1897 Human Embryology 15

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter XV. The Amnion And Proamnion

Definition of the Amnion. — The ainnion is a thin, ixjlhicid, non-vascular membrane, and is the innermost of the envelojKiS enclosing the embryo or foetus. Its origin and formation have been described already, p. '281. MorpluJogically it is a yniTt of the bodywall (somatopleuro) of the foetus, and therefore consists, as we have seen, of two layers, one epithelial continuous with the ectoderm (sen epidermis) of the embryo, the second of loose connective tissue continuous with the somatic mesoderm (outer loiif of mesoderm after the api)earance of the body-cavity). The epithelial layer is turned toward the embryo, and the connective-tissue layer consetiuently lies upon the outside of the amnion away from the embryo, and toward the chorion and the uterine wall.

Growth of the Amnion. — Concerning the growth of the amnion I know of no exact measurements. During the first three weeks it stimds off a little from the embryo, but during the fourth week the latter grows so rapidly that it takes up nearly the whole of the amniotic cavity; during the second month the amnion enlarges rapidly so as to leave considerable space for the amniotic fluid; the amnion continuc^s, of course, to expand during all the following months, but after the fourth month it fits pretty closely around the embryo, but is kept distended by the amniotic fluid.

The amnion does not grow around the allantois-stalk or umbilical cord of man as it is commonly stated to do, but, on the contrary, springs from the stalk in the same manner as from the body of the embryo, and is separated from the stalk in the course of development, as is descrilK^d more fully below, in connection with the histology of the allantois-stalk.

Histology of the Amnion. — For a certain period after it is first formed the amnion, in all embryos I have been able to examine, consists of two layers of cells, both very thin and with the nuclei considerable disfemces apart, but sometimes in little groups; between the two layers is a distinct space. The ectodermal layer is the most regular and the l>est defined as to its inner boundary. The mesodermal layer is more or less irregular and sends at intervals a process across the spa(*e betwe^'n the two layers to be attached to the ectoderm.

In a human amnion of a normal two months' embryo. Fig. 191, the !nesoderm has tecome vers' much thicker, and is readilv seen to be separated into two parts, the thin mesothelial layer, msthy covering the surface of the amnion toward the chorion, and a mesenchymal layer, mes, which makes up the greater part of the membrane; the mesenchyma is ])robably deriveil from the mesothelium by proliferation and migration ; I have noticed many indications of the process, but have never studied it carefully. The ectoderm, Ec, is very much in the condition just deflcribed for the enrlier stage, hut in apecimene of three onths amn a 1 1 as becon e tl cker and its cells are beginning to cliange nto the cubo dal fun of later stages No Blood vessels or nen es are known to ex st in the amnion of

in very early stages I short distance into

Fig. 101. -Set

the human embryo, although in sheep embryos ii the vessels havel)ec'u noticed by Bonnet to extend n the aranioii fmm the IxKly-wall.

Histological Differentiation.— The tissues of the amnion do notprc^ess beyond an early embryonic stage; thewtixlenn remain^ ing at the one-layered stage,

the mtfsoderm preserving » much of the primitive matrix. Emory (" Arch. Ital. Biol.,"' III., 37) 1ms directed IBS attention to the primitive homogeneous matrix of the vertebrate mesoderm, and esI>ecially to the separate subepidermal layer of the embrj-o, which amtaius no cells Ecl at first. In the human amnion there is a non-cellular layer mider the epithelium, res iia is wull shown in Fig. 192, A and B. Sometimes this layer is invaded to a certain extent by connective tissuecells, B; in other cases the mi' P<'rtion of the matrix towjud wi' the chorion actpiires a fibrillar character, A, as if partially resorbed, but in no case have I seen the matrix entirely altered from its primitive character. The cells of the mesoderm lie in hicunie; they are flattened in the plane iwrallel to the surface, and hence in vertical sections, Fig. I'.ri, appear more or less fusifonn. They present no siiecial features, so far nn I have ol)served. to distinguish them from other embryonic coimective-tissue cells. Their IkxHos have little affinity for coloring-matters, hence it is diffirulf to follow the processes by which the cells ai-e tmited. Their nuclei are at first round or oval. After the thinl month they often sliow a great variety of alterations in shape and size. Figs. l'.)3, I'.tl; some of the nuclei are tlion veiy lurKc, with a distinct network, (J: others are smaller and differ butslightlyfrom tin- nonnal; some ai-e very irregular, b, imd others again strangely elongatotl, «; many other forms

A, from an embryo ot III tri, Ertodertn ; ui'i. Div dtfrmlc cella. X S4!>dliiu


19S.— A Natural Group of Nuclei from the Meso Ftjetus of the Fifth month.

derm of the Anmiou of X \VS& dianis

beside those represented in Fig. 193, are to be found. The changes indicated I consider of a degenerative character, and in fact many of the nuclei are breaking down, for one finds in soiyie specimens every stage between a nucleus and scattered granules — ^nuclei, nuclei with indistinct membranes, nuclei without membranes, masses of granular matter, clusters of granules crowded together, and finally other clusters more or less scattered. This degenerative process may be compared with that described by Phisalix (Arch. Zool. ExpL, Ser. II., T. III., 382) as occurring in the blood-cells of the spleen of teleosts. Compare also the chromatine degeneration observed by Flemming to occur in ova of the vertebrate ovary (His and Braune's ArchiVy 1885, 221-244). In the human amnion the nuclear degeneration described is not always to be recognized so clearly, although the nuclei in all amnia older than three months, which I have observed, are more or less irregular and distorted. Finally it is to be added that not infrequently the cells form a distinct epithelioid layer upon the surface of the amnion next the chorion as represented in Fig. 102, B, a. The epithelium of the amnion varies in appearance, as seen in transverse sections. Usually the cells are cuboidal or low cylinders. Fig. 192, A, each with a rounded top, in which is situated the more or less nearly spherical nucleus; sometimes, however, the nuclei lie deeper

Less frequently the epithelium is thiu, Fig. 192,  j^g nuclei, which are transversely elongated, lie further apart. It is probable that those differences are not structural, but conditional upon the greater or less degree to which the amnion is stretcheil. I have observed no constant differences between the placental and the remaining amnion. The most interesting peculiarity of the epithelium is best seen in surface views; namely, the intercellular bridges. They display themselves with a clearness which I have never seen in other epithelia; see Fig. 195.

Fig. 194. — Mesodermic Nuclei of the Amnioo of an Ehnbryo of about Four Months. X 718 diaraa

The nuclei, nii, are relatively large, nmndcd, with distinct outlines; they have ft more or less well niarkeil iiitra-nuclejir network, with thickened nodes, and a stnall number of deeply stained granules, which are probably chromatin. Each nucleus is surrounded by a cell-body, pi, and tho adjacent cell-bodies are sejiaratcd from one another \n clear spaces With higli powers, us lepie^cnted m the figure, one sees that these spaces are sepanited fiom one anothei by threads of muteiial, pi, stretchlug acio- - ab bridges, tiinuetting neighboring cells Examined atteutneh, the protc)pliism ot the cells ovliibits a vacuolated Appearance. One la thu"i ltd to new the epithelium as a ipongc'-w ork of protot))nsm bimiewhat con(leMhed around each nucleus , according t*> this interjiretation the intercellular spaces are large uii'sIk s ot the spougewoik, and the inter ' cellular budges are l)rotoplaMnic A re cent iwpor* hj- lloiisn ur M.uidle Idc, h hich 1 ov\ o to the kindness of the author, bungs n merits of i ileit'stiiig observations to show that the lutenvUulai biidg*> of the rele Malpighi of the mammahan epideiiiii'j are not protoplasmic, but processes of the cell membnincK This paiipr has led me to examine my itrep.irations of the .imniotic epithelium, but I lia\e been ttnable to find in them an\ indications of mcrabnim'S an>mid the cells or reasons for considering the mtenvltiilar hinlges a« other thm piotophiamatic in constitution Whether this result is due ti> the imiieitec tioii of my prepaiatiun, oris ill accordance w ith the truth must l)e decided bjfurther nn estigatiun Wiimgriidow has callcni .ittentKin, 72.1, to clear spaces .imoug the epithelial cells, these "jLice^ rcHinble vesicles, and in hardened si)ecimens ba^e granular contents, thej area little larger than the neighboi iiig < ells, and --eein t< > hav e no nucleus As to the nature of these sjiiict's I can express no ilefimte opinion; the\ an' pniluiblv what somi' authors have desciibwl as stoni.ita. The ci ttxlcrmal cells &eem to p,ntiallv degenerate during the latter half ot pregnane;, for the cell Uiundaries beMuiie less distinct and the nuclei become more and more difficult to stain, but the constancy and extent of these changes have never been investigated.

Meola, 84.1, ascribes a much more complex structure to the amnion than his predecessors, in which he is followed by Viti,86.1. Both of these authors sulxlivide the mesodermic stratum into three layers: a lamina connetivale^ next the ectoderm, a sostanza intermediay and a membrana Umitante. As to the histological details, Viti differs somewhat from Meola, but agrees with him in finding a histological distinction l)etween the three layers enumerated. The extent to which I can distinguish three layers is indicated by the description of the mesoderm given above ; I have been unable to find the marked structural differences affirmed by Viti. Viti's paper is to be commended for its excellent historical reviews, particularly for his summary of the various theories as to the origin of the amniotic fluid. Wiuogradow, 72.1, has described in chloride of gold preparations a finft network of clear spaces, which suggest the existence of lyinph channels in the mesodermic layers.

union of the Amnion and Chorion. — At first there is a considerable distance bt>tweeu the amnion and chorion, which condition is maintained in man during the first two months, but during the third month the amnion gradually comes to lie against the chorion, a jd after that a loose connection is established between the two membranes, their mes<Hlermic layers becoming gently agglutinated. The connection remains always very slight, so that the amnion can always te readily peeled oflf. As to the nature of the connection nothing definite is known; sections show that there is a space between the amnion and chorion filled with a transparent matrix, wliich, at least in hartlened specimens, sometimes presents a somewhat fibrillar api^arance; in this matrix are scattered a few cells, but whether they are connective tissue (mesenchyma) or wandering colls, and whether they are derived from the amnion or. the chorion, 1 do not know. The layer in <iuestion is designat^Kl by KiUliker (" Entwickelungsgeschichte," '^^I'l) as GallertgeirehPyfind his opinion, witli which I agree, is that the layer probably belongs to the chorion.

Amniotic Fluid. — the amniotic fluid, known as the liquor amtn'i^ the Fruchtfms.scr ot German writers, is a serous li(]uid, whidi entirely fills the cavity of the amnion, and bathes the embryo on all sides. We consider in this article — 1, its functions; 2, its quantity; 3, its comjx)sition ; 4, its origin.

FrxcTioxs. — The amniotic fluid obviously serves for the mechanical prote<'tion of the fcetus against sudden shocks, blows, or pressure; assists in the maintenance of a constant temperature, and alTords the fct^tus scope for its movements in lit era. When deficient in rjuantity it maj' no longer prevent the pressure of the uterine walls from acting on the child, in wliich case deformities may result. It keeps the skin of the fcetus moist and <loes the same for the genital passiiges of the mother during deliverj-; it is, however, not essential to the act of birth, as is shown bv, 1, the deliven'^ in some cases s(»veral houi-s after the outflow of the fluid, imd, 2, the delivery of thi» child with the membranes intact.

The chief fun(»tion of the fluid, however, appears to \ie to ser^'^e as a water-supply to the embryo. It is probable that during the early stages of foetal life, possibly during the greater part or even the whole period of intra-uterine existence, the embryo absorbs considerable quantities of fluid directly through the skin, but of this absorption we have no direct certain proof. On the other hand, the swallowing of the liquor ainnii by the foetus per as is well established, firsts by direct observations of the bird's ovum ; second^ by the finding in the mammalian digestive tract of remnants of foetal epidermis, hairs, and of the vernix caseosa, which can have reached their site only by being swallowed while floating in the amniotic fluids. That the embryo chick swallows the amniotic liquid was known to Harvey (1G51), and is said to have been observed by Haller ; renewal and extension of these observations is much needeil. As regards the swallowing by the mammalian foetus there are many observations. Needham, Haller, Moriggia, and many others have found meconium in the stomach of the foetus; the pi'esence of epidermal scales in the foetal digestive tract appears to be constant; the presence of hairs and fat {remix caseosa) or of fatty acids derived from the fattv vevu ix. is verv common in the meconium. The fact that the foetus does swallow is established, and analogy with the bird suggests that it swallows constantly the liquor amnii, together with such detritus tis may be suspended in it. As the fluid contains only one to two jier cent solids, it can hardly serve as nourishment to the embryo. The above considerations, taken collectively, render the supposition plausible that the foetus obtains much of its water from the amniotic fluid.

Quantity. — The amount of amniotic fluid at full term has been estimated by Fehling, 79.1, andLevison, 76.1. The former burst the enveloi)e with the finger or with a trcx^ar, collected and measured tlio outflowing liquid ; the after-flow was collected upon a tared linen lying on a waterproof sheet. The minimum obtained in any case was '205 c. c. , the maximum t>300 c. c. (certainly abnormal) . The average amount at full term was (J80 c.c. ; for foetus from the middle of the nintli to the middle of the tenth lunar month, 4'23 c.c. Fehling obscFN^ed thirty-four cases. Levison found the average of twentytwo cases, 8*21 gms. ; Gassner the average of thirty-five cases, 1730 gms. ; but as Gassner 's results seem to deserv^e less confidence, we may safely conclude that at full tenn there is usually under one litre of aniniotic fluid, while it must be remembered that the amount is extremely variable. Richard Haidlen, 85.1, gives a table of fortythree observations of the amount of the amnit^tic fluid determined ju'cording to Fehling's method, rich' sftprct, and has recorded also for each case the sex, length, and weight of the child, the weight of the after-birth, the length of the lunbilical cord, etc. Combining his observations witli those of Fehling (thirty-four cases), he is still unable to detec*t anv constant relation l>etween the amount of the fluid and the weight of the child, tlio weight of the aft(»r-birth, or the length of the umbilical cord. Haidlen's method of tabulation, however, hardly corresj)onds to the reiiuirements of rigid statistics, and it is 7)ossible that a reworking of his figures will give different results. I find the average of his observations to he 71 i c.c. of fluid; taking out two isolated extreme observations, one of 50 c.c., and one of TMMO c.c. (hydramnios), the average of forty -one observations is only 677 c.c. Haidlen failed to find any proportion between the percentage of solids and the amount of the fluid.

The amount during development gradually increases, but no exact proportion exists between the stage of development of the foetus and the amount of fluid. Fehling attempted to show a relation between the lengtii of the umbilical cord and the quantity, but Krukenberg, 84.1, demonstrated from Fehling's own figures, 79.1, that this conclusion was untenable.

Doderlein, 90.1, has shown that in the cow the quantity of the fluid increases during the early part of pregnancy and diminishes during the latter part; the exact figures are given in the table below. It is probably that a similar variation occurs in man.

Composition. — The liquor amnii has the character of a serous fluid. Levison found its specific gravity to vary ftom 1.0005 to 1.0070, while, according to Prochownick, 77. 1, it varies from 1.0069 to 1.0082. The latter found it to contain between 1.07 and 1.60 per cent dry solids, giving 0.51 to 0.88 per cent ash. With the increase of Quantity there is no constant diminution of the percentage of solids. The following table compiled from Vogtand Scherer, 49.1, indicates the little that is known concerning the changes in composition during gestation :

3 months

4 montlui

5 months

6 months

10 months


Albumen and Mucin


7.28 9.25


10. TT



975. W 7.fl7 7.24 9.25

990.29 6.07 O.M 2.70







Doderlein, 90.1, has investigated the amniotic fluid of the cow; his work appears painstaking and reliable. His chief results are eml)odied in the following tables :


Averaare p^r <*«»nt O.ftHrt

No. of ol>s 10

Amniotic Fluid. NaO.

KO. O.Olil)


0.014 10

0.0088 9


ibr>'(>.s. 2.



Fluid. 5.







\\ t. ^riii.s.



cent of

Total N.

l»roteld N.

K-Prot. N.

cc. contents.




of I'inh. 833

• • . •

• • .

• « • •


1 • • • •





• •mm

. • . •

• « • •

• • • •













• • • •

• • a •

• • • •






• • • »

• • • ■

• • • •

1 • • • •






• » • *

• • ■ •

' 20







1 80





• • • •

  •  » a »

• • . •









1 100


1 .M23














, 250








' 180








1 400




15 ,








8 !



, 0.072


1 800


In the cow at term the per cent, of albuminoids in eight observa\\ tions was 0.164, 0.4G4, 0.280, 0.440, 0.268, 0.010, 0.24T, making an

average of 0.348 per cent. These figures show that the fluid can have practically no nutritive value.

It is clear that there is a great diminution in the amount of albumen, especially toward the last month, and there is apparently a small t : diminution in the percentage of salts. The salts are such as are usually contained in serous tluids. In connection with the albumen

1 J it may be remarked that the fluid contains no fibrin-forming material,

ii] as has been shown by Gusserow, 78. 1. There is a small quantity ot'

J I urea, but not more than is found in other serous fluids; hence, the

j'l presenci^ of urea is no argument in favor of the view that the amni otic fluid is an excretion of the foetal kidney. Early in gestation the amount is small, but it gradually increases until the ninth mouth, 0.030 j)er cent, and tenth, 0.045 \^y cent (Fehling). The figures of various authors differ greatly — sometimes no urea being found ((;/. Preyer, **Si>ecielle Physiologic des Embryos," p. 289). I Finally wo have to note the i)rosence of lymph-corpuscles, but

j whether they are always pre^sent, and, if so, in what numbers, is unknown ; in a few cases they have l)een found in large numbers.

I Origin. — It is a hyiH)thesis of long standing that the liquor amnii

is an excretion of the foetus, and opinion has inclined to regard it as the produ^.t of the fcetal urinary apparatus. There is, however, no satisfactory argimient of any kind in favor of this view, but, on the contrary, there are many forcible objections to it, and, moreover, there is strong evidence to sliow that it is derived from the mothet by direi^t transudation. It is to be considered, firtftlij, that the licjuor has the (•oni]>osition of a serous fluid, transuded from the blood-vessels, and d<H»s not resemble urine; like other serous fluids it contiiins a small amount of urea, l)ut this is no indication what4*oever of the urinary origin of the fluid; .srro;/(////, that the f<etal penis is completely closed during the gre^itcr part of embryonic life, l)ec"ause after the closure of the ra])he on the stalk the glans remains long imiKn'forate, so that in the male, at h»ast, the direct discharge of the urine into the amniotiir cavity is impossible; unless, therefore, we ar<* readv to attribute the formation of the fluid to dift'erent sour(*es in the two sexes, w(» cannot assume the kidn(»v to he the source of the flui<l in either sex; fh/'nUt/, that th(» Huid is not excreterol)a])le that the foetus, which constantly re(juirt»s water for its own use, should excr*'te a large (juantity only to swallow it again.

That the li(inor transudes dire(*tly from the uterine^ wall or from the chorion through the anuiion into the amniotic cavity is indicated, first, by the com]M>sition of the fluid; stH-inuL by (^\j>eri mental evidence that certain salts can pass directly from tlu* mother into the fluid without passing through th<» fcetus, at least during the latter ])art of pregnancy. Zuntz, PfliigerV Archif\ XVI., Tils, was the first to make such an experiment; he injected an aqueous solution of sulph-indigotate of sodium into the jugular vein of a pregnant rabbit; the liquor amnii showed a distinct blue color, while no trace of blue was found in any part of the foetus. Wiener, 81.1, repeated and extended this observation, and G. Ejrukenberg made similar exi)eriments with like results, with iodide of potassium. R. Haidlen, 86. 1, also repeated Krukenberg's experiment of giving women iodide of potassium in the early stages of labor, and also small repeated doses for several days before labor ; in each case he found the salt in the amniotic fluid, and also in the first urine of the child. This experiment, therefore, does not show whether the diffusion takes place from the uterine wall or the foetus into the amniotic cavity.

All the facts taken collectively led Minot (Buck's "Handbook," I., 141) to the theory that the liquor amnii is a product of the osmotic function of the amnion ; that, during the earliest period, the osmosis takes place from the fluid in the space between the amnion and chorion ; that during a certain interval, namely, while the superficial capillaries of the chorion maintain an active circuhition in that membrane (r/. Chorion), the fluid may come from the chorion, and, therefore, indirectly from the foetus; and finally that during at least the latter half of pregnancy the transfusion occurs from the decidua through the chorion and amnion both. That the amnion itself produces the liquid it encloses is highly probable, but the exact source of supply upon which the amnion draws is much more uncertain.

Prbamnion. — This convenient term was intnxluced bv Ed. van Beneden to designate that part of the area emhnjoiuiUs at the sides and in front of the head of the developing embryo, which remains without mesoderm for a considerable period, so that the ectoderm and entoderm are l)rought in the region of the prcmmnion into immeiliate contact. As found in one stage of the rabbit, it has already been figured in this work, p. 183, Fig. 100. A later stage in the rabbit, as seen in longitudinal section, is figured b}" Kolliker in his '^Grundriss," 2te Aufi., 107. We find that it had been obser^'ed in the chick by Remak, His, 68.1, 0, Gjusser, 77.3, 4013, and Kolliker. Strahl was the first to direct special attention to it. But the earliest description of the proamnion known to me is that of C. Daresto, 66. 1, who gives a very exact account of the expansion of the mesoderm {fefdllet vasctflaire) in such a manner as to leave an area in front of the head without mescxlerm. Dareste is, therefore, to be considered the discoverer of the proamnion. It has since l)een observed by various writers : Van Beneden and Julin have described it in the rabbit, Heaj)e in the mole, and recently its exact historj' has Ix^en admirably worked out in the chick by Ra^^l. The proamnion, then, has l)een ol)served in representatives of the classes Reptilia, Aves, and Mammalia; hence we may conclude that it is common to all amniota. It will \ye remembered that the mesoderm grows out in all directions fn)m the blastopore, or hinder end of the primitive streak. In a chick of twenty-seven hours the front edge of the mesoderm is a somewhat irregular transverse line, which crosses the genninal area alniut at the front border of the head. This line is well shown in His' drawings, lor. c«Y.,Pl. XII., Fig. 14. As the inesoilemi expands it does not grow forward in the median line, but does grow forward at the sides of the area pellucida in front of the head of the embryo, p. 160. A space is thus enclosed between the mesoderm on each side; this space later becomes the proamnion; it contains no mesoderm. Later on the lateral portions of the mesoderm approach the median line again, some distance in front of the head, so that now the proamniotic area is completely surrounded by mesoderm, Fig. 150, Pr.a, We see, as the next phase of development, the head amniotic fold arising in such a position that the proamnion is embraced between the arc of this fold and the head of the embryo ; the proamnion, therefore, constitutes the floor of the pit fonned by the upgrowth of the head amnion. The appearances at tliis stage, as seen in longitudinal sections, Fig. 100, are extremely characteristic ; the proamnion. Pro-am y springs from the wall of the pericardial chamber and pjisses round the head of the embryo; the l)roamniotic ectodenn passes upward on the embryo, and its entodenn piisses backward under the heart, as a thin layer of ^ells, Eu, which joins the lining of the archenteron. In the chick the proamnion never acquires any considerable development, but gradually disa Pilars by encrojichments of the mesoderm upon all sides, as has been well descrilKni by Riivn, whose Fig. 3, loc. cit.y PI. XXI., will servo to give a clear general notion of the relation of the proiunnion to the head and to the true amnion in the chick. The disapix?arance of the proamnion in the chic^k involves some curious appearances in sections of embr^'os, which have not been understood hitherto, but which Riivn had correctly and fully elucidated, so far as I can judge; see also the loss thomugh observations of Shore and Pickering, 89. 1.

In the rabbit, according to Van Beneilen and Julin, wliose observations have been confirmed to a certain extent by KiUliker and Heape, the role of the pr<jamnion is more considerable. The history of the proamnion, as given by Van Beneden, may be followed easily by the aid of the accompanying diagrams. Fig. 11)0, copied from Van Beneden. In A, the proamnion, pro. A, is very small, and the allant<)is, Al, is just growing out. In B, the embryo, which for greater clearness has lxK?n shaded with stippling, ha« grown very much, and the anterior half of its Ixwly is bent down at a sharp angle into the yolk-sac. The enibr3'o, however, remains sej)arated from the cavit}', Y, of the yolk-sac by the pnwunnion, which forms, as it were, a IkkxI, p/v>. A^ over the anterior extremity of the embryo. The anniion proper is as yet develojwd only over the posterior end of the embrvo. For the further historv of tlicj amnion see above. The proamnion, as can Ix) seen in C and D, retains its imiM)rtance as a fcetal covering for a considerable jxn-iud, during which the amnion, «/>?, and allantois, Al, are rapidly pni-suing their development. After the stage shown in Fig. lOfJ, D, l)y the expansion of the cavity marked Coe\ the amnion proper, a/;^ encroaches more and moreu|K)n the proamnion, pro. A, until at last the embrvo is (entirely covennl by the true amnion, and the ])roamnion is altogether lost. It is to l)e noted especially that the amnion develops jirinci pally ov(n* the iM)sterior end of the embryo, and grows forward. To this fact reference will be made again directly.

Sp far as at present known the greatest dev€»lopment of the j)roamnion is in the opossum, Fig. 202, where it covers ultimately the entire embryo; at first there is a true aiiinion over the caudal half of the rump of the embryo, but this gradually disappears and the proamnion replaces it. As in the rabbit the proamnion projects into the hollow yolk-sac, hence in the opossum the embryo may be said to lie in the proamniotic pocket, inside the yolk-sac, as it were. It must not be forgotten, however, that the cavity of the proamnion is

Fig. IOC.— Diafrrnin of the ixevelopnient of the Fietal Adnexaiu tht* Rabbit. CAft«T Van B»*neden and Julin. ) A. R ('. I). SucoeHKive stances; tmt. A, pro-umuion : ^r. area vaMculoHa: (\>f, ccttloin; Coe\ On", extra cmbr>'onic i>ortion of the cuflom; En, eutodennic cavity <»f the enibrvo: Knt, extra-embr)'oulc enttxierm: Ec, ectoderm: Mes, mesoderm; A pi, area phicentalis; At, allantoic; T, terminal Kinus of the area vaaculosa : Y, yolk-sac; Am, amnion: Am', iK>rtion of the amnion miitinl witli the wall of the allantois; CA, chorion.

morpliologically outside the yolk-sac, tis is clearly shown in the diagrams of Fijij. 1<)0.

In certain mammalia there is no proamnion, owing probiibly to modifications in the early development of the mes<xlerm, leading to a precocious invasion of the proamniotic area by the middle germlayer. This seems to be the case in all rodents with inverted germlavers (guinea-pigs, rats, etc., c/. p. 141), also in the sheep. Bonnet, 89.1, 19, and probably other ruminants, and finally in man. The earliest stages of human development which we have yet obtained show us the mesoderm completely developed around the entire ovum and separated over the whole of the extra-embryonic region into a somatic (or chorionic) and splanchnic (or yolk-sac) layer, the amnion fully formed, and no proamnion. It is quite possible at at an early stage in the formation of the mesoderm there is an area corresponding to the proamnion, but it must be obliterated almost immediately.

Solution of the Amnion. — That the amnion is a modification of part of the extra-embryonic somatopleure (primitive chorion) is certain from its development, but beyond this nothing definite is known as to its evolution or phylogenetic origin. Nor do the speculations of Balfour, '* Comp. Embryol.," II., 309, nor of Van Beneden and Julin, 84. 1, 4'25, nor of J. A. Ryder, 86.3, seem satisfactory. Balfour says : *' The origin of the amnion is more difficult to explain than that of the allantois ; and it does not seem possible to derive it from any pre-existing organ. It api)ears to me, however, very probable that it was evolved, pari puissUy with the allantois, as a simple fold of the somatopleure round the embryo, into which the allantois extended itself as it incresised in size and became a respiratory organ. It would be obviously advant^igeous for such a fold, having once started, to become larger and larger, in order to give more and more room for the allantois to spread into."

Vmi Beneden and Julin say : '* Dans iu)tre opinion la cause determinante de la formation de renvelloi)e amnioticjue reside dans la descente de Tembryon determinee elle meme par le pois du corps. C'est par une acceleration du developpemont que la cavite amniotique en est venu a so former quand Tembryon ne possede encore qu'un pois insignifiant." Essentially the same view has been advocate<l by Shore and Pickering, 89. 1,10. The chief objection to this theor\" is that it really gives no cause for the expansion of the somatopleure and chorion ; there is no prcx)f that a mere strain of weight can cause the cells of a membrane to proliferate, and since such proliferation is the immediate cause of the growth of the anmion. Van Beneden and Julin must assume for their theory that the strain of w^eight does cause proliferation ; but this assumption lacks support. Moreover they give no evidence to show that the embryo, in vfero, is situated in the primitive amniota ujxm the upper side of the ovum, although it is probable such was the case.

Ryder's theory, 86.3, of the origin of the amnion, like that of Van Beneden and Julin, to which ho does not refer, is purely niechmiical ; but Ryder seeks the cause in a rigid z(ni(t rucJiata forcing the embryo down into the yolk. See his summary, loc. cit., p. 1S4. So far as we know, however, the embryo of tho Sauropsida caiuiot be said to sink into the yolk, and so load to the devoloinnent of an anmion ; but, on the contrary, the amniotic folds rise up clear alH:)ve the yolk. Moreover, the formation of the anmion is really a very (*omplex process, part arising from the proamnion, part by a dilation of the pericardial cavity (Par-ietal-Hohle), and part as the extra-embryonic tail folds. These facts speak in my judgment miequivocally against the amnion having arisen by the sinking of the embryo into the yolk-sac. Nor is there any justification, I think, for seeking these simple mechanical explanations, which are worthy of Herbert Spencer, since the formation of the amnion depends upon inequalities in the growth power of the germ-layers, and only such explanation can be valid as explains that inequality — which Byder's Hypothesis fails to do, so far as I can see.

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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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