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=Chapter II.  -  The Ovum of Dasyueus=

==1. Structure of the Ovarian Ovum==

The full-grown ovarian ovum of Dasyurus (PI. 1, fig. 1) appears as a rounded, or more usually, ovalish cell, the diameter of which varies in section in ten eggs measured from '28 X â€˜126 mm. to '27 x '26 mm. (average, '24 mm.), and is therefore large relatively to the ova of Eutheria. It is enclosed by a tliin, but very definite I'efractive membrane or zona (vitelline membrane of Caldwell) of an approximate thickness of '002 mm. (fig. 1, z-p-), on which the cells of the discus proligerus (fig. I, d.p.) directly abut, a differentiated corona radiata and syncytial layer being absent. It appears to be identical in its relations and optical characters with the membrane investing the monotreme ovum, and never shows in section any trace of radial striations (though I believe I have detected an extremely faint appearance of such in the fresh zona), or of the extension into it of protoplasmic processes from the adjacent cells of the discus â€˜proligerus, such as Caldwell figures in the case of the ovum of Phascolarctus (cf. his PI. 29, fig. 5). Within the zona the peripheral cytoplasm of the ovum is differentiated to form an exceedingly thin but distinct bounding layer or egg-membrane (vitelline membrane, sensu stricto).


The cytoplasmic body of the ovum exhibits a very obvious and striking differentiation into two regions in correspondence with the presence in it of two definitely localised varieties of deutoplasmic material, respectively granular and fluid. Peripherally it consists of a relatively narrow cytoplasmic zone of practically uniform width, dense and finely granular in appearance owing to the presence in it of numei'ous particles of deutoplasmic nature. This we may distinguish as the foi'mative zone (fig. 1, f.z.). In it lies embedded the large vesicular nucleus (about â€˜06 x â€˜03 mm. in diam.). Centrally and forming the main bulk of the ovum is a mass of greatly vacuolated cytoplasm presenting the appearance of a clear wide-meshed reticulum. Its framework is coarser peripherally where it passes over without definite limit into the formative zone, with which it is structurallv identical, but much finer and wider-meshed centrally, so fine, indeed, that it almost invariably breaks down under the action of fixatives, and appears in sections as an irregular space, perhaps crossed by a few fine interlacing strands (fig. 1, d.z.). The meshes of this reticulum are occupied by a clear fluid which must be held to constitute the central deutoplasm of the egg. We may accordingly designate this central reticular area as the deutoplasmic zone.


If we pass now from the full-grown to the ripe ovarian ovum (PI. 1, figs. 2 and 3), i. e. an ovum in which either thefirst polar spindle has appeared or the first polar body hasalready been separated off, it at once becomes evident that important changes have occurred in the disposition and relative proportions of the two constituent regions of the eggcytoplasm. The full-grown ovum is of the centrolecithal type, the central deutoplasmic zone forming its main bulk and being completely surrounded by the thin formative zone. The ripe ovum, on the other hand, exhibits an obvious and unmistakable polarity, and is of the telolecithal type, as the following facts show. The cytoplasmic body evidently consists of the same two regions as form that of the full-gi-own ovum, but here the dense formative region now forms its main bulk, and no longer surrounds the clear deutoplasmic region as a uniform periphei*al layer. It has not only increased considerably in amount as compared with that of the full-grown egg, and at the expense apparently of the moreperipheral coarser portion of the deutoplasmic zone, but it has undergone polar segregation, with the result that it now occupies rather more than one hemisphere of the egg as a dense finely granular mass, with vacuoles of varying sizesparsely scattered through it (figs. 2 and 3, /.z.). It accordingly defines one of the ovular poles. The opposite pole is just as markedly chai'acterised by the presence immediately below it of a more or less rounded clear mass, eccentrically situated, and composed of an extremely finecytoplasmic reticulum with wide fluid-filled meshes. It is completely surrounded by formative cytoplasm (though over the polar region the enclosing layer is so extremely thin that it here almost reaches the sui'face), and its cytoplasmic framework is perfectly continuous with the same, the line of junction of the two being abrupt and well defined. So delicate, however, is this framewoi'k that it breaks down more or less completely under the action of fixatives of such excellence even as the fluids of Flemming and Hermann, and thus in sections usually all that represent it ai'e a few irregular cytoplasmic strands crossing a large, sharply defined clear space (figs. 2 and 3, d.z.). The mass in question has thus all the characters of the deutoplasmic zone of the fullgrown ovum, and it must undoubtedly be held to represent the central portion of that which has not been utilised in the upbuilding of the formative cytoplasm, and which has been forced to take up an excentric position immediately below the polar region of one hemisphere, owing to the increase of the formative cytoplasm and its segregation in the other hemisphere.


The ripe ovum of Dasyurus thus possesses a polarity which in its way is equally as striking as that of the Monotreme egg. Towards the one pole the main mass of the ovum is composed of dense, slightly vacuolated formative cytoplasm, in which the polar spindle is situated peripherally, but nearer the equator than the formative pole. Toward the opposite pole and practically reaching the surface is a rounded mass of greatly vacuolated deutoplasmic cytoplasm. Eoughly, the formative cytoplasm constitutes about two-thirds of the bulk of the ripe egg, the deutoplasmic the remaining third. Such being the structure of the ripe ovarian egg, if we classify it at all, we must place it, it seems to me, with eggs of the telolecithal type. My view of the significance of this marked polar differentiation of the constituent materials of the ripe ovum of Dasyurus I shall presently indicate. Meantime I would lay special emphasis on the fact that the eccentric mass of deutoplasmic cytoplasm represents matei'ial, surplus deutoplasmic material which has not been utilised in the upbuilding of the formative cytoplasm.


The fact of the occurrence in the Eutherian ovum of a polar differentiation of its constituent materials is now definitely established, thanks especially to the valuable researches of Prof. 0. Van der Stricht and his pupils - H. Lams and the late J. Doorme. In this connection I wish to refer here in some detail to the extremely interesting observations of Van der Stricht ['03, '05] on the structure and polarity of tlie ovum of the bat (Vesperugo noctula), since these observations are in essential agreement with my own on the ovum of Dasyurus, and enable me to affirm that the polar differentiation herein recorded for the first time for the Marsupial ovum is attained as the result of vitellogenetic processes, which essentially correspond with those of the ovum of the bat. Van der Stricht, as is well known, has made a special study of the process of vitellogenesis in the Eutherian ovum, and is, indeed, at the present time the foremost authority on this particular subject, so that his views are worthy of all respect.


Study of the oocyte of Vesperugo during the period of growth shows, according to Van der Stricht, that â€œ a un moment donne du developpement du jeune oeuf, les boyaux et amas vitellogenes [derived, according to him, from â€˜ une couche vitellogene, mitochoudriale,' present in the young oocyte in the first stage of growth] disparaissent au profit du vitellus, dont la structure pseudo - alveolaire s'accentue graduellement.â€ The full-grown oocyte at the stage just prior to the appearance of the first polar spindle is characterised by the presence of this â€œ pseudo-alveolar structui'e â€ throughout the extent of its cytoplasmic body. The alveoli or vacuoles are of variable size, are filled by a clear liquid, and â€œ correspondent incontestablement au deutoplasma de I'oeuf. A ce stade du developpement de I'oocyte, ce vitellus nutritif, auquel s'ajoutent bientot des granulations graisseuses, est repandu uniformement dans toutes les profondeurs du cytoplasme. Nulle part on ne constate une zone deutoplasmique distincte d'une zone de vitellus plastique.â€ In Dasyurus the stage in vitellogenesis which almost exactly coiTesponds with that of the full-grown oocyte of Vesperugo just described is seen in oocytes not quite full-gi*own. In fig. 4 is shown an oocyte of Dasyurus (â€¢26 x â€˜20 inm. in diameter), in which the same pseudo-alveolar sti'ucture as described by Van der Stricht for the Vesperugo oocyte is perfectly distinct. Here, however, fatty particles are not apparent, and the peripheral portion of the cytoplasm tends to be free from vacuoles. In Dasyurus the formation of these deutoplasmic vacuoles begins in oocytes about '2 mm. or less in diameter. This characteristic pseudo-alveolar â€ stage is followed in both Vesperugo and Dasyurus by one in which there is recognisable in the cytoplasmic body of the ovum a differentiation into a dense peripheral zone and a central vacuolated area. In Vesperugo this stage is attained about the time of appearance of the first polar spindle, whilst in Dasyurus it is attained somewhat earlier, always prior to the formation of the latter. So close is the agreement between the two forms that Van der Stricht's desci'iption of the bat's egg at the time of appearance of the first polar spindle might equally well be applied to the full-grown ovum of Dasyurus. He writes ['03, p. 43] : â€œVers I'epoque de I'apparition du premier fuseau de maturation, le vitellus prend un autre aspect. La partie centrale deutoplasmique conserve une structure pseudo-alveolaire, mais dans le voisinage immediat du premier fuseau et daus toute I'etendue de la couche peripherique du protoplasme, apparait une mince zone de vitellus compact et dense, plus ou moins homogene ou les vesicules claii'es font defaut. . . . A ce moment, on


distingue daus I'odcyte de V. noctula une zone centi'ale tres etendue, riche en deutoplasme et une zone corticale tres mince, riche en vitellus plastique.â€ This centrolecithal phase, as we may term it, is followed in Vesperugo during fertilisation and the separation of the second polar body by a telolecithal phase characterised by a distinct polarity. â€œ La zone de vitellus plastique s'epaissit encore, mais surtout a un pole de I'oeuf, a celui oppose au pole ou se detachent les deux globules polaires. Ce pole, ou s'accumule graduellement le vitellus formateur, merite le nom de pole animal. II est oppose au pole d'expulsion des globules polaires, vers lequel est refoule le deutoplasme, et qui se comporte desormais comme le pole vegetatif. Pendant que les deux pronucleus male et femelle se ferment, le vitellus plastique augmente graduellement en abundance au pole animal, tandis qu'il diminue an p61e vegefcafcif, et le deutoplasme, parseme d'un plus grand nombre de boules graiss6uses, constitue une masse spberique excenfcriqne, voisine des deux globules polairesâ€ (Van der Stricbfc, '03, pp. 44 - 45). It is evident, then, that the fertilised ovum of Yesperugo exhibits a polarity comparable with that of the ripe ovarian ovum of Dasyurus, and that the vitellogenetic processes in the ova of these two widely separated forms proceed along lines almost identical, at all events so far as their broad outlines are concerned. In both we find during growth a progressive vacuolisation of the egg-cytoplasm consequent on tlie elaboration of a deutoplasmic fiuid. In both, the '^pseudo-alveolar â€ condition so engendei-ed is followed by one in which there is recognisable a differentiation into a peripheral "formative â€ zone rich in deutoplasmic granules, and a central " deutoplasmic â€ zone rich in fluid yolk, and finally in both there occurs a segregation of the granular "formativeâ€ and fiuid yolk-constituents to opposite I'egions of the egg, with resulting attainment of a definite polarity. In view of the close general agreement in the vitellogenetic processes, and in the constitution of the ova in Vesperugo and Dasyurus, it might be expected that the poles would accurately correspond, but such is not the case if Van der Stricht's determination of the poles in the ovum of Vesperugo is correct. In the latter, according to Van der Stricht, the deutoplasm is located at that pole from which the polar bodies are given off; at the opposite pole the "plasticâ€ vitellus accumulates, and close to it the two pronuclei unite and the first cleavage spindle is formed. Accordingly Van der Stricht concludes that "le premier pole correspond au pole vegetatif, le second au p61e animal des oeufs a deutoplasme polaire (0. Hertwig).â€ In Dasyurus, on the other hand, I am perfectly convinced (and adequate reason for my conviction will be forthcoming in the course of my description of the processes of cleavage and germ-layer formation) that the pole of the ripe ovum in relation to the mass of deutoplasmic cytoplasm is not the vegetative pole, but represents morphologically the upper or animal pole of the egg, the opposite pole in relation to which the formative cytoplasm is situated being' the lower or vegetative. The deutoplasmic cytoplasm thus lies in the upper hemisphere, whilst the formative cytoplasm occupies the lower. If Van der Stricht's determination of the poles of the ovum of Vesperugo be accepted, then we must conclude that the poles of the Dasyurus ovum are exactly reversed as compared with those of the bat^s egg. In this connection it may be recalled that Lams and Doorme ['07] have demonstrated the occurrence in Cavia of an actual reversal of the original polarity of the ovum, pi'ior to the beginning of cleavage. These facts may well give us pause before we proceed to attach other than a purely secondary significance to the exact location of the formative and deutoplasmic constituents in the Metatherian and Eutherian ovum. But besides this apparent difference in the location of the deutoplasmic constituents of the ova of Dasyurus and Vesperugo, there exists yet another which concerns the fate of these constituents in the respective eggs. In Vesperugo, Van der Stricht shows that the â€œ deutoplasm â€ remains an integral part of the egg, and I'etains its polar distribution in the blastomeres up to at least the 4-celled stage. ^ In Dasyurus, on the other hand, the fate of the deutoplasmic mass is a very different, and, indeed, a very remarkable one. It does not remain an integral part of the segmenting egg as in Vesperugo, but prior to the completion of the first cleavage furrow it becomes bodily separated off, apparently by a process of abstriction, from the formative cytoplasm as a clear rounded mass which takes no further direct part in the developmental processes. As soon as its elimination is effected, the remainder of the cytoplasmic body of the ovum, formed of the formative cytoplasm alone, divides into the first two equal-sized blastomeres, the first cleavage plaue being coincident with the polar diameter and at right angles to the plane of separation of the deutoplasmic mass, or â€œyolk-body â€ as we may term it (PI. 2, figs. 14-16, 19, ij.b.), so that it is this formative zone of the ovum which is filone concerned in tlie production of the embryo and its foetal membranes.


Vide, however, â€œAddendumâ€ (p. 121).

We have but to recall the conclusion already reached that the clear vacuolated zone at the upper pole of the ripe ovum of Dasyurus consists of surplus material, mainly in the form of fluid of deutoplasmic nature which has not been utilised in the upbuilding of the formative cytoplasm, and the signiflcance of this remarkable and, so far as the Mammalian ovum is concerned, absolutely unique occurrence becomes at once manifest.'^ We have to do here with an actual elimination of surplus deutoplasmic material by the Marsupial ovum - a phenomenon only paralleled elsewhere, so far a,s I am awai'e, and even then but distantly, by the curious temporary separation of the so-called yolk-lobe which occurs during the cleavage of the yolk-laden eggs of certain Molluscs (Nassa, Ilyanassa, Modiolaria, Aplysia, Dentalium) and Annelids (Myzostoma, Chjetopterus) . In these forms cleavage of the ovum into the first two blastomeres is accompanied by the sepai'ation of a portion of the ovular substance in the form of a non-nucleated mass or so-called yolk-lobe. This latter, which has been shown to be connected Avith the formation of determined organanlagen, reunites Avith one of the tAvo blastomeres, and then the same process of abstriction and reunion recurs at the second cleavage.^ We have here evidently a purely adaptive phenomenon, the object of which no doubt is to permit of the total cleavage of the yolk-laden ovum on Avhat are presumably the old ancestral lines, and I believe a comparable explanation Avill be found applicable to the elimination of surplus yolkmaterial by the Marsupial ovum.

As regards the significance of the occurrence of the deutoplasmic zone in the ovum of Dasyurus, holding the views that I do as to the phylogeny of the Marsupialia (viz. that the Metatheria and Eutheria are the divergent branches of a common stock, itself of Prototlierian derivation), and bearing in mind the occurrence of an undoubted repi'esentative of the shell round the Marsupial ovum, I venture to see in the fluidmaterial of the deutoplasmic zone the partial and vestigial equivalent of the yolk-mass of the monotreme egg. In other words, I would regard the deutoplasmic fluid as the product of an abortive attempt at the formation of such a solid yolkmass. The objection will no doubt be forthcoming that this interpretation cannot possibly be correct since the supposed equivalent of the yolk-mass in the Dasyure ovum is located, on my own showing, at the wrong pole - at the upper instead of at the lower. But its precise location does not seem to me to be a matter to which we need attach any great importance, since it has doubtless been adaptively determined in correlation with the special character of the cleavage process.


Yide â€œAddendumâ€ (p. 121), in Avliich reference is made to the discovery by Prof. Van der Stricht of the elimination of deutoplasm in the ovum of Yesperugo.
Â® Of. Korschelt u. Heider, â€˜Lehrbuch d. vergl. Entwicklungsgeschichte,' Lief. 3, p. 107, 1909.


The belief that the minute yolk-poor ovum of the Eutheria is no pure primarily holoblastic one, but that it has only secondarily arrived at the total type of cleavage as the result of the all but complete loss of the yolk ancestrally present in it, consequent on the substitution of the intra-uterine mode of development for the old oviparous habit, is now widely held amongst Mammalian embryologists. Hubi'echt, however, is an exception, wedded as he is to a belief in the direct derivation of the Eutheria from Protetrapodous ancestors with yolkpoor, holoblastic eggs. Whether the interpretation I have put forward, viz. that the non-formative or deutoplasmic zone of the Dasyure ovum is the reduced and partial equivalent of the yolk-mass of the Monotreme egg, be accepted or not, I venture to think that my discovery of an actual elimination of deutoplasmic material by the Marsupial ovum affords a striking confirmation of the truth of the prevailing conception as to the phylogeny of the Eutherian ovum, and I further venture to think that the facts I have brought forward in the preceding ])ages justify us in' regarding the ripe ovarian ovum of Dasyurus as being potentially of the yolk-laden, telolecithal type, and the uterine ovum, by bodily casting out the superfluous part of its deutoplasm, as becoming at the same time secondarily lioniolecithal and secondarily lioloblastic. Ihe Marsupial ovum presents itself to my mind as tlie victim of tendencies conditioned by its ancestry, and in particular it appears as if its inherited tendency to elaborate yolk had not yet been brought into accurate correlation with the other changes (reduction in size, intra-uterine development), which it has undergone in the course of phylogeny. As the consequence it manufactures more yolk than it can utilise, and so finds itself under the necessity of getting rid of the surplus. Whether or not a comparable elimination of deutoplasmic material occurs in the ova of other Marsupials, future investigation must decide. I should be quite prepared to find variation in this regard, correlated perhaps with the size of the egg. In the case of Phascolarctus, Caldwell gives the diameter of the ovum as '17 mm., and his figure of a (horizontal ?) section of the uterine ovum (here produced as text-fig. 1, p. 27) shows a differentiation of the cytoplasmic body of that into vacuolated and granular zones quite comparable with that of the Dasyure ovum. From the few measurements of ova of other marsupials that I have been able to make, it would appear that the ovum of Trichosurus approximates in size to that of Dasyurus, whilst that of Perameles and pi-obably also that of Macropus are smaller. From Selenka's figure I have calculated that the ovum of Didelphys measures about 'IS x â€¢12 mm. in diameter. In the smaller ova it is quite likely that yolk-formation may not proceed so far as in the relatively large ovum of Dasyurus.

2. Maturation and Ovulation.

The details of the maturation process have not been fully worked out, owing to lack of material. As in the Eutheria (Sobotta, Van der Stricht, Lams and Doorme, and others), the first polar body is separated off in the ovary, the second apparently in the upper part of the- Fallopian tube where entrance of the sperm takes place. The first polar figure (late anaphase observed, fig. 5) lies in the formative cytoplasm, close below and afc right angles to the zona. Its exact site is subject to some slight variation, and is best described as adjacent to the equatorial region of the egg, sometimes nearer the lower pole, more usually, perhaps, nearer the upper. Centrosomes and polar radiations were not observed. The heterotypical chromosomes (gemini) have the form of somewhat irregular, more or less angular granules. I have not been able to determine their number. The figure is barrel-shaped, and almost as broad as long, measuring

015 X â€˜OIS mm. The first polar body (fig. 6, is small
relatively to the size of the egg, its diameters varying round â€¢03 X '01 mm., and its shape is that of a flattened bi-convex disc. In uterine eggs thex'e is some evidence pointing to the probability of its having undergone division.

The second polar spindle (figs. 3 and 7) lies immediately subjacent to the first polar body in the fully ripe ovarian ovum. It is shorter than the first, measuring '013 mm., and xnuch narrower. The second polar body measures about '015 X '01 mm. in diameter, and is thus smaller than the first. I have only seen the second polar body in uterine ova, and therefore can only presume that it is sepai'ated off in the upper part of the Fallopian tube, subsequently to the penetration of the sperm, as in Eutheria.

Ovulation takes place irrespective of whether copulation has occurred or not, and it is a fact Avorbhy of record that, even if the ova be not fertilised, the pouch and mammary glands undergo the same series of growth changes as are characteristic of, at all events, the earlier stages of normal pregnancy.

The follicular cells of the discus proligerus investing the ovum are already in the ripe follicle in a state of disruption, and I believe they separate completely from the ovum at the moment of dehiscence, so that, except for the zona, the ova are quite naked Avhen they enter the tube. I have no evidence of the existence outside the zona of a layer of proalbumen such as Caldwell describes round the ovum of Phascolarctus. Apparently the ova are shed almost simultaneously, and they must pass with considerable rapidity down the tubes to the uteri wliere cleavage begins, for I have only once found a tubal ovum, and that one had evidently been retarded for some reason, and was polyspermic.


3. The Secondary Egg-membranes: Albumen and Shell membrane

During the passage of the ovum down the tube it is fertilised, and becomes enclosed externally to the zona by two secondary layers formed as secretions by the cells of the oviducal lining. First of all, the ovum becomes surrounded by a transparent to semi-transparent laminated layer of albumen, â€¢015 to â– 02'2 mm. in thickness, composed of numerous very delicate concentric lamellae, and having, normally, numbers of sperms imbedded in it (figs. 8-11, alb., sp.). Then outside the albumen layer there is laid down a definite, but at first very thin, double-contoured membrane (figs. 8 and 10, s.m.), which, following Caldwell, I have no hesitation in homologising with the shell-membrane of the Monotreme egg. Caldwell in 1887 described and figured a definite membrane enclosing the uterine ovum of Phascolarctus, externally to, and quite distinct from the albumen, which he interpreted as the representative of the shell-membrane of the Monotremata, but owing apparently to the fact that Selenka altogether failed to recognise its true nature in Didelphys, since he regarded it as a derivative of the follicular epithelium, and termed it the â€œ granulosa-membran,â€ this highly significant discovery of Caldwell has been largely ignox'ed. Such a membrane is constantly present and easily recognisable in all the Marsupials (Dasyurus, Perameles, Trichosurus, Macropus, Petrogale, Phascologale, Acrobates, Phascolarctus, Bettongia), of which I have had the opportunity of studying early developmental stages. It is laid down in the Fallopian tube, is perfectly distinct from the albumen, and increases in thickness in the uterus, and if it has not the significance which Caldwell has suggested, then I must leave it to those who decline to accept Caldwell's interpretation to put forward an alternative one, since I am unable to do so.

The shell-membrane of Dasyurus (PI. 1, figs. 8-11; PI. 2, figs. 17, 18, s.m.) is a ti*ansparent, perfectly homogeneous layer, highly refractive in character and of a faint yellowish tint. When fully formed it possesses firm, resistant properties, recalling those of chitin, and is doubtless composed of a keratin base. It is distinguishable at once from the albumen by its optical characters and staining reactions, so that there is not the slightest justification for the supposition that it may represent simply the specially differentiated outermost portion of that layer. In ova which have just passed into the uterus (fig. 10) the shell-membrane is extremely delicate, its thickness being only about '0016 mm., but even before cleavage begins it has increased to â€002 mm. (fig. 12) ; in the 2-celled stage (fig. 18) it has reached â€˜005 mm., in the 4-celled stage (fig. 22) '0072 mm., whilst in the 16-celled stage (figs. 24-26) it has practically attained its maximum thickness, viz., â€˜0075'008 mm. Caldwell's measurements in the case of Phascolarctus agree closely with the above (shell of unsegmented ovum from the uterus, '0015 mm. thick, that of the '3 mm. â€˜â€˜ovum,â€ '01 mm.). Its presence renders the thorough penetration of ova and early blastocysts with pai'affin a capricious and frequently troublesome operation, and its resistant shell-like nature becomes only too obvious in the process of section-cutting, since it cracks with the utmost readiness (cf. PI. 3, figs. 32, 37).

The occurrence of a shell-membrane round the Marsupial ovum is a feature of considerable phyletic significance, as I need hardly point out. It shows us that the ancestors of the Metatheria must have been oviparous, or must themselves have come from an oviparous stock, which there is no valid reason for supposing was other than Prototherian in its characters. It also renders untenable the views of Hubrecht to the effect that the Metatheria are the descendants of Eutheria, whilst the Eutheria themselves have been directly derived from some presumed viviparous group of hypothetical Protetrapods, unless we are to suppose that the Metatheria are even novv on the way to acquire secondarily the oviparous habit, much in the same way as the Mouotremes, according to Hubi-echt, have long since succeeded in doing.

q'be occurrence of a shell-membrane round the Marsupial ovum has also aai important ontogenetic significance in relation to the mode of formation of the blastocyst, as I shall endeavour presently to show.


4. The Uterine Ovum.

The unsegmented ovum from the uterus (figs. 8-13) consists of the following parts :

(1) The shell-membrane externally, 'OOlfi - '002 mm. in thickness.

(2) The laminated layer of albumen, '015 - "022 mm. or more in thickness.

(3) The zona, about -0016 mm. in thickness.

(4) The perivitelline space, between the zona and the ovum, occupied by a clear fluid which coagulates under the action of certain fixatives, e. g. Hermann's fluid (fig. ll,p.s.), and which has diffused in from the uterus. The minute polar bodies lie in this space, usually nearer the upper pole than the lower.

(5) The ovum proper.

The entire egg is spherical in form, and varies in diameter in the fresh state from about '3 mm. to '36 mm. (average about â€˜32 mm.).

The ovum itself is ovoidal, its polar diameter always slightly exceeding the equatorial. Its average diametrical measurements in the fresh state run about '25 x '24 mm., though I have records of ova measuring as much as â€˜3 x â€˜29 mm., and I find that there is an undoubted slight variation iu the size of the ova of even one and the same batch, as well as iu those from different females.

The uterine ovum exhibits the same marked polarity as cliaracterises the ripe ovarian ovum (the upper pole being marked by the vacuolated deutoplasmic zone (figs. 8-11, d.z.), and so far as its cytoplasmic body is concerned it shows no essential difference from that.

Examined fresh in normal salt solution, the formative cytoplasm forming the bulk of the ovum appears dense, finely granular, and of a very faint lightish-brown tint, its opacity being such that the two pronuclei situated in its central region ' can just be made out. In section, this central region is distinguishable from the peripheral zone by its uniform, more finely granular character and by the absence of the fluid-filled vacuolar spaces which are generally present in the latter figs. 10 and 12). The deutoplasmic zone at the upper pole, which is only partially visible in the entire egg owing to the way in which it is enclosed by the formative cytoplasm (figs. 8, 9, d.z.), presents a characteristically clear or semi-transparent vacuolated appearance in the fresh state, but may have einbedded in it a small dense mass (fig. 8, cf. also figs. 11 and 14), evidently formed by the transformation of a portion of its fluid constitutent into the solid state, and so to be regai'ded as compai'able with a bit of formative cytoplasm.

In most of the unsegmented uterine ova at my disposal the male and female pronuclei have attained approximately tlie same size and lie in proximity in the central more homogeneous region of the formative cytoplasm (figs. 10-12). The transformation of the sperm-head into the male pronucleus probably takes place during the passage of the ovum down the tube, and was not observed, and I am as yet uncertain whether the pronuclei unite to form a single cleavage nucleus or give origin directly to the chi'omosomes of the first cleavage figure.

Caldwell figures ('87, PI. 30, fig. 5) a section through the uterine ovum of Phascolarctus which I reproduce here as Text-fig. l,in order to facilitate comparison with my figs. 11 and 12, with which it shows an essential agreement, apart from the pi'esence of follicular cells in the albumen which I have never observed in Dasyurus, and making allowance for the difference in sectional plane. The figure is stated to represent â€œthe seventeenth section of a vertical longitudinal seines of thirty-five sections through the segmenting ovum, containing two nuclei, taken from the uterus and measuring T7 mm. in diameter.â€ Caldwell has, I think, fallen into several errors in his interpretation of the structural features seen in this

Text-fig. 1.


Section of uterine ovnni of Phascolarctus cinereus. (After Caldwell.)

figuie. In the first place, the sectional plane appears to me not to be vertical as in my own figs. 11 and 12, but horizontal, and to have passed through the lower portion of the deutoplasmic zone, shown in the figure as a central markedly vacuolated area. Then there is no evidence to be derived from the figure in support of the description of the ovum as segmenting. The part inside the zona {vm.) labelled and described as protoplasm with finest jolk-granules,â€ I would interpret simply as coagulum in the perivitelline space, whilst the so-called â€œ segmentation nuclei â€ Ua) situated in it are probably the polar bodies or their derivatives. The part labelled yÂ«, and designated â€œ white yolk,â€ I would regard as the ovum itself. It exhibits an obvious differentiation into a central vacuolated area and a peripheral, dense, granular zone with scattered vacuoles, and I thiuk there can be little doubt but that the former corresponds to the deutoplasmic zone of the Dasyure ovum, the latter to the formative zone. It is these errors of intei-pretatiou apparently which misled Caldwell into making the statement, now widely quoted in the text-books, that cleavage in Phascolarctus is of the meroblastic type.
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