig. .5) and of amphioxns, to the segmentatiou of which the above descriptitm may be said to apply. Strictly speaking, the cells are not of exactly equal site, tiiose in the region of the vegetative (H)le of the egg being slightly larger than those at the animal pole. Contrasted with this is
(b) The total unequal cleavage of amphibian ova, whose figments are of unequal size (Fig. 19), These eggs being
B beginning tn be dtvlcJeil
Mted sDrftce of the cfni a
eher In pmtopl««n; rf, thi
Indie (HcnwiB).
gg: j<, stage of (tie firitdlvfginn, . The four segnipnls ofthesepontl Bt»gc of division ijr nn equatorial farrow into elaht Btginents ; p, pigthe nnlmal pole: pr, [he pari iif the e^ wblch ii part which Is richer In deiitoplaam ; ip. nudeac
elolecithal, the ligmer protoplasmic animal \xAe is directed Ipwanl, while the deutoplasmic vegetative pole is underneath. The inequality of the resultinjij segments, as well as the <lirection of the cleavaj^e planes, may be appreciated by reference to Fig. 1 9, whicli reprevseuts a frog's ovum.
2. Meroblastic ova are those in which the segmentation is partial, division being limited to the formative yolk, or pn)toplasm.
(a) Partial discoidal cleavage is the variety of meroblastic cleavage that occurs in those tololecithal ova having a germdisk (Fig. 8), to which latter the segmentation is limited. This method of s(»ginentation is seen in the eggs of birds, re[)tih;s, and fishes. In the L'^^y; of the bird, which may be tiiken as a typical example, the germ-disk, in whatever position the i'\r^ may be placed, floats on the top of the yolk. The beginning of the first segmentation is indicate*! by a furrow in the center of the surface of the germ-disk (Fig. 20). This furrow deepens, cutting vertically from the
Fig. 21).— Surfiioo view i»f the first stntrcs <«f rli-avnir*' in tin; hen'B epij (after Coiito) : o. IwnKT of tlui jftTm-disk ; />, vertiful furrow ; r, sinuU central segiiieut ; d, Innr^' lK»riplu»ml neinnent.
upjM^r to the h)wer surface (»f the germ-disk, dividing it into
two iHpial l>Jirts. Another groove, crossing th(» first at a
right angle, bisiHi^t^ each of the two segments, and each of
thi^» is in turn bisected by a radial furrow, so that the
p^niwlisk now consists of eight sector-shap<'d cells. (Voss
fumnvs, api>oaring near the center of the disk, vwi off the
apiixv^of the sectors, adding small central .s<'gments. CVll d\vi*\on cimtinues until the germ-disk consists of a flatt(Mied
ntt.^ of Ml*, ^veral strata thick, lying on the surfiice of the
volk,
TV wcond methoil of meroblastic segmentation is ^>^ ¥«i|lMnl ctomttt, which occurs in the eentn^leeithal
bered, tbe nutritive-yolk isccDtrally placed and is surrounded
by the fonnative-yolk. The segmentation-nucleus lies in the
center of the nutritivc-yolk, and in this position undei^oes
division and subdivision. The new nuclei now migrate into
the peripherally placed formative-yolk, when the latter divides into as many jiarta as there are nuclei, and thus the
central unsegmented nutritive-yolk becomes enclosed in a
sac composed of small cells.
The Stage of the Blastodermic Vesicle
The blaatomeres of the morula soon show a diiTereutiation into two groups of cells, a peripheral or outer and a central or inner group (Plate I., Fig. 2, left figure). This differentiation is indeed foreshadowed by the fact that of the first
Fig. 21,— Orom of the bsl, ihiiw- Fiq. ■."■i-Oi-ura of (he bat, nhowl
IntC vacuulallon of tlii; in.f!mpntod olallon iif Uii; sugintiiiwl I'Kg lo form the
eggtfifomi theblaslodenuiccavily. blaatndennJu oavltf . y IJUO(Van BeDedea]. X 500 (Van Btneden).
two blastomercs, one is slightly larger than the other. Vacuoles now appear in some of the central cells, and these, becoming larger, finally coalesce to form a fissure-like space, the cleavage-cavity or segmentation-cavity, the lecithocele of Van Beneden (Figs. 21 and 22), The ovum is now in the stage of the blastodermic vesicle.
It will be profitable to compare this stage of the mammalian ovum with the corresponding blastnla stage of the
\aw:i-\':\, tit 3iii|ilit<>\ii:s iaiict'olatus, out: of the lowestvcrte\iTaH:^, a fi.-li-likf unimal several iiiuhos in length inliabiting
tli<- M<i]iu:rniii<-aii S«.-a. The blastnla in this ca^ id a simple
N«; <-"rii]»fi-<-il of cells which siirniiiii(] the cleavage -cavity as
fi .-itifrU; layer (Fig. 2(i, A), The celU in the region of the
vi-({**tativc |>ol<; are larger and niore tiirltid, liecatise more
<leiit(i[ila^niii', (h:in those at the animal pile, as shown in the
«.m.-lipirf.
The mammalian blaatwdennic veBlcle, varying in shape in
'ItfTert'iit .-[H-cie:*, i-un^i^ts of a layer of i^omowhat flattcDcd
(ri'IU, the enveloping or subzonal layer (the "outer cell-mass"),
MirroinnHng a sp:ic<-, the cleavage -cavity or aegmentation'
cavity, and of an irn'guhir mass of more spherical cells, the inner cell-mass, whi<-Ii hitter is attached at one jioint to the
enveloping hiyer and encroaehes upni the >|Kice {Fig. 23
iind Plate I., Fig- 1). The pleavage-cavity contains an
ailMiniiii'iiiH fluid. It is during this stai.f that the germ, in
the eiiH! of mammals, n'aches tin- ntenis. As a pecnliarity
of mammalian development the hlast<Hleritii<- vesicle now
rapidly ineniaws in size, the cleavage-civity l.e.-oming relatively much larger. The zi>nii pcUiioiila, \vhi< li siill snrroimds
iIk! ovnm, in hy this time i|uite attcnnated unci is called
iIk. ptodiorion.' The cells of the cnv<loping hiycr tliin
•Ml I't constitute what is known as Eauber's layer.= In
II,*' rahhil *mil.r>'o, Hauler's layer, heing fniicih.oless, .lisnpjo-ar- «t alfiiit the seventh day ; in most mamnials, how»-..^, it i^rrrisls to play a part in fnrtiicr development
7- vr* /..•rffcwwn U bIho av.pliea tn * c.ntir« i.f »lhijtiiiiinii« niiiKr'9. •f*'i, -iit •nam mtivB* ■« >l pawe* »'""K "^ nviilmt.
— .,. (wVirm 4^«Mr Itaubci'rii-1. -Am tkr cmbrraoic ihieM
(Van Bcnedeu). the form of the blastnia of amphibians
and of the Snuropsida (birds and reptiles) is greatly nio<li(ied
by the relatively abundant niitritivc-yolk with which their
ovR are endowed. An
amphibian ovum in the
blastula glage is shown
in Fig. 24. It will be
iieen that its walls consist of several layers of
cells, and the cleavagecavity is encroached
npon to a considerable
extent by the large and
abundant cells of the
vegetative pole, which
lire esnecially rich in t^c ^t.—BiaituU of trftun Mnlatiu:/'!. leg
~\ ^ nicnUtlon-MTlty : n. mnrKln^ mae; di. MIi
deutoplasm. In the with abundBnl yoU (Herlwlg).
eggs of birds and reptiles — that is, in the telolecithal eggs that undergo partial discoidal segmentation — the blastula fiirm is so markedly modified as to be scarcely recognizalile. In this case, as shown in Fig. 25, the cleavage-cavity is a narrow fissure
Fig. 25— Mi!i]iBnstM.-IJnn through Bgerm-iUtik of prlmluriu la the blutnlaiUga
(kIIi-t JtUekert): B,cavily of the blululH: b. ■egmentvilgcnn: <U, Buely gninnlar
yoU with yolk-nuclel.
whose niof is the perm-disk, and whose floor is the unsegmented nutritive-yolk, which latter corresponds therefore to the large vegetative cells forming the floor of the amphibian egg shown in V\^, 24.
References
- ↑ Recent investigations by Peters, of Vienna, upon an ovum of three or four days, already embedded in the uterine nnicosa, would indicate that less time than this is occupied in traversing the oviduct.
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
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