|
|
Line 536: |
Line 536: |
| and some of them multinucleate. In this abnormal egg it | | and some of them multinucleate. In this abnormal egg it |
| appears as if the formative cells had divided in fairly normal | | appears as if the formative cells had divided in fairly normal |
| fashion, whilst the nou-formative cells had failed to do so. | | fashion, whilst the nou-formative cells had failed to do so. |
| | |
| | |
| | |
| ==Chapter IV. - Growth oe the Blastocyst and Differentiation OF THE Embryonal Ectoderm and the Entoderm==
| |
| | |
| 1. Growth of the Blastocyst.
| |
| | |
| In the preceding chapter we have seen that the cleavage
| |
| process in Dasyurus results in the formation of a small
| |
| spherical vesicle, about '4 mm. in diameter, Avhich consists,
| |
| internally to the investment formed by the apposed zona and
| |
| shell-membrane, simply of a cellular wall, unilaminar throughout its entire extent, and enclosing a fluid-filled cavity
| |
| normally devoid of any cellular elements. The stage of the
| |
| just completed blastocyst is followed by a period of active
| |
| growth of the same, and it is a noteworthy featui'e in the
| |
| development of Dasyurus that during this time the blastocyst
| |
| undergoes no essential structural change, but remains unilaminar until it has reached a diameter of from 4'5 to 5'5 mm.
| |
| Even during cleavage, the egg of Dasyurus increases in
| |
| diameter, partly owing to the thickening of the shell membrane, partly, and, indeed, mainly, as the result of the accumulation of uterine fluid under pressure within the egg-envelopes,
| |
| but the increase due to these causes combined is relatively
| |
| insignificant, being only about '1 mm. As soon, however, as
| |
| the cellular wall of the blastocyst is completed, rapid growth
| |
| sets in, under the influence of the hydrostatic pressure of the
| |
| fluid, which tensely fills the blastocyst cavity, with the result
| |
| that the small relatively thick-walled blastocyst becomes
| |
| convei'ted into a large extremely thin-walled vesicle, but
| |
| beyond becoming very attenuated, the cellular wall during
| |
| this period of actjve growth uudei'goes no essential change,
| |
| and retains its unilaminar character until the blastocyst, as
| |
| already mentioned, has reached a diameter of from 4'5 to 5‘5
| |
| mm. In vesicles of about this size there become differentiated
| |
| from the formative cells of the upper hemisphei-e the embryonal ectoderm and the entoderm, and this latter layer then
| |
| gradually spreads round inside the non-formative (extraembiwonal ectodermal) layer of the lower hemisphere so as to
| |
| | |
| | |
| 44
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| form a complete lining' to tlie blastocyst, whicli thereby
| |
| becomes bilaminar. Sucli a marked enlargement of the blastocyst prior to the differentiation of the embryonal ectoderm
| |
| and entoderm as is here described for Dasyurus does not
| |
| apparently occur, so far as known, in other Marsupials : in
| |
| Perameles, for example, the embryonal ectoderm and the
| |
| entoderm are in process of differentiation in vesicles a little
| |
| over 1 mm. in diameter (v. p. 77), in Macropus these two layers
| |
| are already fully established in a vesicle only *8 mm. in
| |
| diameter (v. p. 79), and much the same holds good for Ti'ichosurus and Petrogale. It is pai'alleled by the marked growth
| |
| which in the Monotremes follows the completion of the blastocyst and which precedes the appearance of embryonal diffei-entiatiou. It must be remembered, however, that the growing
| |
| blastocyst in the Monotreme is bilaminar and not unilaminar
| |
| as in Dasyurus, owing to the fact that the entoderm is established as a complete layer at a very much earlier period than
| |
| is the case in the latter. I am nevertheless inclined to regard
| |
| the attainment by the Dasyurus blastocyst of a large size,
| |
| prior to the differentiation of the embi'yonal ectoderm and the
| |
| entoderm, as a more primitive condition than that found in
| |
| other Marsupials. The pronounced hypertrophy which the
| |
| uteri of Dasyurus undergo during the early stages of gestation, an hypertrophy which appears to be proportionately
| |
| greater than that met with in other forms,^ is no doubt to be
| |
| correlated with the presence in them of such a considerable
| |
| number of actively growing blastocysts.
| |
| | |
| Selenka states (Heft 5, p. 180) that he examined seven
| |
| blastocysts of Dasyurus “-f mm.†in diameter, taken from a
| |
| female fifteen days after copulation. He describes their
| |
| structure as follows : “ Man unterscheidet (1) eine sehr
| |
| | |
| zarte aussere, homogene Haut (Granulosamembran), (2)
| |
| | |
| ' Por example, the uteri of a female (5, 18 . vii . '01) from which 1
| |
| obtained twenty-one normal vesicles, 4'5-6 mm. in diameter, with the
| |
| embryonal area definitely established, measured as follows : Left uterus,
| |
| 4'5 X 4'7 X 1'4 cm. (fourteen vesicles) ; right uterus, 4'5 X 4'2 x 1‘45 cm.
| |
| (seven vesicles and one shrivelled).
| |
| | |
| | |
| THE EAELY DEVELOPMENT OF THE MARSHPIALIA. 45
| |
| | |
| | |
| darmiter ein Lagei' von Ektodermzellen, welche im Gebiete
| |
| des Embryonalschildes prismafcich, am gegeniiberliegenden
| |
| Pole nahezu kubisch, im iibrigen abgeplattet erscbeinen, (3)
| |
| ein inneres zusammenliangeudes Lager von abgeflacbten Entodermzellen.†This description, apart from the reference to
| |
| the thin shell-membrane, is entii'ely inapplicable to blastocysts
| |
| of Dasyurus of the mentioned size which I have studied.
| |
| | |
| I have examined a practically complete series of vesicles of
| |
| Dasyinms ranging from '4 mm. to 4 mm. in diameter and all of
| |
| them without exception are unilaminar.
| |
| | |
| Of vesicles under 1 mm. diameter I possess serial sections
| |
| of more than two dozen, I'anging from '5 mm. to '8 mm. in
| |
| diameter, and obtained from three different females. These
| |
| differ structurally in no essential respect from the just completed blastocysts. A surface view of a blastocyst '6 mm. in
| |
| diameter is shown in fig. 63, PI. 6; in this the difference in
| |
| the cytoplasmic chai'acters of the cells of opposite hemispheres
| |
| is clearly brought out, the non-formative cells of the lower
| |
| hemisphere having much more marked perinuclear zones of
| |
| dense cytoplasm (deutoplasm) than the formative cells of the
| |
| upper hemisphere ; moreover, the former cells tend to be of
| |
| larger superficial extent than the latter. Pig. 34, PI. 3,
| |
| represents a section of a blastocyst '57 mm. in diameter, and
| |
| fig. 35 a section of one '73 mm. in diameter. These blastocysts differ in no essential way from the '43 mm. blastocyst
| |
| represented in fig. 33. As in the latter, the cellular wall is
| |
| unilaminar throughout, but both it and the shell-membrane
| |
| have undergone considerable attenuation. Moreover in these
| |
| blastocysts, apart from the clue afforded by the shrivelled
| |
| yolk^body, it is practically impossible to determine from the
| |
| sections which is morphologically the upper hemisphere and
| |
| which the lower. In fig. 36, from a '6 mm. blastocyst, on the
| |
| other hand, the cells of the hemisphere opposite the yolk-body
| |
| (y.b.) are larger than those of the hemisphei'e adjacent to
| |
| which that body is situated. In the '57 mm. blastocyst the
| |
| shell-membrane has a thickness of ‘0052 mm., in the -73 mm.
| |
| blastocyst it measures -0045 rnrh., and in a -84 mm. blastocyst
| |
| | |
| | |
| 46
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| •0026 mm. The zona is now no longer recognisable as an
| |
| independent membrane. In blastocysts of this stage of
| |
| growth a variable number of small spherical cells or cellfragments are frequently met with in the blastocyst cavity,
| |
| usually lying in contact Avith the inner aspect of the cellular
| |
| wall (fig. 34, i.c.). In some blastocysts such structures are
| |
| absent, in others one or two may be present, in yet others
| |
| numbers of them may occur. They raa,y be definitely nucleated,
| |
| but this is exceptional; more usually they contain one or more
| |
| deeply staining granules (of chromatin ?), or are devoid of
| |
| such. They ai'e of no morphological importance, and I think
| |
| thei*e can be no doubt that they represent cells or fragments
| |
| of cells which have been separated off from the cellular wall
| |
| during the process of active growth. They are of common
| |
| occurrence in later blastocysts, and it is possible the so-called
| |
| “ yolk-balls †observed by Selenka in Didelphys are of the
| |
| same nature.
| |
| | |
| If we pass now to vesicles from 1 to 3 or 3'5 mm. in
| |
| diameter, we find the wall still unilaminar, but considerably
| |
| more attenuated than it is in the blastocysts last referred to.
| |
| In a vesicle with a diameter of 1'24 mm. the shell-membrane
| |
| has a thickness of about '0015 mm., whilst the cellular wall
| |
| has a thickness of only '0045 mm. In a 3'5 mm. vesicle the
| |
| shell-membi'ane measures about •0012 mm., Avhilstthe cellular
| |
| wall ranges from •OOlS to •OOIS mm. in thickness. A small
| |
| portion of the wall of a vesicle, 2^4 mm. in diameter, is shown
| |
| in PI. 6, fig. 64. In these later vesicles I have failed to detect,
| |
| either in surface examination of the vesicles in to to or in
| |
| sections, any regional differences between the cells indicative
| |
| of a differentiation of the wall into upper or formative, and
| |
| lower or non-formative, hemispheres. Everywhere the wall
| |
| is composed of flattened, exti'emely attenuated cells, polygonal
| |
| in surface view, and all apparently of the same character. It
| |
| might therefore be supposed that the polarity, which is recognisable in early blastocysts, and which is dependent on the
| |
| pronounced differences existent between the cells of the
| |
| upper and lower rings of the 16-celled stage, is of no funda
| |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| | |
| 47
| |
| | |
| | |
| mental importance, since it apparently becomes lost at an
| |
| early period during the growth of the blastocyst. Such an
| |
| assumption, however, would be very wide of the maxk, as I
| |
| hope to demonstrate in the next section of this paper, and,
| |
| indeed, in view of the facts already set forth, is an altogether
| |
| improbable one.
| |
| | |
| Reappearance of Polar Differentiation in the
| |
| Blastocyst Wall. - Following on the period of what may
| |
| be termed the preliminary growth of the blastocyst, in the
| |
| course of which the original polar differentiation in the
| |
| blastocyst wall apparently becomes obliterated, is an
| |
| extremely interesting one, during which that differentiation
| |
| again becomes manifest. In view of the fact (1) that the
| |
| fourth cleavage in Dasyurus is of the nature of a qualitative
| |
| cytoplasmic division, and (2) that approximately one half or
| |
| rather less of the unilaminar vosicle wall is formed from the
| |
| eight smaller and less yolk-rich cells of the upper ring of the
| |
| 16-celled stage, and its remainder from the eight larger
| |
| more yolk-rich cells of the lower ring, it thus becomes a
| |
| question of the first importance to determine if we can the
| |
| significance of that differentiation.
| |
| | |
| Amongst the Eutheria, it has been conclusively shown by
| |
| various observers (Van Beneden, Heape, Hubrecht, Assheton,
| |
| and others) that there occurs during cleavage an early
| |
| separation of the blastomeres into two more or less distinctly
| |
| differentiated groups, one of which eventually, by a process
| |
| of overgrowth, completely encloses the other. The peripheral
| |
| cell-group or layer forms the outer extra-embryonal layer of
| |
| the wall of the later blastocyst (the trophoblast of Hubrecht,
| |
| or trophoblastic ectgderm as I prefer to term it). It therefore
| |
| takes no direct part in the formation of the embryo, and may
| |
| be distinguished as non-formative. The enclosed cell-group,
| |
| termed the inner cell-mass or embryonal knot, gives rise, on
| |
| the other hand, to the embryonal ectoderm as well as to the
| |
| entire entoderm of the vesicle, and may accordingly be distinguished as formative. May it not be, then, that we have
| |
| here at the fourth cleavage in Dasyurus a separation of the
| |
| | |
| | |
| 48
| |
| | |
| | |
| J. P. HIPL.
| |
| | |
| blastomeres into two determinate cell-groups, respectively
| |
| foi'mative and non-formative in significance, entirely compar-,
| |
| able with, and, indeed, even more distinct than that which
| |
| occurs during cleavage in the Eutheria ? I venture to think
| |
| that the evidence brought forward in this paper conclusively
| |
| justifies an answer in the affirmative to that question.
| |
| | |
| If we assume that the upper cell-ring of the 16-celled stage
| |
| in Dasyurus is formative in destiny and the lower cell-ring
| |
| non-formative, then we might naturally expect to find in the
| |
| unilaminar wall of the later blastocyst some differentiation
| |
| indicative of its origin from two distinct cell-groups, and
| |
| indicative at the same time of the future embryonal and
| |
| extra-embryonal regions. Now just such a differentiation,
| |
| does, as a matter of fact, become evident in vesicles 3'5 to
| |
| 4'5 mm. in diameter. We have already seen that the wall in
| |
| early blastocysts '4 to '8 mm. in diameter exhibits a wellmarked polar differentiation in correspondence with its mode
| |
| of oi'igin from the diffei-entiated cell-rings of the 16-celled
| |
| stage, its upper hemisphere or thereabouts consisting of
| |
| smaller cells, poor in deutoplasm, its remainder of larger
| |
| cells, rich in deutoplasm. .In later blastocysts, 1-3 mm. or,
| |
| more in diameter, it is no longer possible to recognise this
| |
| distinction - at all events I have failed to observe i't - but if
| |
| we pass to blastocysts 4-5 mm. in diameter, in which the wall
| |
| is still unilaminar, we find on careful examination of the
| |
| entire vesicle under a low power that there is now present a
| |
| definite continuous line^ which encircles the vesicle in theequatorial region so as to divide its wall into two hemi-,
| |
| spherical areas (PI. 4, fig. S8,j.L). If we remove and stain;
| |
| a portion of the wall of such a vesicle, including this line,)
| |
| and examine it microscopically (figs. ,42-46.), it becomes
| |
| apparent at once, from the .disposition of the cells on either
| |
| side of the bns, that we have to do with a sutural line or line
| |
| of junction produced by the meeting of twp sets- of cells,'
| |
| which are pursuing their .own, independent courses of growth
| |
| and division. ; The, cells never cross the demax'cation line
| |
| from the one side tn the other, but remain strictly confined
| |
| | |
| | |
| 49
| |
| | |
| | |
| THE EAELY DEVELOPMENT OF THE MARSUPIAL [A.
| |
| | |
| to their own territory, so that we are justified in regarding
| |
| the vesicle wall as composed of two independently growing
| |
| zones. Now tlj^ existence of two such independent zones in
| |
| the unilaminar wall is, to my mind, only intelligible on the
| |
| view that they are the products of two originally distinct,
| |
| predetermined cell'groups, and if this be admitted, then I
| |
| think we are justified in concluding, in view of the facts
| |
| already set forth, that tlie two zones in question are derived,
| |
| the one from the upper cell-ring of the 16-celled stage, the
| |
| other from the lower ring ; that, in other words, they represent respectively the upper and lower hemispheres of the
| |
| early blastocysts.
| |
| | |
| If, now, we find that the embryonal ectoderm and the entoderm arise from one of these two regions of the unilaminar
| |
| wall, whilst the other directly forms the outer extra-embryonal
| |
| layer of the later (bilaminar) vesicle, then we must designate
| |
| the former region as the upper or formative, and the latter as
| |
| the lower or non-formative. Further, bearing in mind the
| |
| characters of the cells of the two rings of the 16-celled stage,
| |
| T think we are justified in holding that the formative region
| |
| is derived from the ring of smaller, less yolk-rich cells, and
| |
| the non-formative region from the ring of larger, more yolkrich cells, even if it is impossible to demonstrate an actual
| |
| genetic continuity between the constituent cells of these two
| |
| rings and those forming the independently growing areas of
| |
| the later blastocyst. I have recently re-examined a series of
| |
| vesicles, measuring 1'5-1'8 mm. in diameter, obtained from a
| |
| female killed in 1906, and I have so far found it impossible,
| |
| either in the entire vesicle or in portions of the wall stained
| |
| and mounted on the flat, to distinguish between the cells over
| |
| opposite hemispheres. Thus the only actual guide Ave have
| |
| for the determination of the poles in such vesicles is the
| |
| yolk-body, and though the latter is liable to- displacement, it
| |
| is Avorthy of record that I have several times found it in
| |
| relation to the formative area in vesicles 4‘5-6 mm. in
| |
| diameter, but never in relation to the non-formative region.
| |
| This evidence is, therefore, so far as it goes, confirmatory of
| |
| VOL. 56, PART 1. - NEAV SERIES. 4
| |
| | |
| | |
| 50
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| the conclusion reached above, viz, that the formative hemisphere is derived from the smaller-celled ring of the 16-celled
| |
| stage. On that conclusion is based my interpretation of the
| |
| poles in the unsegmented ovum, and of the two cell-rings o£
| |
| the 16-celled stage as respectively upper and lower.
| |
| | |
| Of vesicles ov'er 1 mm. in diameter, the smallest in which I
| |
| have been able to detect the sutural line above referred to
| |
| measure 3'25 mm. in diameter. In three lots of vesicles, 3'5
| |
| mm. in diameter from three different females, I have failed to
| |
| X'ecognise it, whilst in two other lots, respectively 3'75 mm.
| |
| (average) and 4 mm. in diameter, the line appears to be in
| |
| course of differentiation as in the 3'25 mm. vesicles. A
| |
| portion of the wall of one of the 3'5 mm. vesicles just referred
| |
| to is shown in PL 4, fig. 41, and a portion of the wall of the
| |
| 3'25 mm. stage, including the sutural line, in fig. 42. Both
| |
| vesicles were fixed in the same fluid, viz. picro-nitro-osmic
| |
| acid. Comparison of the two figures reveals the existence, quite
| |
| apart from the presence of the junctional line in fig. 42, and its
| |
| absence in fig. 41, of certain more or less obvious differences
| |
| between them. In fig. 41 the cells are larger, and their cytoplasmic bodies are inconspicuous, being fairly homogeneous
| |
| and lightly staining. In fig. 42, on the contrary, the cellbodies are strongly marked, the cytoplasm being distinguishable into a lighter-staining peripheral zone, and a much more
| |
| deeply staining perinuclear zone, showing evidence of intense
| |
| metabolic activity. This latter zone is more or less vacuolated,
| |
| and contains, besides larger lightly staining granules, numerous
| |
| smaller ones of varying size, stained brown by the osmic acid
| |
| of the fixative. In the 4 ram. vesicles the cells show pi-ecisely
| |
| the same characters; in the 3'75 mm. vesicles, which were
| |
| fixed in a picro-corrosive-acetic fluid, the granules ax'e absent
| |
| from the cytoplasm, otherwise the cells are similar to those
| |
| of the other two. Mitotic figures are common. The sutural
| |
| line is recognisable in all three sets of vesicles (3'25, 3'75, and
| |
| 4 mm.) (fig. but I cannot be certain that it runs con
| |
| tinuously round, and it appears to have a rather more sinuous
| |
| course than in later blastocysts. The cells of the two regions
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| | |
| 51
| |
| | |
| | |
| of the bliistocyst wall, separated by the sutural line, differ
| |
| somewhat in tlieir characters. On one side of the line (fig.
| |
| 42, tr.ect.) the cells appear to be on the whole slightly larger,
| |
| and of more uniform size than they are on the other, and they
| |
| also stain somewhat more deeply. Comparison with later
| |
| blastocysts shows that the region of more uniform • cells is
| |
| non-formative, that of less uniform, formative. At^this stage,
| |
| however, the differences between the cells of the two regions
| |
| are as yet so little pi'onounced that it is practically impossible
| |
| in the absence of the sutural line to say to which hemisphere
| |
| an isolated piece of the wall should be referred.
| |
| | |
| I am inclined to regard the sutui'al line in these vesicles as
| |
| being in course of differentiation, and judging from the disposition of the cells on either side of it, I think its appearance
| |
| is to be correlated with the marked increase in the mitotic
| |
| activity of the cells of the two hemispheres which sets in in
| |
| vesicles of 3-4 mm. diameter. The preliminary increase in
| |
| size of the blastocyst up to about the 3 mm. stage might be
| |
| described as of a passive character, i.e. it does not appear
| |
| to be effected as the result of the very active division of the
| |
| wall-cells, but is characterised rather by a minimum of mitotic
| |
| division and a maximum of increase in surface extent of the
| |
| cells, due to excessive stretching consequent on the rapid
| |
| imbibition of uterine fluid. Once, however, the requisite size
| |
| has been attained, the cells of the unilaminar wall commence
| |
| to divide activel}', and doubtless as the outcome of that
| |
| wave of activity, the sutural line makes its appeai-ance
| |
| between the two groups of independently growing cells.
| |
| | |
| On the inner surface of the blastocyst wall, especially in
| |
| the region of the formative hemisphere, there are present
| |
| in these vesicles numbers of small deeply staining cells of
| |
| spherical form, and containing osmicated granules similar
| |
| to those in the wall-cells. They may occur singly or in groups,
| |
| and appear to me to be of the same nature as the inteimal cells
| |
| of the earlier blastocyst. In addition to these cells, there are
| |
| present clusters of cytoplasmic spheres, staining similarly to
| |
| the spherical cells, and apparently of the nature of fragmeiita
| |
| | |
| 52
| |
| | |
| | |
| J. P, HILL.
| |
| | |
| | |
| tion products formed either directly from the â– \vall-cells or
| |
| from these internal cells.
| |
| | |
| 2. Differentiation of the Embryonal Ectoderm and
| |
| | |
| the Entoderm.
| |
| | |
| After the preliminary growth in size of the blastocyst is
| |
| completed, the next most important step in the progressive
| |
| development of the latter is that just dealt with, involving
| |
| the appearance of the sutural line, with resulting re-establishment of polar differentiation in the blastocyst wall. Following
| |
| on that, we have the extremely important period during
| |
| which the embryonal ectoderm and the entoderm become
| |
| definitely established.
| |
| | |
| For the investigation of the earlier phases of this critical
| |
| period I have had at my disposal a large number of
| |
| unilaminar blastocysts derived from three females, distinguished in my notebooks as (3, 25 . vii . '01, with fifteen
| |
| vesicles of a maximum diameter of 4‘5 mm. ; 8 . vii . '99, with
| |
| twelve vesicles, 4‘6 .mm. in diameter ; and 6 . vii . '04, with
| |
| twfenty-two vesicles, 4‘5 and 5 mm. in diameter. These three
| |
| lots of vesicles may for descriptive purposes be designated
| |
| as '01, '99, and '04 respectively.
| |
| | |
| The '01 vesicles are distinctly less advanced than the
| |
| other two. The sutural line is now, at all events, definitely
| |
| continuous, and can readily be made out in the intact vesicle
| |
| with the aid of a low-power lens (PI. 4, fig. 38, j.L), but
| |
| the differences between the cellular constituents of the two
| |
| hemispheres which it separates are much less obvious than
| |
| they are in the '99 and '04 vesicles. Here, again, one
| |
| hemisphere forming half or perhaps rather more of the entire
| |
| vesicle is distinguished from the other by the greater uniformity and the slightly deeper staining character of its
| |
| constituent cells (figs. 43 and 44, tr. ecL). This hemisphere,
| |
| subsequent stages show, is the lower or non-formativ^
| |
| hemisphere. It is characterised especially by the striking
| |
| 'uniformity in the size of its cells. Over the opposite hemisphere, the upper or formative one (figs. 43 and 44, the
| |
| | |
| | |
| THE EARLY DEVELOPMENT. OF- THE MAR8UPIALIA.
| |
| | |
| | |
| 53
| |
| | |
| | |
| cells are more variable iu size, the nuclei thus appearing less
| |
| uniformly and less closely arranged, and they stain,. on the
| |
| whole, somewhat less deeply than those of the lower hemisphere. The non-formative cells are on the average smaller
| |
| than the largest of the formative cells, but they are more
| |
| uniform iu size, and their nuclei thus lie at more regular
| |
| distances apart, and appear more closely packed. They are
| |
| also richer in deutoplasmic material, and so stain rather more
| |
| deeply than the formative cells. Sections show that the
| |
| cellular wall is unilaminar throughout its extent, and that,
| |
| whilst it is somewhat thicker than that of 3‘5 mm. vesicles,
| |
| it is still very attenuated, its thickness, including the shellmembrane, ranging-from ‘004 to '008 mm. I have examined
| |
| a number of series of sections taken through portions of the
| |
| wall known to include the sutural line, and find it quite
| |
| impossible to locate the position of the- latter; indeed, I
| |
| cannot certainly distinguish between the formative and nonformative regions.
| |
| | |
| In the blastocyst cavity, lying in contact with the inner
| |
| surface of the wall, and most abundant in the region of the
| |
| formative hemisphere, there are present numbers of deeply
| |
| staining spherical cells with relatively small nuclei similar to
| |
| those described in connection with the 3'25 mm. vesicles.
| |
| They occur singly or in groups, and may appear quite normal
| |
| or may show more or less evident signs of degeneration.
| |
| Their nuclei may stain deeply and homogeneously, or may be
| |
| represented by one or two deeply staining granules, vacuoles
| |
| may occur in their cytoplasm, and spherical cytoplasmic masses
| |
| of very variable size, with or without deeply staining granules
| |
| of chromatin) may occur along with them. In sections and
| |
| preparations of the wall of these, and other 4*5 mm. vesicles
| |
| there are to be found, in both the formative and non-formative
| |
| hemispheres, small localised areas from which such spherical
| |
| cells are being proliferated off in numbers together. PI. 5,
| |
| fig. 47, from the formative hemisphere of an ^04 vesicle shows
| |
| One of the most marked examples of such proliferative. activity
| |
| that I have encountered. A similar but smaller proliferative
| |
| | |
| | |
| 54
| |
| | |
| | |
| J. ?. HILL.
| |
| | |
| | |
| area occurs on the non-formative hemisphere of the same
| |
| vesicle.
| |
| | |
| These spherical cells are, I am convinced, of no morphological importance, and are destined sooner or later to degenerate. They have certainly nothing to do with the
| |
| entoderm, the parent-cells of that layer arising exclusively
| |
| from the formative hemisphei'e and not from cells such as
| |
| these, which are budded ofE from both hemispheres. The fact
| |
| that they are, in unilaminar vesicles, more numerous over the
| |
| formative hemisphere may perhaps be taken as an indication
| |
| of the greater mitotic activity of the formative as compared
| |
| with the non-formative cells.
| |
| | |
| The Primitive Entodermal Cells. - Following closely
| |
| on the stage represented by these '01 blastocysts is the extremely important one constituted by the '99 and '04 vesicles
| |
| before referred to. This stage is the crucial one in primary
| |
| germ-layer formation, and marks the transition from the unilaminar to the bilaminar condition, since in it the entodermal
| |
| cells are not only distinctly recognisable as constituents of the
| |
| formative region, but are to be seen both in actual process of
| |
| separation from the latter and as definitely internal cells, frequently provided with, and even connected together by,
| |
| pseudopodial-like processes of their cell-bodies. Such cells
| |
| are already present in the '01 vesicles (fig. 71), and probably
| |
| also in the blastocysts in which the sutural line first makes
| |
| its appearance, but are much less conspicuous than in these
| |
| older blastocysts.
| |
| | |
| The '99 blastocysts are distinctly more advanced than the
| |
| '01 batch and are just a little earlier than the '04 lot. The
| |
| former measui'ed, as already mentioned, 4'5 mm. in diameter,
| |
| the latter 4'5 and 5 mm. (the majority being of the latter
| |
| size). In my notes, on the intact '99 vesicles I find it stated
| |
| that one hemisphere, forming rather less than half of the
| |
| entire extent of the vesicle wall, appeared somewhat denser
| |
| than the other, the sutural line marking the division between
| |
| the two. I naturally inferred at the time that the denser
| |
| hemisphere corresponded to the embryonal region of the
| |
| | |
| | |
| 55
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| Eutherian blastocyst and the less dense to the extra-embryonal region of the same, but just the reverse proves to hold
| |
| true for the '04 vesicles, the formative hemisphere in these
| |
| appearing less dense than the non-formative. I cannot now
| |
| test my former inference by direct observation since I do not
| |
| appear to have any of the '99 vesicles left intact, but amongst
| |
| my in toto preparations of the vesicle wall I find one
| |
| labelled as from the “ lower pole †which unmistakably
| |
| belongs to the formative hemisphere, hence I conclude that
| |
| the denser and slightly smaller region which I originally
| |
| regarded as formative is really non-formative, a conclusion
| |
| which brings the '99 vesicles into agreement with the '04
| |
| batch.
| |
| | |
| In these latter vesicles the sutural line and the two regions
| |
| of the wall can be quite readily made out on careful examination under a low power with transmitted light. The one
| |
| region appears slightly denser (darker) and has more closely
| |
| arranged nuclei (i. e. is composed of smaller cells) than the
| |
| other. On the average this denser region appeal's to be
| |
| rather the less extensive of the two ; the two regions may be
| |
| about equal ; on the other hand the denser may be the smaller.
| |
| Examination of stained preparations of the wall demonstrates
| |
| that the darker hemisphere is non-formative, the lighter,
| |
| formative. It would therefore seem that in certain of these
| |
| '04 vesicles the formative region has grown more rapidly than
| |
| the non-formative.
| |
| | |
| In stained preparations of the wall both of the '99 and '04
| |
| vesicles, the differences between the two hemispheres are now
| |
| so well marked that there is no diflBculty in referring even an
| |
| isolated fragment to its proper region. The non-formative
| |
| hemisphere differs in no essential way from that of the '01
| |
| vesicles, and as in these, is readily distinguishable from the
| |
| formative by the much greater uniformity in the size and
| |
| staining properties of its cells (fig. 45), as well as by the fact
| |
| that there are no primitive entodermal cells such as occur in
| |
| relation to the formative hemisphere, in connection with it.
| |
| Its constituent cells are on the average distinctly smaller than
| |
| | |
| | |
| 56
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| the largest of the formative ; their nuclei lie nearer each other,
| |
| with the result that in surface examination of the blastocyst
| |
| the non-formative region appears rather denser than the
| |
| formative. In in toto preparations of the wall the former
| |
| usually stains darker than the latter (fig. 45), but this is not
| |
| always the case ; in fig. 46, from an '04 vesicle, there is
| |
| practically no difference in this respect between the two
| |
| regions ; in yet others of my preparations of '99 vesicles the
| |
| formative region has stained more deeply than the nonformative.
| |
| | |
| The formative hemisphere in the earlier blastocysts of this
| |
| particular developmental stage was described (ante, p. 51) as
| |
| differing from the non-formative in that its constituent cells
| |
| were much less uniform in chai*acter than those of the latter.
| |
| This same feature, but in much enhanced degree, characterises
| |
| the formative region of the vesicles under consideration, for it
| |
| can now be definitely stated that the latter I'egion is constituted by cells of two distinct varieties, viz. (1) moi*e lightly
| |
| staining cells which form the chief constituent of the formative region, its basis so to speak, and which are on the
| |
| average larger than those of the other variety, and (2), a less
| |
| numerous series of cells, distinctly smaller than the largest
| |
| cells of the former variety, and with denser, more granular and
| |
| more deeply staining cytoplasm, and frequently met with in
| |
| mitotic division (cf. PI. 6, fig. 65). The two varieties of cells are
| |
| intermingled promiscuously, the smaller cells occurring singly
| |
| and in groups but in a quite irregular fashion, so that here
| |
| and there we meet with patches of the wall composed exclusively of the larger cells.
| |
| | |
| The evidence presently to be adduced shows that the larger
| |
| cells furnish the embryonal ectoderm, and that the smaller
| |
| cells give origin to the primitive entodermal cells from which
| |
| the definitive entoderm arises. The smaller cells may therefore be regarded as entodermal mothei'-cells. Whether these
| |
| latter cells are progressively formed from the larger cells
| |
| simply by division, or whether the two vaifieties become
| |
| definitely differentiated from each other at a particular stage in
| |
| | |
| | |
| THE EAHLY DEVELOPMENT OP THE MARSUPIALTA.
| |
| | |
| | |
| 57
| |
| | |
| | |
| development, must for the present be left an open question. Of
| |
| the actual existence in tlie unilaminar formative region of these
| |
| '99 aud '04 blastocysts of two varieties of cells, respectively
| |
| ectodermal and entodermal in significance, there can be no
| |
| doubt. In preparations of the formative region, however,
| |
| whilst one can without hesitation identify certain cells as
| |
| being in all probability of ectodermal significance and others
| |
| as prospectively entodermal (cf. figs. 65, 66), it must be
| |
| admitted that one is often in doubt as to whether one is
| |
| dealing with small ectodermal cells or with genuine entodermal mother-cells. It is, therefore, hardly to be wondered
| |
| at that I have not yet been able to satisfactorily determine
| |
| at what precise period the entodermal mother-cells first
| |
| become differentiated, though judging from the facts that
| |
| in the eai-liest vesicles in which the sutural line is recognisable one region of the wall already differs from the other in
| |
| the less uniform size of its constituent cells, and that internally
| |
| situated entodermal cells are already present in small numbers
| |
| in the '01 vesicles (fig. 71), I incline to the belief that it
| |
| will probably be found to about coincide with the first
| |
| appearance of the sutural line. To this question I may
| |
| perhaps be able to return at some future time.
| |
| | |
| In addition to the presence of these entodei'mal mothercells, which enter directly into its constitution, the formative
| |
| region of the '99 and '04 blastocysts is. characterised by the
| |
| occurrence on its inner surface of definitely iuteimal cells,
| |
| which generally agree with the former cells as regards size
| |
| and staining properties and are evidently related to them. It
| |
| is these internally situated cells which directly give origin to
| |
| the definitive entoderm of the later blastocysts, and one need,
| |
| therefore, have no hesitation in applying to them the designation of primitive entodermal cells. They are exclusively found
| |
| in relation to the formative hemisphere, and appear in in toto
| |
| prepai'atious as flattened, darkly staining cells closely applied
| |
| to the inner surface of the unilaminar wall, and disposed quite
| |
| irregularly, singly, and in groups. They vary greatly in
| |
| number in blastocysts of even the same batch, but on the
| |
| | |
| | |
| 58
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| | |
| wholo are most abundant in the ^04 series, and they also
| |
| exhibit a remarkable range of variation in shape. They may
| |
| have a perfectly distinct oval or rounded outline (figs. 67, 71,
| |
| 72), or, as is more frequently the case, they may lack a
| |
| determinate form and appear quite like amoeboid cells owing
| |
| to their possession of cytoplasmic processes of markedly
| |
| pseudopodial-like character (fig. 69). Frequently, indeed,
| |
| the cells are connected together by the anastomosing of these
| |
| processes, so that we have formed in this way the beginnings
| |
| at least, of a cellular reticulum (figs. 68, 69,70).
| |
| | |
| The question now arises. How do these primitive entodermal cells originate from the small, darkly staining cells of
| |
| the unilaminar formative region designated in the foregoing
| |
| as the entodermal mother-cells ? I can find no evidence that
| |
| the primitive entodermal cells are formed by the division of
| |
| the mother-cells in planes ta.ngential to the surface ; on the
| |
| contrary, all the evidence shows that we have to do here with
| |
| an actual inward migration of the mother-cells, with or without previous mitotic division, such inward migration being
| |
| the outcome of the assumption by the mother-cells, or their
| |
| division products, of amoeboid properties ; in other words, the
| |
| evidence shows that the formation of the entodei'm is effected
| |
| here not by simple delamination (using that term in the sense in
| |
| which it was originally employed by Lankester), but by a process involving the inward migration, with or without previous
| |
| division, of certain cells (entodermal mother-cells) of the unilaminar parent layer, a process comparable with that found in
| |
| certain Invertebrates (Hydroids) and distinguished by Metschnikoff as '^gemischte Delaminatiou.â€
| |
| | |
| In this connection it has to be remembered that the cells of
| |
| the unilaminar wall of the blastocyst are under considei'able
| |
| hydrostatic pressure, and, in correlation therewith, tend to
| |
| be tangentially flattened, though the flattening in this stage
| |
| is much less than in the earlier blastocysts. From a series of
| |
| measurements made from an '04 vosicle, I find that over the
| |
| formative region the ratio of the breadth to the thickness of
| |
| the cells varies Horn 6 : 1 to 2 : 1, and even to 3 : 2. On the
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MAESUPIALIA. 59
| |
| | |
| | |
| whole cells of the type indicated by the ratio 6 : 1 predominate,
| |
| and we should hardly expect to find such cells dividing tangentially. In fact, the only undoubted examples of such division I
| |
| have met with occur in the single abnormal vesicle present in
| |
| the '04 batch. In this particular vesicle, which had a diameter
| |
| of 3 mm. and was thus smaller than the others, thei'e was
| |
| present on what appeared to correspond to the formative
| |
| hemisphere of the normal blastocyst a well-defined and conspicuous ovalish patch, 1'23 x '99 mm. in diameter.^ Sections
| |
| show that over this area the cells of the unilaminar wall are
| |
| much enlarged and , more or less cubical in form, their thickness varying from ‘012 to ‘019 mm. These cubical cells
| |
| exhibit distinct evidence of tangential division, both past and
| |
| in progress. But in normal vesicles, whilst mitotic figures are
| |
| quite commonly met with in the cells of the formative region
| |
| (in which, indeed, they are more numerous than in those of
| |
| the non-formative region), I have failed to find in my sections
| |
| after long-continued searching even a single spindle disposed
| |
| directly at right angles to the shell-membrane ; the mitotic
| |
| spindles lie disposed either tangentially to the surface or
| |
| obliquely thereto.
| |
| | |
| For the determination of the mode of origin of the
| |
| primitive entodermal cells, it is absolutely necessary to
| |
| study both in to to preparations of the formative region,
| |
| i.e. small portions of the unilaminar wall stained and
| |
| mounted on the flat, and sections of the same. Sections alone
| |
| are, on the whole, distinctly disappointing so far as the
| |
| question under discussion is concerned, and, indeed, give one
| |
| an altogether inadequate idea of the primitive entodeimial cells
| |
| themselves, seeing that practically all one can make out is that
| |
| | |
| 1 Curiously enough, amongst the '99 vesicles there also occurred
| |
| a single small one, likewise 3 mm. in diameter, and with a thickened
| |
| patch 1-28 X 1 mm. in diameter, quite similar in its character to that
| |
| described in the text. I am as yet uncertain whether the thickened
| |
| area in these two vesicles represents the whole of the formative hemisphere of normal blastocysts or only a hypertrophied part of the same,
| |
| or whether, indeed, it may not represent the I'etarded non-formative
| |
| hemisphere.
| |
| | |
| | |
| 60
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| there are present, in close apposition with the inner surface of
| |
| the unilaininar wall, small, darkly staining cells, apparently
| |
| quite isolated from each other and usually of flattened form
| |
| (figs. 73, 74, 76, ent.). One has only to glance at a wellstained in to to preparation of the formative region (cf.
| |
| fig. 70) to realise how inadequate such a description of the
| |
| primitive entoderm cells really is.
| |
| | |
| Sections nevertheless do yield valuable information on
| |
| certain points. Besides affording the negative evidence of
| |
| the absence of tangential divisions and the positive evidence
| |
| that the primitive entodermal cells are actually internal (figs.
| |
| 73, 74, 76), they show that growth of the wall iu thickness
| |
| has already set in, and that it is most marked over the
| |
| formative region, though the thickness attained by the cells
| |
| is as yet very unequal (figs. 73-76). Measurements takeu
| |
| from an '04 vesicle show that over the non-formative region
| |
| (fig. 77) the cells vary in thickness from *006 to '009 mm.,
| |
| whilst over the formative region the range of variation is
| |
| greater, viz. from ‘006 to ‘013 mm., so that we may conclude
| |
| that the latter region is on the average thicker than the
| |
| former (cf. figs. 73-76, with fig. 77 depicting a small portion
| |
| of the non-formative region). It is still impossible to determine the position of the sutural line, even in sections of
| |
| fragments of the wall known to contain it.
| |
| | |
| The entodermal mother-cells are not very readily recognisable in sections. In fig. 75, however, which is drawn
| |
| from an accurately transverse section through the formative
| |
| region of an '04 vesicle, there is depicted what is undoubtedly
| |
| an entodermal mother-cell {ent.). The interesting point
| |
| about this particular cell is that its cell-body, whilst still
| |
| intercalated between the adjoining cells of the unilaminar
| |
| wall, has extended inwards so as to directly underlie one of
| |
| the wall-cells. ' Division of such a cell as this would necessarily result in the production of an internally situated cell
| |
| with all the relations of one of the primitive entodermal type.
| |
| The inwardly projecting spheroidal cell situated immediately
| |
| to the left (in the figure) of the one just refeiTed to, I also
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 61
| |
| | |
| regard as an entodermal mother-cell. Cells of this type are
| |
| not infrequently met with in sections; they nsually stain
| |
| somewhat deeply, and are often found in mitosis.
| |
| | |
| The evidence obtainable from the study of in to to preparations conclusively proves that some at all events of the
| |
| primitive entodermal cells are actually derived from the entodermal mother-cells, much in the-way suggested above, whilst
| |
| others of the primitive entodermal cells are directly formed
| |
| from mother-cells which bodily migrate inwards.
| |
| | |
| Fig. 65, PI. 6, represents a small portion of the formative
| |
| region of an '04 vesicle viewed fPom the inner surface. In
| |
| the centre of the figure, surrounded by the larger, lighter
| |
| staining (ectodermal) cells of the wall, is a smaller cell in the
| |
| telophases of division, the cytoplasm of which is granular and
| |
| stains deeply. That cell unmistakably forms a constituent of
| |
| the unilaminar wall. I regard it as an entodermal mothercell. Fig. 66 shows another cell of the same character in the
| |
| anaphases of division, which likewise forms a constituent of
| |
| the unilaminar wall, but which differs from the corresponding
| |
| cell in fig. 65 in that its cytoplasmic body has extended out
| |
| on one side (lower in the figure), so as to directly underlie
| |
| part of an adjacent ectodermal cell. In other words we have
| |
| here a surface view of the condition represented in section in
| |
| fig. 75, only the entodermal mother-cell depicted therein is not
| |
| actually in process of division. Fig. 67, taken from the same
| |
| preparation as fig. 65, shows what I take to be the end result
| |
| of the division of such a cell as is i-epresented in the two
| |
| preceding figures. Here we see two small deeply staining
| |
| cells towards the centre of the figure, which from their disposition and agreement in size and cytological characters
| |
| are manifestly sister-cells, and the products of division of
| |
| just such an entodermal mother-cell as is represented in fig.
| |
| 65, or, better, fig. 66. The one cell (upper in the figure) is
| |
| more angular in form and manifestly still lies in the unilaminar wall; the other (lower in the figure) is ovalish in form
| |
| and is no longer a constituent of the unilaminar wall, but is
| |
| on the contrary a free cell, definitely internal both to the
| |
| | |
| | |
| 62
| |
| | |
| | |
| J. P. HILL. â– . â– :
| |
| | |
| latter and to its sister-cell. It is, in fact, a primitive entodermal cell, as comparison with fig. 68 proves, and that it has
| |
| been formed by the division of a mother-cell situated in the
| |
| unilaminar wall can hardly, I think, be doubted. Its sistercell, which is still a constituent of the wall, would presumably
| |
| have migrated inwai-ds some time later.
| |
| | |
| It is to be noted that the primitive entodermal cell referred
| |
| to above and those depicted in figs. 71 and 72 are definitely
| |
| contoured, ovalish and I'ounded cells, entirely devoid of processes. In these respects they differ markedly from the entodermal cells shown in fig^. 68-70, which are very variable in
| |
| form owing to their possession of more or less elongated
| |
| pseudopodial-like processes. It might thex'efore be inferred
| |
| that the formation of these processes only takes place after
| |
| the entodermal cells have become definitely internal. Such
| |
| an inference, however, would be incorrect, for I have abundant
| |
| evidence showing that such processes may be given off from
| |
| the entodermal mother- cells whilst they are still constituents
| |
| of the wall. In in toto preparations, it is often difficult to
| |
| determine with certainty whether a particular entodermal cell
| |
| still enters into the constitution of the unilaminar wall or not.
| |
| In the portion of the formative region of a '04 vesicle depicted
| |
| in fig. 70, however, I am satisfied that all the entodermal
| |
| cells therein shown (they are readily distinguishable by their
| |
| smaller size and more deeply staining character) are, with the
| |
| possible exception of the one on the extreme right, at least
| |
| partially intercalated between the larger ectodermal cells of
| |
| the wall. Some of them are entirely situated in the wall ;
| |
| others have extended inwards in varying degree so as to
| |
| partially underlie the ectodermal cells. It is these latter
| |
| entodermal cells in particular which exhibit the cytoplasmic
| |
| processes above referred to. As the figure shows, these processes have all the characters of pseudopodia,; they vary in
| |
| size, form, and number from cell to cell, individual processes
| |
| may be reticulate and their finer prolongations may anastomose with those of others, and they are formed of cytoplasm,
| |
| less dense and rather less, deeply staining than that of the
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 63
| |
| | |
| | |
| cell-bodies from which they arise. Attention may be specially
| |
| directed to the cell towards the left of the hgure (mai'ked ent.).
| |
| Here we have an entodermal cell whose cytoplasmic body is
| |
| evidently still partially intercalated between the cells of the wall,
| |
| but which is, at the same time, prolonged inwards (towards
| |
| the left) so as to underlie the adjoining ectodermal cell.
| |
| From this inward prolongation there are given off two slender
| |
| processes, one short and tapering, the other very much
| |
| longer ; this latter, after becoming vei'y attenuated, gradually
| |
| widens to form an irregular fan-shaped expansion, suckerlike in appearance, and produced into several slender
| |
| threads, which is situated adjacent to the nucleus of
| |
| the ectodermal cell on the extreme left. Then from the
| |
| right side of the same cell there is given off a small inwardly
| |
| projecting bulbous lobe which may well be the start of just
| |
| such another process as arises from the left side. Processes
| |
| of the peculiar sucker-like type just described, formed of a
| |
| slender elongated stem and a distal expanded extremity from
| |
| which delicate filamentous prolongations are given off, are
| |
| abundantly met with in preparations, and strikingly recall the
| |
| pseudopodia of various Ehizopoda. They are seen in connection with other entodermal cells in fig. 70, and with many
| |
| of those in fig. 68. I regard them as veritable pseudopodia.
| |
| Towards the right side of fig. 70 the two entodermal cells
| |
| there situated stand in direct protoplasmic continuity by
| |
| means of two slender connecting threads, whilst the upper of
| |
| these two cells is again joined by a very fine process to the
| |
| irregular pseudopodial expansion which arises from one of
| |
| the two entodermal cells situated nearer the middle of the
| |
| figure, and that same expansion is directly connected with the
| |
| second of the two entodermal cells just mentioned, so that we
| |
| have here established the beginning of a cell-network, prior
| |
| to the complete emancipation of its constituent entodermal
| |
| elements from the unilaminar wall. We have, then, clear
| |
| evidence that the entodermal elements in Dasyurus, prior to
| |
| their separation from the unilaminar formative region ai*e
| |
| capable of exhibiting amoeboid activity, since not only may
| |
| | |
| | |
| 64
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| | |
| they send lobose prolongations of their cytoplasmic bodies
| |
| inwards below the adjacent ectodermal cells, but they may
| |
| emit more or less elongated processes of indubitable pseudopodial character, which similarly lie in contact with the inner
| |
| surface of the wall-cells. Furthermore, we have evidence
| |
| that these pseudopodial processes may anastomose with each
| |
| â– other so as to initiate the formation of an entodermal reticulum,
| |
| whilst the cells from which they arise are still constituents of
| |
| the unilaminar wall - an especially noteworthy phenomenon.
| |
| Certain of the primitive entodermal cells, as we have seen,
| |
| are at first devoid of such processes, but since they all
| |
| eventually form part of a continuous reticulum, it is evident
| |
| that the entodermal elements are capable of emitting pseudopodial processes as well after as before their separation from
| |
| the formative region.
| |
| | |
| Finally, in view of the fact that the entodermal mothei'-cells
| |
| depicted in fig. 70 are not actually in process of division, and
| |
| therein differ from those of figs. 65 and 66, we may conclude
| |
| that the formation of the primitive entodermal cells is effected
| |
| either with or without the pi*evious division of the mother-cells.
| |
| | |
| If Ave admit, as I think on the evidence we must admit,
| |
| that the entodermal cells in Dasyurus are endowed with
| |
| amceboid properties, then Ave are relieved of any further
| |
| difficulty in regard to the mechanism of their inAvard migration
| |
| from the unilaminar Avail. Doubtless, in the case of those
| |
| entodermal mother-cells Avhich do not undergo division, the
| |
| precocious formation of the above-described pseudopodial
| |
| processes which spread out from the cells like so many
| |
| suckers considerably facilitates their direct detachment from
| |
| amongst the cells of the Avail. In the case of those primitive
| |
| entodermal cells Avhich originate as the direct products of
| |
| division of the mother-cells, it no doubt depends on a variety
| |
| of circumstances (e.g. actual form of the dividing cell,
| |
| direction of the spindle, etc.) whether they exhibit amoeboid
| |
| activity precociously (i.e. before their actual i separation), or
| |
| only at a later period.
| |
| | |
| The entoderm varies considerably in its degree of diffe
| |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| | |
| 65
| |
| | |
| | |
| rentiation iu different vesicles of this stage, and even in
| |
| different parts of the formative region of one and the same
| |
| vesicle. In some vesicles there are relatively few primitive
| |
| entodermal cells, in othei*s they are much more abundant.
| |
| Fig. 68, from the formative region of an ^04 vesicle, shows a
| |
| typical patch of them and illustrates very well the highest
| |
| stage of differentiation which they attain in these vesicles. The
| |
| entodermal cells therein depicted all appear to be definitely
| |
| internal, and it is especially worthy of note that the portion
| |
| of the unilaminar wall in relation to them is composed exclusively of the larger, lighter staining cells. It is these cells
| |
| which directly form the embryonal ectoderm of the blastocysts
| |
| next to be described. The entodermal cells are obviously
| |
| amoeboid in character (obsei've especially the cells near the
| |
| middle of the figure), and are in active process of linking
| |
| themselves together into a cellular reticulum. In fig. 69 is
| |
| shown a small portion of the formative region of another ^04
| |
| vesicle. A single entodermal mother-cell in process of
| |
| division occurs in position in the unilaminar Avail, which is
| |
| otherwise composed of ectodermal cells, whilst internally there
| |
| are present three entodermal cells, already linked together by
| |
| their pseudopodial processes. ^Jfiie two lowermost cells afford
| |
| especially striking examples of amoeboid activity, the elongated
| |
| pseudopodial process of the cell on the left terminating iu a
| |
| well-marked reticulation in definite continuity Avith the corresponding, but shorter and thicker process of the cell on the
| |
| right.
| |
| | |
| 3. Establishment of the Definitive Embryonal
| |
| | |
| Area.
| |
| | |
| FolloAving directly on the stage represented by the '04
| |
| blastocysts described in the preceding section is one designated in my list as 5, 18 . vii . 01 and referred to here as 5, '01.
| |
| It comprises twenty-two blastocysts obtained from a female
| |
| killed fifteen days after coition and all normal, Avith the
| |
| exception of one Avhich Avas shrivelled, and all in precisely
| |
| | |
| VOL. 56, PAllT 1. NEW SERIES. 5
| |
| | |
| | |
| 66
| |
| | |
| | |
| .T. P. HILL.
| |
| | |
| | |
| the same stag-e of development. They measured from 4‘5 to
| |
| 6 mm. in diameter.
| |
| | |
| In this stage the formative region of the preceding blastocysts has become transformed into the definitive embryonal
| |
| area (embryonic shield, Hubrecht) as the result of the completion of that process of inward migration of the entodermal
| |
| mother-cells which we saw in pi-ogress in the vesicles last
| |
| described, and the consequent establishment of the entoderm
| |
| as a continuous cell-layer undeidying and independent of, the
| |
| embryonal ectoderm constituted by the larger passive cells of
| |
| the original unilaminar formative layer.
| |
| | |
| In the entii*e blastocyst (PI. 4, fig. 39) the embryonal area
| |
| is quite obvious to the naked eye as the more opaque, hemispherical region, forming rather less than half the entire
| |
| extent of the vesicle wall ; the larger remainder of the same
| |
| is formed by the much more transpai-ent, non-formative or
| |
| extra-embryonal region. Sections of the entire blastocyst
| |
| show (1) that the embryonal area is bilaminar over its entii-e
| |
| extent, its outer layer consisting of embryonal ectoderm,
| |
| already somewhat thickened, its much thinner inner layer
| |
| consisting of entoderm, partly still in the form of a cellular
| |
| reticulum, and (2) that the extra-embryonal region is still
| |
| unilaminar throughout and composed of a relatively thin
| |
| layer of flattened cells (extra-embryonal or trophoblastic ectoderm, trophoblast [Hubrecht])^ (PI. 8, fig. 78). The entoderm
| |
| is co-extensive at this stage with the embryonal ectoderm,
| |
| and terminates in a wavy, irregularly thickened, free, edge
| |
| (PI. 5, fig. 49), which over most of its extent either directly
| |
| underlies or extends very slightly beyond the line of junction
| |
| between the embryonal and extra-embryonal ectoderm. The
| |
| junctional line is thus not very easily seen. In fig. 48, however,
| |
| | |
| ' In consonance witli my conviction that this layer is homologous
| |
| both Avith the so-called trophoblast of Eutheria and the exti-a-embryonal
| |
| ectoderm of Prototheria, and in view of the theoretical signification
| |
| which Hubrecht now insists should be attached to the term “ trophoblast.†and which I am wholly unable to accept, I venture to suggest as
| |
| an alteiTiative name for this layer that of “ tropho-ectoderni.
| |
| | |
| | |
| THE EARLY DEVELOPMEI^T OF THE MARSUPIALIA.
| |
| | |
| | |
| 67
| |
| | |
| | |
| a small portion oP the line shows with sufficient distinctness, I
| |
| think, to demonstrate its identity with that of the preceding
| |
| stage.
| |
| | |
| The vesicle wall in all my sections of this stage appears
| |
| to be somewhat thinner than that of the '04 blastocysts, but
| |
| apart from this apparently variational difference the present
| |
| blastocysts are almost exactly intermediate between the latter
| |
| and those next to be described.
| |
| | |
| The embryonal ectoderm (fig. 78, emb. ect.) appears in
| |
| section fairly uniformly thickened, though its cells are still of
| |
| the flattened type. In surface view in in toto preparations
| |
| (cf . fig. 48), they exhibit the same polygonal form and lightly
| |
| staining qualities as the larger cells of the formative region
| |
| of the '04 blastocysts, which we have already identified as
| |
| prospective embryonal ectodermal cells. The junctional line
| |
| between the embryonal ectoderm and the extra-embryonal is
| |
| now for the first time readily distinguishable in sections
| |
| (fig. 78). The extra-embryonal ectoderm (tropho-ectoderm)
| |
| (PI. 5, figs. 48 and 49, PI. 8, fig. 78, tr. ect.) differs in no
| |
| essential respect from the corresponding layer in the '04
| |
| blastocysts.
| |
| | |
| The entoderm in these blastocysts is exceedingly closely
| |
| adherent to the inner surface of the embryonal ectoderm and
| |
| cannot be removed therefrom by artificial means. It varies
| |
| slightly in its character in different vesicles and in different
| |
| parts of its extent in the same vesicle. Mostly it appears as
| |
| a continuous thin cell-layer (figs. 49 and 78, ent.), but here and
| |
| there patches occur in which the cells form a reticulum quite
| |
| similar to that shown in fig. 68 of the preceding stage.
| |
| | |
| The next stage (designated in my list as 8 . vi . 01), and the
| |
| last of Dasyurus that need be described in the present communication, comprises eleven vesicles (5-5'5 mm. in diameter),
| |
| in which the embryonal area is conspicuous and distinctly in
| |
| advance of that of the preceding vesicles, but is still devoid
| |
| of any trace of embryonal differentiation (PI. 4, fig. 40;
| |
| PI. 8, fig. 79).
| |
| | |
| The embryonal area is hemispherical in form (its greatest
| |
| | |
| | |
| 68
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| diameter varying' from 3'5 to 4 mm.) in all except two of the
| |
| blastocysts, in which it is elongate, with longer and shorter
| |
| diameters. It occupies about a third or less of the entire
| |
| extent of the vesicle wall, and thus appears relatively smaller
| |
| than that of the preceding (.5, '01) vesicles. The entoderm now
| |
| extends for a distance of about 1 mm. beyond the limits of
| |
| the area, so that in the entire vesicle (fig. 40) three zones
| |
| differing in opacity are distinguishable, viz. the dense hemispherical zone at the upper pole, constituted by the embryonal
| |
| area; below that, a less dense, narrow annular zone, formed of
| |
| extra-embryonal ectoderm and the underlying peripheral
| |
| extension of the entoderm ; and finally, the still less dense
| |
| hemispherical area, forming the lower hemisphere of the
| |
| blastocyst and constituted, solely by extra-embryonal ectoderm. Thus approximately the upper half of the blastocyst
| |
| is bilaminar, the lower half unilaminar. Sections show that
| |
| the embryonal ectoderm (fig. 79, emh. ect.) is now a quite
| |
| thick layer of approximately cubical cells, whilst the extraembryonal ectoderm {tr. ect.) is formed of relatively thin
| |
| flattened cells. The line of junction between the two is perfectly obvious, both in sections (fig. 79) and in surface view
| |
| (PI. 5, fig. 50). The embryonal ectodermal cells, though
| |
| much thicker than the extra-embryonal, are of less superficial
| |
| extent; their nuclei therefore lie closer together than those
| |
| of the latter, moreover they are larger, stain more deeply, and
| |
| are more frequently found in division, all of which facts
| |
| testify to the much greater growth-activity of the embryonal
| |
| as compai'ed with the exti-a-embryonal ectoderm at this stage
| |
| of development (cf . fig. 50, emh. ect. and tr. ect.-, in the preparation from which this micro-photograph was made the entoderm
| |
| underlying the embryonal ectoderm has been removed, whilst
| |
| it is still partially present over the extra-embryonal ectoderm).
| |
| | |
| The entoderm (fig. 79, ent.) over the region of the embrvonal area is readily separable as a quite thin membrane,
| |
| and is then seen to consist of squamous cells, polygonal in
| |
| outline, and either in direct apposition by their edges or connected together by minute cytoplasmic processes. Beyond the
| |
| | |
| | |
| THE EARLY DEVELOPMENT OE THE MAESUPIALIA.
| |
| | |
| | |
| 69
| |
| | |
| | |
| embryonal area, liowever, its peripliei'al extension below the
| |
| extra-embryonal ectoderm is much less easily separable in the
| |
| intact condition (cF. fig. 50), because oF its greater delicacy
| |
| due to the fact that it has here largely the form of a cellular
| |
| reticulum. In this extra-embryonal region the entodermal
| |
| cells are frequently found in mitosis. Ic would appear, then,
| |
| that the entoderm is first laid down in the region of the embi'yonal area as a cellular reticulum, which later becomes
| |
| ii'ansformed into a continuous cell-membrane, and that its
| |
| peripheral extension over the inner surface of the extraembryonal ectoderm is the result of the growth and activity
| |
| oF its own constituent cells.
| |
| | |
| This peripheral growth continues until there is formed
| |
| eventually a complete entodermal lining to the blastocyst
| |
| cavity. The rate of growth appears to be somewhat variable.
| |
| In a series of primitive streak vesicles (6-6'75 mm. in diameter)
| |
| the lower third oF the wall is, I find, still unilaminar. In
| |
| another series of vesicles of the same developmental stage
| |
| (4'5-6 mm. in diameter) a unilaminar area is present at the
| |
| lower pole, varying from I x ‘5 mm. in diameter to as much
| |
| as 4 mm. Even in vesicles 7-7'5 mm. in diameter a unilaminar patch may still occur at the lower pole, but in vesiqles
| |
| 8'5 mm. in diameter (stage of fiat embryo) the entodermal
| |
| lining appears always to be complete.
| |
| | |
| The Origin of the Entoderm in Eutheria. - The
| |
| remai'kable facts relative to the origin of the entoderm in
| |
| Dasyurus which I have been able to place on record in the
| |
| jireceding pages, thanks to the large size attained by the
| |
| blastocyst prior to the differentiation of the formative germlayers and to the circumstance that the formative cells are
| |
| not arranged, as they are in Eutheria, in the form of a more
| |
| or less compact cell-mass, but constitute a thin unilaminar
| |
| cell-layer of relatively great extent which can easily be cut
| |
| up with scissors, and which, after staining and mounting on
| |
| the fiat can be examined under the highest powers, throw, it
| |
| seems to me, a new and unexpected light on the mammalian
| |
| entoderm, and at the same time help to fill the considerable
| |
| | |
| | |
| 70
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| gap whicli has hitherto existed in our knowledge of its early
| |
| ontogenesis. Although the mode of origin of the entoderm
| |
| in Dasyurus would appear, in the present state of our knowledge, to find its closest parallel, not amongst vertebrates, but
| |
| in certain invertebrates (cf . the mode of origin of the entodermal cells from the wall of the blastula in Hydra as
| |
| described by Brauer^), the observations of Assheton ('94)
| |
| on the early history of the entoderm in the rabbit, when
| |
| viewed in the light of the foregoing, seem to me to afford
| |
| ground for the belief that phenomena comparable with those
| |
| hei'e recorded for Dasyui'us will eventually be recognised as
| |
| occurring also in Eutheria.
| |
| | |
| Hubrecht ('08), in his recent treatise on early Mammalian
| |
| ontogeny, deals very briefly with the question of the origin
| |
| of the entoderm in the latter group, merely stating that
| |
| “ from the inner cell-mass arises by delamination a separate
| |
| lower layer which we designate as the entoderm of the
| |
| embryo. These entoderm cells wander in radial direction
| |
| along the inner surface of the trophoblast, which in many
| |
| cases is thus soon transformed into a didermic structure.
| |
| | |
| . . . When the entoderm has separated off by delamina
| |
| tion from the embryonic knob, the remaining cells of the
| |
| latter form the ' embryonic ectodei'm,' which is thus situated
| |
| between the entoderm and the trophoblast.â€
| |
| | |
| Assheton, in the paper just referred to, has given a careful
| |
| account of the first appearance of the entodermal cells in the
| |
| rabbit, and of what he believes to be the mode of their
| |
| peripheral extension below the trophoblastic wall of the
| |
| blastocyst. He shows that the inner cell-mass, at first
| |
| spherical, gradually, as the blastocyst enlarges, fiattens out
| |
| below the “ covering layer †of the trophoblast until it forms
| |
| an approximately circular plate “ nowhere more than two
| |
| cells thick.†During the process of flattening, cells are seen
| |
| to jut out from the periphery of the mass; these eventually
| |
| separate, and appear as rounded cells scattered irregularly
| |
| over the inner surface of the trophoblast and ‘^extending
| |
| ' ‘ Zeitschi'. f. wiss. Zool.,' Bd. Hi, 1891.
| |
| | |
| | |
| THE EAHLY DEVELOPMENT OF THE MAPSUPIALIA. 71
| |
| | |
| | |
| over an arc of about 60° from the upper pole in all directions.â€
| |
| These “ straggling†cells, as Assheton terms them, as well as
| |
| the innermost cells of the now flattened inner cell-mass, are
| |
| regarded as hypoblastic and the outermost cells of the same
| |
| as epiblastic (embryonic epiblast). “The hypoblast, as a
| |
| perfectly definite layer, is formed by the time the blastodermic vesicle measures '5 mm. in diameter, that is, about the
| |
| 102nd hour after coition. It is not, however, as yet by any
| |
| means a continuous membrane ; it is a network or fenestrated
| |
| membrane. For this reason, in section it appears to be
| |
| represented by isolated cells lying beneath the embryonic
| |
| disc (v. fig. 29, Sy.)†(cf. Dasyurus). In considering the
| |
| question how the peripherally situated (“ straggling â€) entodermal cells, which are undoubtedly derived from the inner
| |
| cell-mass, “apparently Avander round the inside of the blastodermic vesicle,†he I'eaches the conclusion that this is not the
| |
| result of amoeboid activity or growth “in the sense of migration †on the part of these cells, but “ is only an apparent
| |
| growth round produced by the more rapid growth of a
| |
| zone of the [trophoblastic] wall of the vesicle immediately
| |
| surrounding the embryonic disc, in which zone the marginal
| |
| cells of the inner mass lie.†He is unable to find any
| |
| evidence of the production of pseudopodial processes by
| |
| these pei'ipheral entodermal cells, the majority of them
| |
| appearing at first to be quite isolated from each other and
| |
| approximately spherical. “Certain of the cells here and
| |
| there are connected by threads of protoplasm, but this, I
| |
| think, is not a sign of pseudopodic activity, but merely
| |
| indicates the final stage in division betAveen the tAvo cells.â€
| |
| By the sixth day the hypoblast of the embryonic disc has
| |
| assumed the lorm of a continuous membrane, composed of
| |
| completely flattened cells, Avhilst the peripheral hypoblast
| |
| cells have become more numerous, and “many of them,
| |
| possibly all of them, are noAV undoubtedly connected by more
| |
| or less fine protoplasmic threads.†Such, in brief, is
| |
| Assheton's account of the early history of the entodenn in
| |
| the rabbit; it presents obvious points of agreement with my
| |
| | |
| | |
| 72
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| own for Dasyunis, and I ventui'o to think the agreement is
| |
| even greater than would appear from Assheton's conclusions.
| |
| In adopting- the view that the more active growth of the
| |
| region of the blastocyst wall immediately surrounding the
| |
| inner cell-mass is the sole causal agent in effecting the separation and peripheral spreading of the entodermal cells, I cannot
| |
| but feel, in view of his own description and figures and of my
| |
| own results, that he has attributed a much too exclusive importance to that phenomenon and a much too passive role to the
| |
| entodermal cells themselves. In Dasyurus the inward migration and the later peripheral spreading of the entodermal
| |
| cells is effected without any such marked unequal growth of
| |
| tlie blastocyst wall as occurs, according to Assheton, in the
| |
| rabbit, as the direct outcome of their owu inherent activity,
| |
| and I believe the possession of a like activity characterises
| |
| the entodermal cells of the rabbit. The evidence of Assheton's
| |
| own fig. 40, which shows in surface view a portion of the
| |
| vesicle wall with the peripheral entodermal cells in relation
| |
| thereto, and which should be compared with my figs. 68 and
| |
| 69, conclusively demonstrates, to my mind, the possession by
| |
| these cells of amoeboid properties, and thus support is
| |
| afforded for the belief that the separation of the entodermal
| |
| cells from the formative cell group (inner cell-mass) is here
| |
| also the expression of an actual migration. Whether or not
| |
| the strands of protoplasm which Assheton ('08, '09) describes
| |
| as present in the sheep, pig, ferret, and goat, connecting the
| |
| inner lining of the inner mass to the wall of the blastocyst,
| |
| and which he interprets as tending “ to show that the inner
| |
| lining of the inner mass is of common origin with the wall of
| |
| the blastocyst,†are of any significance in the present connection, I cannot certainly determine.
| |
| | |
| 4. Summary.
| |
| | |
| The results and conclusions set forth in the preceding
| |
| pages of this chapter may be summarised as follows;
| |
| | |
| (1) The unilaminar wall of the blastocyst of Dasyurus con
| |
| | |
| THE EARLY DEVELOPMENT OE THE MARSUPIALIA.
| |
| | |
| | |
| 73
| |
| | |
| | |
| sists of two regions distinct in origin and in destiny, viz. an
| |
| upper or formative region, derived from the upper cell-ring
| |
| of the 16-celled stage, and destined to furnish the embryonal ectoderm and the entoderm and a lower or nonformative region derived fi-om the lower cell-ring of the
| |
| mentioned stage, and destined to form directly the extraembryonal or trophoblastic ectoderm (tropho-ectoderrn) of the
| |
| bilaminar vesicle.
| |
| | |
| (2) The formative region, unlike the non-formative, is
| |
| | |
| constituted by cells of two varieties, viz. : (i) a more
| |
| | |
| numerous series of larger, lighter-staining' cells destined
| |
| to form the embryonal ectoderm, and (ii) a less numerous
| |
| series of smaller, more granular, and more deeply staining
| |
| cells, destined to give origin to the entoderm and hence
| |
| distinguishable as the entodermal mother-cells.
| |
| | |
| (3) The entodermal mother-cells, either without or subsequently to division, bodily migrate inwards from amongst the
| |
| larger cells of the unilamiuar wall and so come to lie in
| |
| contact with the inner surface of the latter. Tkey thus give
| |
| origin to the primitive entodermal cells from which the
| |
| deKnitive entoderm arises. The larger passive cells, which
| |
| alone form the unilamiuar wall after the inward migration of
| |
| the entodermal cells is completed, constitute the embryonal
| |
| ectoderm.
| |
| | |
| (4) The entodermal cells as well before as after their
| |
| migration from the unilamiuar wall are capable of exhibiting
| |
| amoeboid activity and of emitting pseudopodial processes, by
| |
| tlie anastomosing of which there is eventually formed a
| |
| cellular entodermal reticulum underlying, and at first coextensive with, the embryonal ectoderm.
| |
| | |
| (5) The definitive entoderm thus owes its character as a
| |
| connected cell-layer primarily to the formation of secondaiy
| |
| anastomoses between the pseudopodial processes emitted by
| |
| the primitive entodermal cells (or entodermal mothercells).
| |
| | |
| (6) The assumption by the entodermal cells of amoeboid
| |
| j^roperties whilst they are still constituents of the unilamiuar
| |
| | |
| | |
| 74
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| wall affords an intelligible explanation of the mechaiiisin of
| |
| their inwai'd migration.
| |
| | |
| (/) The entoderm is first laid down below the formative or
| |
| embryonal region of the blastocyst; thence it extends gradually by its own growth round the inner surface of the uuilaramar non-forrnative region so as to form eventually a
| |
| complete entodermal lining to the blastocyst cavity. In this
| |
| way the blastocyst wall becomes bilaminar throughout.
| |
| | |
| (8) The bilaminar blastocyst consists of two reguous, respectively embryonal and extra-embryonal. The embryonal
| |
| region (embryonal area) is constituted by an outer layer of
| |
| embryonal ectoderm and the underlying portion of the entoderm, and the extra-embryonal, of the extra-embrvonal or
| |
| trophoblastic ectoderm (tropho-ectoderm), which is separated
| |
| from the embryonal by a well-marked junctional line, together
| |
| with the underlying portion of the entoderm, which is perfectly continuous with that below the embryonal ectoderm.
| |
| | |
| (9) The formative or embryonal region of the blastocyst
| |
| in Dasyurus is from the first freely exposed, and at no time
| |
| daring the developmental period dealt with in this paper
| |
| does there exist any cellular layer externally to it, i. e. a
| |
| covering layer of trophoblast (Deckschicht, Kauber's layer)
| |
| is absent and there is no entypy of the primary germ-layers
| |
| (cf. p. 111).
| |
| | |
| | |
| Chapter V. - Some Early Stages op Perameles and
| |
| | |
| Macropds.
| |
| | |
| The early material of Perameles and Macropus at my
| |
| disposal comprises only a small number of stages, but is of
| |
| special importance, since it enables me to demonstrate that
| |
| so far as these particular stages are concerned, the early
| |
| developmental phenomena in these forms are essentially the
| |
| same as in Dasyurus, and thus affords ground for the belief
| |
| that there is one common type of early development throughout the series of the Marsupialia. Moreover, it is of interest
| |
| since it reveals the. existence of what might be termed
| |
| | |
| | |
| 75
| |
| | |
| | |
| THE EAELY DEVELOPMENT OP THE MAESUPI ALIA.
| |
| | |
| specific differences in the early development of these Marsupials, especially in regard to the time of appearance of the
| |
| entoderm. In Dasyurus, it will be remembered, the primitive
| |
| entoderm cells first become definitely recognisable as internally situated cells in vesicles 4‘5 mm. in diameter. In
| |
| Perameles they occur in vesicles just over 1 rnm. in diameter,
| |
| while in Macropus they are already present in a blastocyst
| |
| only -35 mm. in diameter, so that it would appear that the
| |
| entoderm is differentiated much earlier in the higher, more
| |
| specialised types than in the more generalised forms. This
| |
| difference in time of appearance of the entoderm is perhaps
| |
| to be correlated with a difference in size of the ovarian ova
| |
| in the three genera mentioned.
| |
| | |
| | |
| 1. Perameles.
| |
| | |
| The earliest material of Perameles I possess consists of two
| |
| eggs of P. obesula, which I owe to the skill and enthusiasm
| |
| of my friend Mr. S. J. M. Moreau, of Sydney. Egg -Ameasures '23 mm. in diameter, and egg B, ‘24 x ‘23 mm.
| |
| The former consists of thirty-two cells, the latter of thirty. In
| |
| both the shell-membrane has partially collapsed, but the general
| |
| plan of arrangement of the blastomeres can still fairly readily
| |
| be made out. Fig. 51, PI. 3, represents a micro-photograph
| |
| of a section of egg B, the better of the two. It shows the
| |
| .shell-membrane (nearly '005 mm. thick) externally, considerable remains of the albumen between that and the
| |
| deeply stained zona, and then, closely applied to the inner
| |
| surface of the latter, the blastomeres arranged in the form of
| |
| an inverted D, so as to enclose a central space, open below
| |
| as the figure stands. This latter opening extends through
| |
| the series, and it seems probable that there was a corx*esponding one opposite to it in the intact egg. Evidently we
| |
| have hei'e a stage in the formation of the blastocyst, in which
| |
| the blastomeres are in course of spreading towards one or
| |
| both of the poles of the sphere formed by the egg-envelopes.
| |
| | |
| | |
| 76
| |
| | |
| | |
| J. r, HILL,
| |
| | |
| | |
| â– just as liappeus in the corresponding' stage of Dasyurus (cf.
| |
| fig. 51 with fig. 31j though the latter represents a somewhat
| |
| older stage in Dasyurus). The blastocyst-wall here appears
| |
| relatively more extensive than in the 32-celled stage of
| |
| Dasyurus, an apparent difference which may perhaps be accounted for by the difference in size of the respective eggs
| |
| (•24 mm. as compared with '36 mm.) . The blastomeres situated
| |
| adjacent to the opening and those on the right side of the
| |
| figure tend to be more flattened and of greater superficial extent than the remainder, but I can recognise no
| |
| difference in the cytological characters of the cells. The
| |
| space or cleavage cavity enclosed by the blastomeres is partly
| |
| occupied by a granular coagulum, and towards the opening
| |
| there is present a lightly staining reticular mass, which
| |
| i*ecalls the yolk-body of Dasyurus, though I am not prepared
| |
| to affirm that it is of that significance. The fixation of the
| |
| specimen is not quite perfect.
| |
| | |
| My next stage of Perameles is constituted by a blastocyst
| |
| of P. nasuta, for which I am again indebted to Mr. Moreau
| |
| measuring in the preserved condition '29 x '26 mm. Pig. 52,
| |
| PI. 3, shows a section of this blastocyst. Structurally,
| |
| it corresponds in all essential respects with the '43 mm.
| |
| blastocyst of Dasyurus, figured on the same plate (fig. 33).
| |
| The blastocyst Avail is complete and unilamiuar throughout.
| |
| It is distinguishable into tAvo regions, a more extensive region
| |
| over Avhich the cells are large and flattened and a less extensive,
| |
| composed of smaller but thicker cells (left side of fig. 52).
| |
| In the early blastocysts of Dasyurus, it may be recalled, the
| |
| evidence showed that the region of more flattened cells is
| |
| formative in significance, that of more bulky cells, non-formative. It is possible the same holds good for this Perameles
| |
| blastocyst. On the other hand, the structural condition of
| |
| the stage next to be described rather supports the vieAv that
| |
| the smaller region, composed of plumper cells, is in this case
| |
| formative. That view seems to me the more probable of the
| |
| two, but there is a considerable difference in size betAveen the
| |
| present blastocyst and those next available, so that it is
| |
| | |
| | |
| THE EAKLy DEVELOPMENT OP THE MARSUPIALIA.
| |
| | |
| | |
| 77
| |
| | |
| | |
| impossible to decide this point witli certciinty. The blastocyst cavity is partly occupied by coagulnm. There are no
| |
| cells present in it, but the question of the presence of a yolkbody must remain open. The shell-membrane (‘0045 mm. in
| |
| thickness) and zona are in close apposition.
| |
| | |
| Following this early blastocyst, I have three vesicles of
| |
| P. nasuta, two of them measuring 1‘3 mm. in diameter,
| |
| the other PI mm. In their stage of development they
| |
| agree pretty closely with the 4'5-5 mm. vesicles of Dasyurus,
| |
| referred to in the preceding pages under the designation
| |
| 6, '04, the entoderm being in process of differentiation. The
| |
| formative region was readily distinguishable in the intact
| |
| vesicles as a darker patch occupying about three eighths of
| |
| the surface extent of the wall. In section (PI. 8, figs. 80, 81)
| |
| it is characterised by its greater thickness as compared with
| |
| the non-formative or trophoblastic region, and by the
| |
| presence below it of numbers of primitive entodernial cells.
| |
| Compared with the corresponding stage in Dasyurus, the
| |
| chief difference consists in the relatively much greater thickness of the cells of the formative region in the Perameles
| |
| vesicle. The latter cells are here already more or less definitely cubical in shape, their thickness varying from '09
| |
| mm. to '015 mm., and altogether they form a layer of a much
| |
| more uniformly thickened character than that of the 6, '04
| |
| vesicles of Dasyurus. The trophoblastic ectoderm (figs. 80,
| |
| 81, tr. ect.) is composed of somewhat flattened cells, .varying
| |
| in thickness from ‘005 to '008 mm.
| |
| | |
| The primitive entodermal cells (figs. 80, 81, ent.) are
| |
| present below the formative region in fair abundance, more
| |
| especially around the periphery of the same, which may thus
| |
| appear somewhat thickened (fig. 81). 4'he cells vary in size
| |
| from ‘01 X '007 mm. to '024 x '009 mm., and they stain on the
| |
| whole somewhat more deeply than the formative cells, to
| |
| whose under-surface they are closely applied. They occur
| |
| groups. Mitotic figures are frequently met
| |
| with in the cells of the formative ai'ea (observe the obliquely
| |
| disposed figure in one of the formative cells in fig. 81), and
| |
| | |
| | |
| 78
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| | |
| â– they also occur in the primitive entodermal cells. Examination of the sections leaves no doubt in one's mind as to the
| |
| source of the entodermal cells. They are undoubtedly derived
| |
| from the formative region of the vesicle wall. The 'shellmembrane has a thickness of about '0027 mm.
| |
| | |
| 2. Macro pus.
| |
| | |
| Of Macropus the earliest stage I have examined is a blastocyst of M. ruficollis, -25 x *21 mm. in diameter. It is not
| |
| in a quite perfect state of preservation, but is in a sufficiently
| |
| good condition to enable me to say that the wall is complete
| |
| and unilaminar throughout, just as in the ‘29 x "26 mm.
| |
| blastocyst of Perameles. The shell-membrane has a thickness
| |
| of about -005 mm., and there are still remains of the albumen
| |
| between it and the zona.
| |
| | |
| My next stage (figs. 82-85) is a blastocyst of the same
| |
| species, *35 mm. in diameter. It unfortunately suffered in
| |
| preparation, but practically the whole of the formative area
| |
| of the blastocyst wall and part of the trophoblastic ectoderm
| |
| are comprised in the sections (PI. 9, fig. 82), so that it is still
| |
| possible to make out its chief structural features. In its stage
| |
| of development this blastocyst closely agrees with the last
| |
| described blastocysts of Perameles. The formative area of
| |
| the wall is perfectly distinct in the sections because of its
| |
| greater thickness and the presence below it of the primitive
| |
| entodermal cells. It attains its gi-eatest thickness (*027 mm.)
| |
| peripherally, whilst it is thinnest centrally (*006 mm.), so that,
| |
| taken as a whole, it is not quite such a uniformly thickened
| |
| layer as is that of the Perameles blastocysts. Primitive entodermal cells are present below it, but not in great abundance
| |
| (figs. 82, 84, 85, ent.). In fig. 83, a formative cell is seen in
| |
| division, the axis of the spindle being oblique to the surface.
| |
| The trophoblastic ectoderm (figs. 82, 83, tr. ect.) is composed
| |
| of the usual flattened cells, and varies in thickness from
| |
| *005 to *0067 mm.
| |
| | |
| In the blastocyst cavity, adjacent to the trophoblastic
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP TliE MARSDPIALIA.
| |
| | |
| | |
| 79
| |
| | |
| | |
| ectoderm on the left side of fig. 82, there is visible a small
| |
| spherical cell similar to the degenerate cells met with in
| |
| blastocysts of Dasyurus.
| |
| | |
| My last stage of M. ruficollis comprises an excellently
| |
| preserved blastocyst, measuring '8 mm. in diameter, in which
| |
| the embryonal ectoderm and the entoderm are definitely
| |
| established. It thus corresponds to the 8, '01 stage of
| |
| Dasyurus (blastocysts o - 5'5 mm. diameter). The embryonal
| |
| area is circular and measures '468 mm. in diameter. Its
| |
| constituent cells are cubical and from '008 to ‘OlS mm. in
| |
| thickness, Avhilst the trophoblastic ectoderm is formed of
| |
| flattened cells, -006 ram. in thickness. The entoderm is
| |
| present as a continuous layer of attenuated cells below the
| |
| embryonal ectoderm, and it probably also forms a continuous
| |
| layer below the trophoblastic ectoderm. Entodermal cells are
| |
| certainly pi*esent over the lower polar region of the vesicle,
| |
| but it is difficult to be certain from the sections whether or not
| |
| they form a perfectly continuous layer. The shell membrane
| |
| has a thickness of •0026 mm.
| |
| | |
| I have a corresponding blastocyst of Petrogale penicillata •915 mm. in diameter, with an oval, embryonal area
| |
| •525 X ^45 mm. in diameter, and a later blastocyst of M.
| |
| ruficollis P46 mm. in diameter, with a circular embryonal
| |
| area '57 mm. in diameter.
| |
| | |
| Chapter VI. - General Summary and Conclusions.
| |
| | |
| The observations recoi'ded in the pi'eceding pages and the
| |
| conclusions deducible therefrom may be summarised as
| |
| follows ;
| |
| | |
| (a) Ovum. - The uterine ovum of Dasyurus is characterised
| |
| (1) by its large size relatively to those of Eutheria; (2) by
| |
| the presence externally to the zona of a layer of albumen and
| |
| a shell-membrane, both laid down in the Fallopian tube and
| |
| homologous with the corresponding structures in the Mouotreme ovum, the shell-membrane, like the shell of the latter,
| |
| inci'easing in thickness in the uterus; (3) by its marked
| |
| | |
| | |
| 80
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| polarity, its lower two thirds consisting of formative cytoplasm, dense and finely granular in appearance, owing to the
| |
| presence of fairly uniformly distributed deutoplasmic material,
| |
| and containing the two pronuclei, its upper third being
| |
| relatively clear and transparent, consisting as it does of a
| |
| delicate reticulum of non-formative cytoplasm, the meshes of
| |
| which are occupied by a clear deutoplasmic fluid. Study of
| |
| the pi'ocess of vitellogenesis in ovarian ova demonstrates that
| |
| this fluid represents surplus deutoplasmic material which has
| |
| not been utilised in the upbuilding of the formative region of
| |
| the ovum.
| |
| | |
| The fate of the clear non-formative portion of the ovum is
| |
| a very remarkable one. Prior to the completion of the first
| |
| cleavage, it is separated off from the formative remainder of
| |
| the ovum as a spherical mass or yolk-body, Avhich takes no
| |
| direct part in development, though it becomes enclosed iu the
| |
| blastocyst cavity on completion of the blastocyst wall at the
| |
| upper pole. Its contained deutoplasmic fluid is to be regarded
| |
| as the product of an abortive attempt at the formation of a
| |
| solid yolk-mass, such as is found in the Monotreme ovum.
| |
| By its elimination the potentially yolk-laden telolecithal ovum
| |
| becomes converted into a secondarily homolecithal, holoblastic
| |
| one. All the evidence is held to support the conclusion that
| |
| the Marsupials are descended from oviparous ancestors with
| |
| ineroblastic ova.
| |
| | |
| (b) Cleavage. - Cleavage begins in the uterus, is total, and
| |
| at first equal and of the radial type. The first two cleavage
| |
| planes are meridional and at right angles to each other.
| |
| The resulting four equal-sized blastomeres lie disposed radially
| |
| around the polar diameter like those of the Monotreme (not
| |
| in pairs at right angles to each other as in Eutheria), and
| |
| enclose a segmentation cavity open above and below, their
| |
| upper ends partially surrounding the yolk-body. The third
| |
| cleavage planes are again meridional, each of the four blastomeres becoming subdivided equally into two. The resulting
| |
| eight cells form an equatorial ring in contact with the inner
| |
| surface of the sphere formed by the egg-envelopes. They
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. . »1
| |
| | |
| contain deutoplasmic material, which is, however, located
| |
| mainly in their lower halves. The ensuing fourth cleavages
| |
| are equatorial, and in correlation with the just-mentioned
| |
| disposition of the deutoplasm, are unequal and qualitative,
| |
| each of the eight blastoraeres becoming subdivided into an
| |
| upper smaller and clearer cell, with relatively little deutoplasm fairly uniformly dispersed through the cytoplasm, and
| |
| a lower larger, more opaque cell Avith much deutoplasm,
| |
| mainly located in a broad zone in the outer portion of the
| |
| cell-body. A 16-celled stage is thus produced in which the
| |
| blastomeres are characteristically arranged in two superimposed rings, each of eight cells, an upper of smaller, clearer
| |
| cells next the yolk-body, and a lower of larger, denser cells.
| |
| The former is destined to give origin to the formative or
| |
| embryonal region of the blastocyst wall, the latter to the
| |
| non-formative or extra-embryonal region of the same.
| |
| | |
| (c) Formation of the Blastocyst. - There is in the
| |
| Marsupial no morula stage as in Butheria, the blastomeres
| |
| proceeding directly to form the wall of the blastocyst. The
| |
| cells of the two rings of the 16-celled stage divide at first
| |
| meridionally and then also equatorially, the division planes
| |
| being always vertical to the surface. The daughter-blastomeres so produced, continuing to divide in the same fashion,
| |
| gradually spread towards opposite poles in contact with
| |
| the inner surface of the fii-m sphere formed by the zona and
| |
| the thickened shell-membrane. Eventually they form a complete cellular lining to the said sphere and it is this which
| |
| constitutes the wall of the blastocyst. The latter is accordingly unilaminar at its first origin, and it remains so in
| |
| Dasyurus until it has attained, as the result of active gi'owth
| |
| accompanied by the imbibition of fiuid from the uterus, a
| |
| diameter of 4-5 mm. It consists of two parts or regions,
| |
| distinct in origin and in destiny, and clearly marked off from
| |
| each other in later blastocysts by a definite junctional line
| |
| approximately equatorial in position, viz. an upper, embryonal
| |
| or formative region derived from the upper cell-ring of the
| |
| 16-celled stage, and a lower, extra-embryonal or nonVOL. 56, PART 1. NEW SERIES. 6
| |
| | |
| | |
| 82
| |
| | |
| | |
| J. P, HILL.
| |
| | |
| | |
| formative region derived from tlie lower cell-ring of the same
| |
| stage.
| |
| | |
| (d) Later History of the Two Region s of the Blastocyst Wall (for details see pp. 72-74). - From the embryonal
| |
| region are derived the embryonal ectoderm and the entire
| |
| entoderm of the vesicle. I conclude^ therefore, that it is tlie
| |
| homologue of the inner cell-mass or embryonal knot of the
| |
| Eutherian blastocyst. The extra-embryonal region directly
| |
| furnishes the outer extra-embryonal layer of the vesicle wall,
| |
| i. e. the outer layer of the omphalopleure and chorion of later
| |
| stages. Assuming, as the facts of comparative anatomy and
| |
| palaeontology entirely justify us in doing, that the Mammals
| |
| are monophyletic and of reptilian origin, and further assuming
| |
| that the foetal membranes are homologous structures throughout the Amniotan series (also in my view a perfectly
| |
| justifiable assumption)^, then the homologies of this extraembryonal region of the Marsupial blastocyt are not far to
| |
| seek. It is clearly the homologue of the extra-embryonal
| |
| ectoderm of the Sauropsidan and Monotreme egg, and the
| |
| homologue also of the outer enveloping layer of the Eutlierian
| |
| blastocyst, to which Hubi'echt has given the special name of
| |
| “ trophoblast.†In my view the trophoblast is none other
| |
| than extra-embryonal ectoderm which in the viviparous
| |
| mammals, in correlation with the intra-uterine mode of
| |
| development, has acquired a special significance for the
| |
| nutrition of the embryo.
| |
| | |
| These, then, are my conclusions, and to me they seem on
| |
| general grounds perfectly obvious, viz. : (1) that the embryonal or formative region of the unilaminar Marsupial
| |
| blastocyst is the homologue of the inner cell-mass or
| |
| | |
| * How Assheton can maintain f 09, p. 266) “ that the amnion of the
| |
| rabbit is not more homologous to the amnion of the Sauropsidan than
| |
| the homy teeth of Ornithorhynchns ai-e homologous to the true teeth
| |
| of the mammal or reptile, which they have supplanted,†how he can
| |
| hold this view and yet proceed to utilise the presence of the amnion as
| |
| one of the leading charactei-s distinguishing the Amniota from the
| |
| Anamnia, I fail to comprehend. Surely the presence of a series of
| |
| purely analogous structures in a group is of no classificatory value.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MAllSUPIALIA
| |
| | |
| | |
| 83
| |
| | |
| | |
| | |
| imposed cell-viiigs, respectively and non-formative cell-rings of
| |
| formative (emlDryonal) and non- the Metatherian.
| |
| formative (extra-embryonal) in
| |
| significance.
| |
| | |
| | |
| 84
| |
| | |
| | |
| J. P. HILL
| |
| | |
| | |
| 3
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| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSUPIALIA.
| |
| | |
| | |
| 85
| |
| | |
| | |
| embryonal knob of tlie Eutberiaii blastocyst ; and (2) that the
| |
| extra-embryonal or noii-formative region of the same is the
| |
| homologne of the extra-embryonal ectoderm of the Sauropsida and Monotremata and of the trophoblast of the
| |
| Eutheria.
| |
| | |
| As regards conclusion (1) there is not likely to be much
| |
| difference of opinion, but as regards (2), whilst perhaps the
| |
| majority of embryologists support the obvious, not to say
| |
| common-place view which I here advocate, it seems certain
| |
| that it will prove neither obvious nor acceptable to those
| |
| mammalian embryologists (I refer specifically to my friends
| |
| Professor A. A. W. Hubrecht and Mr. R. Asshetou) who, with
| |
| only Selenka^s account of eaidy Marsupial ontogeny before
| |
| them, have formulated other and quite divergent views as to
| |
| the morphological nature of the outer enveloping layer of the
| |
| Eutherian blastocyst. It is therefore necessary to discuss
| |
| this question further, though I would fain express my conviction that had the observations recorded in this paper been
| |
| earlier available, much vain speculation as to the phytogeny
| |
| of the trophoblast might possibly have been avoided.
| |
| | |
| Chapter VII. - The Early Ontogeny op the Mammalia in
| |
| THE Light op the Foregoing Observations.
| |
| | |
| In entering on a discussion of the bearings of the results
| |
| of my study of the early development of Marsupials on
| |
| current interpretations of early Mammalian ontogeny, and
| |
| especially of the homologies of the germ-layers, I desire at
| |
| the outset to emphasise my conviction that, specialised
| |
| though the Marsupials undoubtedly are in certain features of
| |
| their anatomy, e. g. their dentition, genital ducts, and mammary apparatus, the observations recorded in the preceding
| |
| pages of this paper afford not the slightest ground for the
| |
| supposition that their early ontogeny is also of an aberrant
| |
| type, devoid of signiffcance from the point of view of that of
| |
| other mammals. On the contrary, I hope to demonstrate
| |
| that the Marsupial type of early development not only readily
| |
| | |
| | |
| 86
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| falls into line with that of Eutheria, and with what we know
| |
| of the early development of the Prototheria, but furnishes
| |
| ns with the key to the correct interpretation of that extraordinarily specialised developmental stage, the Eutherian
| |
| blastocyst. In particular I hope to show that the description
| |
| which I have been able to give of the mode of formation of
| |
| the' Marsupial blastocyst, bridges in the most satisfactory
| |
| fashion the great gap which has till now existed in our
| |
| knowledge of the way in which the transition from the
| |
| Monotrematous to the Eutherian type of development has
| |
| been effected.
| |
| | |
| 1. The Early Development of the Monotremata.
| |
| | |
| Our knowledge of the early development of the oviparous
| |
| mammals is admittedly still far from complete. Nevertheless
| |
| it is not so absolutely fragmentary that it can be passed over
| |
| in any general discussion of early mammalian ontogeny, and
| |
| I certainly cannot agree with the opinion of Assheton ('08,
| |
| p. 227) that from it “we gain very little help towards the
| |
| elucidation of Eutherian development.†On the contrary, I
| |
| think that the combined observations of Semon ('94), and
| |
| Wilson and Hill ('07) shed most valuable light on the early
| |
| ontogenetic phenomena in both the Metatheria and Eutheria.
| |
| I propose therefore to give here a very brief resume of the
| |
| chief results of these observers,^ and at the same time to
| |
| indicate how the knowledge of early Monotreme ontogeny
| |
| we possess, limited though it be, does help us to a better
| |
| understanding of the phenomena to which I have just
| |
| referred.
| |
| | |
| The ovum, as is well known from the observations of
| |
| Caldwell. ('87), is Reptilian in its character in all but size.
| |
| It is yolk-laden and telolecithal, the yolk consisting of
| |
| discrete yolk-spheres, and it is enclosed outside the zona
| |
| (vitelline membrane) by a layer of albumen and a definite shell.
| |
| | |
| * In so doinff I have largely utilised the phraseology of Wilson and
| |
| Hill's paper ('07).
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSUPIALIA. 87
| |
| | |
| | |
| At the moment of entering the oviduct it has a diameter of
| |
| 3‘5-4 mm. (2‘5-3 mm. according to Caldwell), and is therefore
| |
| small relatively to that of a reptile of the same size as the
| |
| adult Monotreme, but large relatively to those of other
| |
| mammals, being about twelve times larger than that of
| |
| Dasyurus, and about eighteen times larger than that of the
| |
| rabbit.
| |
| | |
| Cleavage is meroblastic. The first two cleavage planes are
| |
| at right angles to each other, as iii the Marsupial, and divide
| |
| the germinal disc into four approximately equal-sized cells
| |
| (Semon, Taf. ix, fig. 30). Each of these then becomes subdivided by a meridional furrow into two, so that an 8-celled
| |
| stage is produced, the blastomeres being arranged symmetrically, or almost symmetrically, on either side of a median line,
| |
| perhaps corresponding to the primary furrow (Wilson and Hill,
| |
| p. 37, text-figs. 1 and 2). Imagine the yolk removed and the
| |
| blastomeres arranged radially, and we have at once the open
| |
| ring-shaped 8-celled stage of Dasyurus. The details of the
| |
| succeeding cleavages are unknown. Semon has described a
| |
| stage of about twenty-four cells (Semon, Taf. ix, fig.31),inwhich
| |
| the latter formed a one-layered circular plate with no evidence
| |
| of bilateral symmetry, and this is succeeded by a stage also
| |
| figured by Semon (figs. 32 and 33, cf. also Wilson and Hill,
| |
| PI. 2, fig. 2), in which the blastoderm has become sevei'al
| |
| cells thick, though it has not yet increased in surface extent.
| |
| It is bi-convex lens-shaped in section, its lower surface being
| |
| sharply limited from the underlying white yolk. No nuclei
| |
| are recognisable in the latter, either in this or any subsequent
| |
| stage, nor is there ever any trace of a syncytial germ-wall,
| |
| features in which the Monotreme egg differs from the
| |
| Sauropsidan.
| |
| | |
| The next available stage, represented by an egg of Ornithorhynchus, described by Wilson and Hill ('07, p. 38, PI. 2, fig.
| |
| 4), and by an egg of Echidna, described by Semon ('94, p. 69,
| |
| figs. 22 and 33), is separated by a considerable gap from the
| |
| preceding, and most unfortunately so, since it belongs to the
| |
| period of commencing formation of the germ-layers. The
| |
| | |
| | |
| 88
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| | |
| cellular lens-shaped blastoderm of the preceding stage has
| |
| now extended in the peripheral direction so as to enclose
| |
| about the upper half of the yolk-mass, and in so doing it has
| |
| assumed the form, almost exclusively, of a unilaminar thin
| |
| cell-membrane, composed of flattened cells and closely applied
| |
| to the inner surface of the zona. At the embryonic pole,
| |
| however, in the region of the white yolk-bed, there are
| |
| present in the Ornithorhynchus egg a few plump cells,
| |
| immediately subjacent to the unilaminar blastoderm, but
| |
| separate and distinct from it, whilst in the Echidna egg
| |
| Semon's figure (fig. 33), which is perhaps somewhat schematic,
| |
| shows a group of scattered cells, similar to those in the
| |
| Ornithorhynchus egg but placed considei'ably deeper in the
| |
| white yolk-bed. Unfortunately we have no definite evidence
| |
| as to the significance of these internally situated cells. One
| |
| of two possible interpretations may be assigned to them.
| |
| Either they represent the last remaining deeply placed cells
| |
| of the blastodisc of the preceding stage, which have not yet
| |
| become intercalated in the unilaminar blastodermic membrane
| |
| believed by Semon to be the condition attained in eggs of
| |
| about this stage of development, or they are cells which have
| |
| been proliferated off from this unilaminar blastoderm, to
| |
| constitute the parent cells of the future yolk-entoderm. As
| |
| regards Echidna, Semon expresses a definite enough opinion ;
| |
| he holds that these deeply placed cells actually arise by a
| |
| somewhat diffuse proliferation or ingrowth from a localised
| |
| depressed area of the blastoderm at the embryonic pole, and
| |
| that they give origin to yolk-entoderm. This interpretation
| |
| of Semon seems probable enough in view of the mode of origin
| |
| of the entoderm in the Metatheria and Eutheria. Moreover
| |
| in the next available stage, an egg of Ornithorhynchus, just
| |
| â– over 6 mm. in diameter, described by Wilson and Hill, the
| |
| blastoderm is already bilaminar throughout its extent, so that
| |
| we .might veiy Avell expect to find the beginnings of the entoderm in the somewhat younger eggs.
| |
| | |
| In the 6 mm. egg just referred to, the peripheral portion of
| |
| the utjilaminar blastoderm of the preceding stage has grown
| |
| | |
| | |
| 89
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| SO as to enclose the entive yolk-mass in a complete ectodeimal
| |
| envelope, whilst iiiteimally to that a complete lining of yolkentoderm has become established. As tlie result of these
| |
| changes, and of the imbibition of fluid from the uterus, the
| |
| solid yolk-laden egg has become converted into a relatively
| |
| thin-walled vesicle or blastocyst, possessed of a bilaminar
| |
| wall surrounding the partly fluid vitelline contents of the egg.
| |
| Throughout the greater part of its extent the structure of the
| |
| vesicle wall is very simple. It consists externally of an
| |
| extremely attenuated ectodermal cell - membrane closely
| |
| adherent to the deep surface of the vitelline membrane
| |
| (zona), and within that of a layer of yolk-entoderm, composed
| |
| of large swollen cells, containing each a vesicular nucleus,
| |
| and a number of yolk-spheres of varying size. Over a small
| |
| area, overlying the white yolk-bed, however, the ectodermal
| |
| layer of the wall presents a different character to that
| |
| described above. Its constituent cells are here not flattened
| |
| and attenuated, but irregnlai'ly cuboidal in form and much
| |
| more closely packed together; moreover they stand in proliferative continuity with a subjacent mass of cells, also in
| |
| process of division. The irregular superficial layer and this
| |
| latter mass together form a thickened lenticular cake, "5 mm.
| |
| in greatest diameter, projecting towards the white yolk-bed
| |
| but separated from it by the yolk-entoderm, which retains
| |
| its character as a continuous cell-membrane. This differentiated, thickened area of the wall, situated as it is at the upper
| |
| pole of the egg, as marked by the white yolk-bed, must be
| |
| held to represent a part of the future embryonal region.
| |
| Wilson and Hill incline to regard it as in some degree the
| |
| equivalent of the “primitive plate†of Eeptiles and as the
| |
| initial stage in the formation of the primitive knot of latex;
| |
| eggs. This question, however, does ixot closely concern us
| |
| here : the point I wish to emphasise is the relative inactivity
| |
| of the cells composing the embryonal region of the blastoderm
| |
| in the Monotreme as compared with the marked activity displayed by those constituting the peripheral (extra-embryonal)
| |
| region of the same. It is these latter cells which by their
| |
| | |
| | |
| 90
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| rapid growth complete the envelopment of the yolk-mass and
| |
| so constitute the lower hemisphere of the blastocyst.
| |
| | |
| Ihe bilaminar blastocyst of the Monotreme, foi'nied in the
| |
| manner indicated above, is entirely comparable with the
| |
| Marsupial blastocyst of the same developmental stage. There
| |
| are differences in detail certainly (e.g. in the characters,
| |
| time of formation, and rate of spreading of the entoderm,
| |
| in the mode of formation of the blastocyst cavity and in its
| |
| contents, in the apparent absence in the Monotreme of any
| |
| well-marked line of division between the embryonal aud extraembryonal regions of the ectoderm, in the relatively earlier
| |
| appearance of differentiation in the embryonal region in the
| |
| Monotreme as compared with the Marsupial), but the agreements are obvious and fundamental ; in particular, I would
| |
| emphasise the fact that in both the embryonal region is
| |
| superficial and freely exposed, and forms part of the blastocyst wall just as that of the reptile forms part of the general
| |
| blastoderm. Moreover, should future observations confi^rm
| |
| the view of Semon that the primitive entodermal cells of the
| |
| Monotreme are proliferated off from the embryonal region of
| |
| the unilaminar blastoderm, then we should be justified in
| |
| directly comparing the latter with the unilaminar wall of the
| |
| Marsupial blastocyst, and in regarding it also as consisting
| |
| of two differentiated regions, viz. a formative or embryonal
| |
| region, overlying the white yolk-bed, and giving origin to
| |
| the embryonal ectoderm and the yolk-entoderm, and a nonformative region which rapidly overgrows the yolk-mass so
| |
| as to eventually completely enclose it, just as does the less
| |
| rapidly growing extra-embryonal ectoderm of the Sauropsidan blastoderm.^ Meantime I see no reason for doubting
| |
| that this rapidly growing peripheral portion of the unilaminar
| |
| blastoderm of the Monotreme is anything else than extraembryonal ectoderm homogenous with that of the reptile.
| |
| Indeed, I am not aware that any embryologist except Hubrecht
| |
| thinks otherwise. Even Asshetou is, I believe, content to
| |
| | |
| * We should further he justified in concluding that the entoderm is
| |
| similar in its mode of origin in all three mammalian sub-classes.
| |
| | |
| | |
| THE EAHLY DEVELOPMENT OF THE MARSUPIALIA. 91
| |
| | |
| | |
| regard the outer layer of the Monotrerae blastocyst ns
| |
| ectodermal. Hubrecht's view is that the primitive eiitodermal
| |
| cells of Semon give origin, not to yolk-entoderm, but to the
| |
| equivalent of the embryonal knot of Eutheria, whilst the
| |
| uuilaminar blastodermic membrane itself is a larval layer
| |
| - the trophoblast - that portion of it overlying the internally
| |
| situated cells representing the covering layer (Rauber's layer)
| |
| of the Eutherian blastocyst. ‘'For this view,†remarks
| |
| Assheton [^09, p. 283), “1 can see no reason derivable from
| |
| actual specimens described and figured by those four authorsâ€
| |
| (Caldwell, Semon, Wilson and Hill), with which criticism I
| |
| am in entire agreement, as also with the following statement,
| |
| which, so far as the Metatheria are concerned, is based on
| |
| my own results: “Neither in the Prototheria [n ] or the
| |
| Metatheria is there really any tangible evidence of a trophoblast occui*ring as a covering layer over the definitive epiblast
| |
| as in Eutheria†(p. 234).
| |
| | |
| In connection with the peripheral growth of the unilaminar
| |
| blastoderm in the Monotreme, it is of interest to observe that
| |
| this takes place, not apparently in intimate contact with the
| |
| surface of the solid yolk, as is the case with the growing
| |
| margin of the extra-embryonal ectoderm in the Saui'opsidan
| |
| egg, but rather in contact with the inner surface of the
| |
| thickened zona, perhaps as the result of the accumulation in
| |
| the perivitelline space of tiuid which has diffused into the latter
| |
| from the uterus. In other words, the peripheral growth of
| |
| the extra-embryonal ectoderm to enclose the yolk-mass appears
| |
| to take place here in precisely the same way as the spreading
| |
| of the non-formative cells in Dasyurus to complete the lower
| |
| pole of the blastocyst. In my view the latter phenomenon
| |
| is none other than a recapitulation of the former ; on the
| |
| other hand, I regard the spreading of the formative cells in
| |
| Dasyurus towards the upper pole as a purely secondary
| |
| feature, conditioned by the loss of the yolk-mass and the
| |
| attainment of the holoblastic type of cleavage.
| |
| | |
| If it be admitted that the outer extra-embryonal layer of
| |
| the Monotreme blastocyst is homogenous with the extra
| |
| | |
| 92
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| embryonal ectoderm of the Keptile, then it seems to me there
| |
| is no escape from tlie conclusion that these layers are also
| |
| homogenous with the non-formative region of the unilaminar
| |
| Marsupial blastocyst. I need only point out here that the
| |
| chief destiny of each of the mentioned layers, and I might
| |
| also add that of the outer enveloping layer of the Eutherian
| |
| blastocyst (the so-called trophoblast), is one and the same,
| |
| viz. to form the outer layer of the chorion (false amnion,
| |
| serous membrane) and omphalopleure (unsplit yolk-sac wall.
| |
| Hill ['97]),^ and that to deny their homogeny to each other
| |
| implies the nou-homogeny of these membranes and the amnion
| |
| in the Amniotan series, and consequently renders the group
| |
| name Amniota void of all moi'phological meaning.
| |
| | |
| The rapidity Avith which the enclosure of the yolk-mass
| |
| is effected, and the relative tardiness of differentiation in the
| |
| embryonal region are features Avhich sharply distinguish the
| |
| early ontogeny of the Monotremes from that of the Sauropsida,
| |
| and which, in my view, are of the very greatest importance,
| |
| since they afford the key to a correct understanding of the
| |
| peculiar coenogeuetic modifications observable in the early
| |
| ontogeny of the Metatheria and Eutheria. To appreciate the
| |
| significance of these featui-es it is necessary to take account
| |
| of the great difference which exists between the Sauropsidan
| |
| and Monotreme ovum in regard to size, as Avell as of the very
| |
| different conditions under Avhich the early development goes
| |
| on in the two groups. The Sauropsidan egg is large enough
| |
| to contain Avithin its OAvn confines the amount of yolk necessary for the production of a young one complete in all its
| |
| parts and capable of leading an independent existence
| |
| immediately it leaves the shell. Furthermore, it is also large
| |
| | |
| ' In certain Ainniotes the layers in question appear also to participate
| |
| in the formation of the inner lining of the amnion (amniotic ectoderm)
| |
| (cf . Assheton ['09], pp. 248-9), but this does not affect the statement in
| |
| the text. In the Saxu'opsida and Monotremata I think I am coia-ect in
| |
| saying that no sharp distinction is recognisable between the embi'yonal
| |
| and extra-embryonal regions of the ectoderm, hence it is difficult, if not
| |
| imj)ossible, to determine with certainty their relative participation in
| |
| the formation of the amniotic ectoderm.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE JIARSUPIALIA. 93
| |
| | |
| | |
| enough to provide room for tlie development of an embryo
| |
| without any secondary growth in size after it leaves the ovary.
| |
| Moreover we have to remember that after it has become
| |
| enclosed in the shelly it remains but a short time in the oviduct
| |
| and receives little or no additional nutrient material from the
| |
| oviducal walls. The yolk-mass in any case retains its solid
| |
| character; there is no necessity for its rapid enclosure, and
| |
| so enclosure is effected slowly, contemporaneously with the
| |
| differentiation of the embryo.
| |
| | |
| In the Monotreme the conditions are altogether different.
| |
| The ripe ovarian ovum when it enters the oviduct has a
| |
| diameter of about 3-5 to 4 mm., and is thns considerably
| |
| smaller than that of a Eeptile of the same size as the adult
| |
| Monotreme. The amount of yolk which it is capable of containing is not anything like sufficient to last the embryo
| |
| throughout the developmental period, and, moreover, it does
| |
| not provide the space essential for the development of an
| |
| embryo on the ancestral Reptilian lines. As Assheton ('98,
| |
| p. 251) has pointed out, “ the difference in size between
| |
| the fertilised ovum of a reptile or bird or of a mammal
| |
| is very great ; but the difference in size between the
| |
| embryo of, say, a bird with one pair of mesoblastic
| |
| somites and of a mammal of the same age is comparatively
| |
| small. This means that nearly the same space is required
| |
| for the production of the mammalian embryo as of the
| |
| Sauropsidan, and has to be provided.†In the Monotreme
| |
| not only is additional room necessary, but also additional
| |
| nutrient material, sufficient with that already present in the
| |
| egg to last the embryo throughout the period of incubation.
| |
| Both are acquired contemporaneously during the sojourn of
| |
| the egg in the uterine portion of the oviduct, wherein the egg
| |
| increases greatly in size. When it enters the uterus, the
| |
| Monotreme egg has a diameter, inclusive of its membranes, of
| |
| about 4-5 mm. ; when it is laid, it measures in Ornithorhynchus, in its greatest diameter, 16-19 mm., and somewhat
| |
| less in the case of Echidna. Prior to the enclosure of the yolk
| |
| the increase in diameter, due to the accumulation of fluid in
| |
| | |
| | |
| 94
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| the perivitelliue space and between the zona and shell, is but
| |
| slight. But as soon as the yolk becomes suiTonnded by a
| |
| complete cellular membrane, i.e. as soon as the egg has
| |
| become converted into a thin-walled blastocyst, rapid growth
| |
| sets in, accompanied by the active imbibition of the nutrient
| |
| fluid, which is poured into the uterine lumen as the result of
| |
| the secretory activity of the abundantly developed uterine
| |
| glands. The fluid absorbed not only keeps the blastocyst
| |
| turgid, but it brings about the more or less complete disintegration of the yolk-mass, its constituent spherules
| |
| becoming disseminated in the fluid contents of the blastocyst
| |
| cavity. Although a distinct and continuous subgerminal
| |
| cavity, such as appears beneath the embryonal region of the
| |
| Sauropsidan blastoderm, does not occur in the Monotreme
| |
| egg, vacuolar spaces filled with fluid develop in the white
| |
| yolk-bed underlying the site of the germinal disc and appear
| |
| to represent it. As Wilson and Hill remark ('03, p. 317),
| |
| “ one can, without hesitation, homologise the interior of the
| |
| vesicle with the subgerminal cavity of a Saui'opsidan egg,
| |
| extended so as to include by liquefaction the whole of the
| |
| yolk itself.†In the Marsupial the blastocyst cavity has a quite
| |
| different origin, since it represents the persistent segmentation
| |
| cavity, whilst in the Eutheria the same cavity is secondarily
| |
| formed by the confluence of intra- or intei*-cellular vacuolar
| |
| spaces, but no one, so far as I know, has ever v^entured to
| |
| assert that, because of this difference in mode of origin, the
| |
| blastocyst cavity in the series of the Mammalia is a nonhomogenous formation.
| |
| | |
| To return to the matter under discussion, it appeal's to me
| |
| that the necessity which has arisen, consequent on the I'eduction in size of the ovum, for rapid growth of the same in
| |
| order to provide room for the development of an embryo and
| |
| for the storage of nutrient material furnished by the maternal
| |
| uterus, affords a satisfactory explanation of the much more
| |
| marked activity of the extra-embryonal I'egion of the blastoderm as compared with the embryonal, Avhich is such a striking
| |
| feature in the early ontogeny of the Monotremes, and not
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 95
| |
| | |
| | |
| only of them, but, as Assheton has pointed out ('98, p. 251),
| |
| of the higher mammals as well (cf. the process of epiboly and
| |
| the inertness at first displayed by the formative cells of
| |
| the embryonal knot as compared with the activity of the nonformative or tropho-ectodermal cells), an activity which
| |
| results in the rapid completion of that characteristically
| |
| mammalian developmental stage - the blastocyst or blastodermic vesicle.
| |
| | |
| The necessity for the early formation of such a stage,
| |
| capable of rapidly growing in a nutrient fluid medium
| |
| provided by the mother, has profoundly influenced the early
| |
| ontogeny in all three mammalian subclasses, and natui*ally
| |
| most of all that of the Eutheria, in which reduction of the
| |
| ovum, both as regards size and secondary envelopes, has
| |
| reached the maximum. And I think there can be little
| |
| doubt but that it is this necessity which has induced that
| |
| early separation of the blastomeres into two categories,
| |
| respectively formative and non-formative in significance,
| |
| which has long been recognised as occurring in Eutheria, and
| |
| which I have shown also occurs amongst the Metatheria.
| |
| This early separation of the blastomeres into two distinct
| |
| groups is not recognisable in the Sauropsida, and the idea
| |
| that it is in some way connected with the loss of yolk which
| |
| the mammalian ovum has suffered in the course ofphylogeny,
| |
| was first put forward, I believe, by Jenkinson. In his paper
| |
| on the germinal layers of Vertebrata ('06, p. 51) he writes:
| |
| “ Segmentation therefore is followed in the Placentalia by
| |
| the separation of the elements of the trophoblast from those
| |
| destined to give rise to the embryo and the remainder of its
| |
| foetal membranes, and this ^precocious segregation'
| |
| seems to have occurred phylogenetically during
| |
| the gradual loss of yolk which the egg of these
| |
| mammals has undergone.†Whether or not such a
| |
| precocious segregation †has already become fixed in the
| |
| Monotremes,future investigation must decide (cf . ante, p.90).
| |
| | |
| Ihe loss of yolk, with resulting reduction in size which the
| |
| Monotreme ovum has suffered in the course of phylogeny, we
| |
| | |
| | |
| 96
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| must assume to have taken place gi-adually and in correlation
| |
| with the longer retention of the egg in the oviduct, the
| |
| elaboration of the uterine portion of the same as an actively
| |
| secretory organ, and the evolution of the mammary apparatus.
| |
| The Monotremes thus render concrete to us one of the first
| |
| great steps in mammalian evolution so far as developmental
| |
| processes are concerned, viz. the substitution for intra-ovular
| |
| yolk of nutrient material furnished directly by the mother to
| |
| the developing egg or embryo. We see in them the beginnings of that process of substitution of uterine for ovarian
| |
| nutriment which reaches its culmination in the Eutheria with
| |
| their microscopic yolk-poor ova and long intra-uterine period
| |
| of development. The Marsupials show us in Dasyurus an
| |
| interesting intervening stage so far as the ovum is concerned,
| |
| in that this, though greatly reduced as compared with that
| |
| of the Monotreme, still retains somewhat of its old tendencies
| |
| and elaborates more yolk-material than it can conveniently
| |
| utilise, with the result that it has to eliminate the surplus
| |
| before cleavage begins. But as coucerns their utilisation of
| |
| intra-uterine nutriment, they have specialised along their
| |
| own lines, and instead of exhausting the possibilities implied
| |
| by the presence of that, they have extensively elaborated
| |
| the mammary apparatus for the nutrition of the young, born
| |
| in a relatively immature state, after a short period of intrauterine life (cf. Wilson and Hill [T7, p. 580]).
| |
| | |
| In view of the fact that the young Monotreme enjoys three
| |
| developmental periods, viz. intra-uterine, incubatory, and
| |
| lactatory, the question might be worthy of consideration
| |
| whether it may not be that the Marsupial has merged the
| |
| incubatory period in the lactatory, the Eutherian the same in
| |
| the intra-uterine.
| |
| | |
| | |
| 2. The Early Development of the Metatheria and
| |
| | |
| Eutheria.
| |
| | |
| It will have become evident Horn the foregoing that the
| |
| Metatherian mode of early development is to be regarded as
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MAESUPIALIA.
| |
| | |
| | |
| 97
| |
| | |
| | |
| but a slightly modified version of the Prototherian, such
| |
| differences as exist between them being interpretable as coenogeuetic modifications, induced in the Metatherian by the
| |
| practically complete substitution of uterine nutriment for
| |
| intra-ovular yolk, a substitution which has resulted in the
| |
| attainment by the marsupial ovum of the holoblastic type of
| |
| cleavage. In tlie present section I hope to demonstrate how
| |
| the early ontogeny of the Metatlieria enables us to interpret
| |
| that of the Eutheria in terms of that of the Prototheria.
| |
| | |
| If we proceed to compare the early development in the
| |
| Metatlieria and Eutheria, we encounter, from the 4-celled
| |
| stage onwards, such obvious and profound differences in the
| |
| mode of formation of the blastocyst, and in the relations of
| |
| its constituent parts, that the differences seem at first sight
| |
| to far outweigh the resemblances. Nevertheless, apart from
| |
| their common possession of the same holoblastic mode of
| |
| cleavage, there exists one most striking and fundamental
| |
| agreement between the two in the fact that in both there
| |
| occurs, sooner or later during the cleavage process, a separation of the blastomeres into two distinct, pre-determined cellgroups, whose individual destinies are very different, but
| |
| apparently identical in the two subclasses. In tlie Marsupial,
| |
| as typified by Dasyurus, the fourth cleavages are, as we have
| |
| seen, unequal and qualitative, and result in the separation of
| |
| two differentiated groups of blastomeres, arranged in two
| |
| superimposed rings, viz. an upper ring of eight smaller, less
| |
| yolk-rich cells, and a lower of eight larger, more yolk-iuch
| |
| cells. The evidence justifies the conclusion that the former
| |
| gives origin directly to the formative or embryonal region of
| |
| the vesicle wall, the latter to tlie non-formative or extraembryonal region.
| |
| | |
| Amongst the Eutheria the evidence is no less clear. It has
| |
| been conclusively shown by various observers (Van Beneden,
| |
| Duval, Assheton, Hubrecht, Heape, and others) that, sooner
| |
| or later, there occui's a separation of the blastomeres into two
| |
| distinct groups, one of which eventually encloses the other
| |
| completely. The two groups may be clearly distinguishable
| |
| | |
| VOL. 56, PART 1. NEW SERIES. 7
| |
| | |
| | |
| 98
| |
| | |
| | |
| t.C.TTV. (€cj
| |
| | |
| trect.
| |
| | |
| | |
| i.c.nv.ffo.)
| |
| | |
| | |
| | |
| tr.ect.
| |
| | |
| | |
| cunrvTh.c.
| |
| emJb. ect.
| |
| | |
| | |
| Diagrams illustrating the mode of formation of the blastocyst
| |
| in Metatheria (a-d) and Eutheria (1-3). b.c. Blastocyst cavity.
| |
| i.c.m. Inner cell-mass, 'pr.amn.c. Primitive amniotic cavity.
| |
| r.l. Rauber's layer. s.c. Segmentation cavity. For other
| |
| reference letters see explanation of plates (p. 125).
| |
| | |
| | |
| THE EAltLY l.)E VET;01â– '^[ENâ– T OF THE MARSUFIALIA. 99
| |
| | |
| | |
| in eai'lv cleavage stages, owing to diffecences in the characters
| |
| and staining reactions of their cells, and in such cases there
| |
| is definite evidence of the occurrence of a process of overgrowth
| |
| or epiboly, whereby one group gradually grows round and
| |
| completely envelops the other, so that in the completed
| |
| morula a distinction may be drawn between a central cellmass and a peripheral or enveloping layer (rabbit. Van
| |
| Beneden; sheep, Assheton). In other cases, where it has
| |
| been impossible to recognise the existence of these two
| |
| distinct cell-groups in the cleavage stages, we nevertheless
| |
| find, either in the completed moimla or in the blastocyst, that
| |
| a more or less sharp distinction may be drawn between an
| |
| enveloping layer of cells and an internally situated cell-mass
| |
| (inner cell-mass).
| |
| | |
| E. van Beneden, in his classical paper on the development
| |
| of the rabbit, published in 1875, was the first to recognise
| |
| definitely the existence of two categories of cells in the
| |
| segmenting egg of the Eutherian mammal. In this form he
| |
| showed how in the morula stage a cap of lighter blastomeres
| |
| gradually grows round and envelops a mass of more opaque
| |
| cells by a process of overgrowth or epiboly. In his more
| |
| recent and extremely valuable paper on the development of
| |
| Yespertilio ('99), he again demonstrated the existence of two
| |
| groups of blastomeres as well in the segmenting egg as in the
| |
| completed morula, but failed to find evidence of epiboly in all
| |
| cases. Nevertheless he holds fast to the opinion which he
| |
| expressed in 1875 : “ Que la segmentation s'accompagne, chez
| |
| les Mammiferes placentaires, d'un enveloppement progressif
| |
| d'une partie des blastomeres par une couche cellulaire, qui
| |
| commence a se differencier des le debut du developpement,â€
| |
| and states that “dans tons les oeufs arrives a la fin de la
| |
| segmentation et dans ceux qui moutraient le debut de la
| |
| cavite Blastodermique j'ai constamment rencontre une couche
| |
| peripherique complete, eutourant de toutes parts un amas
| |
| cellulaire interne, bien separe de la couche enveloppante.â€
| |
| The latter layer he regards as corresponding to the extraembryonal ectoderm of the Sauropsida, and points out that
| |
| | |
| | |
| 100
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| “ chez tons les Choi'des les premiers blastomeres qui se
| |
| differencient et qui avoisinent le pole animal de I'oeuf sont
| |
| des elements epiblastiqnes. C'est par la couolie cellulaire qui
| |
| resulte de la segmentation ulterieure de ces premiers blastomeres epiblastiqnes que se fait, cbez les Sauropsides, benveloppement du vitellus. Dans Toeuf reduit a n'etre plus
| |
| qu'une sphere microscopiquej bepibolie a pu s'achever des la
| |
| fin de la segmentation, voire meme avant bachevement de ce
| |
| phenomene.†The “ amas cellulaire interne †(embryonal
| |
| knot, inner cell mass). Van Beneden shows, differentiates
| |
| secondarily into “ un lecithophore et un bouton embryonnaire.'' The former is the entoderm of other authors, the
| |
| latter the formative or embryonal ectoderm. Hubrecht, in
| |
| the forms studied by him (Sorex, 'I'upaia, Tarsius^) finds
| |
| a corresponding differentiation. In Tupaia he describes the
| |
| morula stage as consisting of a single central lightly staining
| |
| cell, which he regards as the parent cell of the inner cell-mass
| |
| of later stages, and of a more darkly staining peripheral layer
| |
| which forms the unilaminar wall of the blastocyst. Here,
| |
| then, the parent cells of the two cell-groups would appear to
| |
| be separated at the first cleavage. Hubrecht, like Van
| |
| Beneden, holds that the inner cell-mass furnishes the
| |
| embryonal ectoderm and the entire entoderm of the blastocyst.
| |
| The peripheral layer he has termed the trophoblast ('88, p.
| |
| 511), and in his paper on the placentation of the hedgehog
| |
| ('89, p. 298) he defines the term as follows: “I propose to
| |
| confer this name to the epiblast of the blastocyst as far as it
| |
| has a dix'ect nutritive significance, as indicated by proliferating
| |
| processes, by immediate contact with maternal tissue, maternal
| |
| blood, or secreted material. The epiblast of the germinal
| |
| ai-ea - the formative epiblast - aud that which will take part
| |
| in the formation of the inner lining of the amnion cavity is,
| |
| ipso facto, excluded from the definition.†Thus the name
| |
| | |
| * In Erinacens the entoderm, from Hubrecht's observations, appears
| |
| to be precociously differentiated, prior to the separation of the embryonal
| |
| ectoderm fi'om the overlying trophoblast, but the details of the early
| |
| development in this form are as yet only incompletely known.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSHPIALIA. 101
| |
| | |
| | |
| trophoblasb was originally employed by Hubrecbt as a convenient term designatory of what he at the time regarded as
| |
| the extra-embryonal ectoderm of the mammalian blastocyst.
| |
| In the course of his speculations on the oingin of this layer,
| |
| however, he has reached the conclusion that it is really of the
| |
| nature oP'a larval envelope, an Embryonalhiille (^08, p. 15),
| |
| inherited by the mammals, not from the reptiles (which have
| |
| no direct phylogenetic I'elationship to the latter), but from
| |
| their remote invertebrate ancestors ('Vermiform pi'edecessors
| |
| of coelenterate pedigree, provided with an ectodermal larval
| |
| investment [Laiwenhiille] â€).
| |
| | |
| Assheton, again, although he was unable to convince himself ('94) of the correctness of van Beneden's account of the
| |
| occurrence of a process of epiboly in the segmenting eggs of
| |
| the rabbit, finds in the sheep ('98) that a differentiation into
| |
| two groups of cells is recognisable “ perhaps as early as the
| |
| eight segment stage,†and that one of the groups gradually
| |
| envelops the other. “Let it be noted,†he writes ('98, p. 227),
| |
| “ that we have now to face the fact, based on actual sections,
| |
| that there is in certain mammals a clear separation of
| |
| segments at an early stage into two groups, one of which
| |
| eventually completely surrounds the other,†and instances
| |
| Van Beneden's observations on the rabbit (of the correctness
| |
| of which he, however, failed to satisfy himself, as noted above),
| |
| Duval's observations on the bat, Hubrecht's on Tupaia, and
| |
| his own on the sheep. Assheton thinks this phenomenon
| |
| “ must surely have some most profound significance,â€
| |
| but finds himself unable to accept the interpretations of
| |
| either Van Beueden or Hubrecht, and puts forward yet
| |
| another view, “ based on the appearance of some segmenting
| |
| eggs of the sheep †('08, p. 233), “that in cases where this
| |
| differentiation does clearly occur, it is a division into epiblast
| |
| and hypoblast, the latter being the external layer†('98, p. 227).
| |
| Assheton thus differs from all other observers in holding that
| |
| the inner cell-mass or embryonal knot of the Eutherian
| |
| blastocyst gives origin solely to the formative or embryonal
| |
| ectoderm, and I believe 1 am correct in stating that he also
| |
| | |
| | |
| 102
| |
| | |
| | |
| J. p. mi,L.
| |
| | |
| | |
| differs from all other observers in holding that the outer
| |
| enveloping layer of the same is entodermald
| |
| | |
| The fact, then, of the occurrence amongst Eutheria of a
| |
| “precocious segregation †of the blastomeres into two distinct
| |
| groups, one of which eventually surrounds the other completely, is not in dispute, though authorities differ widely in
| |
| the intei'pretation they place upon it. In the Eutherian
| |
| blastocyst stage, the enveloping layer forms the outer unilaminar wall of the vesicle, and encloses the blastocyst cavity
| |
| as well as the other internally situated group. This latter
| |
| typically appears as a rounded cell-mass, attached ac one spot
| |
| to the inner surface of tlie enveloping layer, but more or less
| |
| distinctly marked off from it. It is genei-ally termed the
| |
| inner cell-mass or embryonal knoc (“ amas cellulaire interne â€
| |
| of Van Beneden). For the enveloping layer Ilubrecht's name
| |
| of “ trophoblast †is now generally employed, even by those
| |
| who refuse to adopt the speculative views with which its
| |
| originator has most unfortunately, as I think, enshrouded this
| |
| convenient term.
| |
| | |
| I have demonstrated the occurrence of an apparently comparable “precocious segregation^^ of the blastomeres into
| |
| two distinct groups in one member of the Metatheria which
| |
| there is no reason to regard as an abeirant type, and I have
| |
| shown beyond all shadow of doubt that from the one group,
| |
| which constitutes what I have termed the formative region
| |
| of the unilaminar vesicle-wall, there arise the embi*youal
| |
| ectoderm and the entire entoderm of the vesicle, both embryonal and extra-embryonal, and that the other group, which
| |
| constitutes the non-formative region of the vesicle-wall,
| |
| directly furnishes the extra-embryonal ectoderm, i.e. the
| |
| ectoderm of the omphalopleui'e and chorion."
| |
| | |
| * Assheton states ('08, p. 233, cf. also '98, p. 220) that his interpretation “ owes ranch also to the theoretical conclusions of Minot and
| |
| Robinson.†However that may be, both Minot and Robinson in their
| |
| most recent writings continue to speak of the chorionic ectoderm.
| |
| | |
| ^ Whether or not it participates in the formation of the ainniotic
| |
| ectoderm future investigation must decide.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPFALIA. 103
| |
| | |
| As resrards Eutheria, we have seen that Van Beneden and
| |
| Hubrecht, though their views in otlier respects are widely
| |
| divero-ent, both ag'ree that the inner cell-mass of the blastocyst furnishes the embryonal ectoderm (as well as the amniotic
| |
| ectoderm wholly or in part) and the entire entoderm of the
| |
| vesicle. That, in fact, is the view of Mammalian embryologists
| |
| generally (Duval and Assheton excepted),^ and if we may
| |
| assume it to be correct, then it would appear that the later
| |
| history of the formative region of the Marsupial blastocyst
| |
| and that of the inner cell-mass of the Eutherian are identical.
| |
| That being so, and bearing in mind that both have been
| |
| shown, at all events in certain Mammals, to have an identical
| |
| origin as a group of precociously segregated blastotneres,^ I
| |
| can come to no other conclusion than that they are homogenous formations. If that be accepted, then this fact by itself
| |
| renders highly probable the view that the so-called trophoblast of the Eutherian blastocyst is homogenous with the
| |
| non-formative region of the Metatherian vesicle, and v?hen
| |
| we reflect that both have precisely the same structural and
| |
| topographical (not to mention functional) relations in later
| |
| stages, inasmuch as they constitute the ectoderm of the chorion
| |
| and omphalopleure (with or without participation in the
| |
| formation of the amniotic ectoderm;, and that both have a
| |
| similar origin in those Mammals in which a precocious segregation of the blastomeres has been recognised, their exact
| |
| | |
| * The view of Duval ['95], based on the study of Vespertilio, that the
| |
| inner cell-mass gives rise solely to entoderm, and that the enveloping
| |
| layer furnishes not only the extra-embryonal but also the embryonal
| |
| ectoderm, is shown by Van Beneden's observations on the same form to
| |
| be devoid of any basis of fact. Assheton's views are referred to below
| |
| (p. 110).
| |
| | |
| - The fact that the phenomenon of the “ precocious segregation†of
| |
| the blastomeres into two groups with deteiminate destinies has already
| |
| become fixed in tlie Marsupial lends additional weight to the view of
| |
| Van Beneden that such a segregation will eventually be recognised as
| |
| occurring in all Eutheria without exception. Without it, it is difficult
| |
| to understand how the entypic condition, characteristic of the blastocysts of Ml known Eutheria, is attained, imless by differentiation in
| |
| situ, which .seems to me highly improbable.
| |
| | |
| | |
| 104
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| homology need no longer be doubted. In the preceding section
| |
| of this paper (ante, pp. 91, 92) I have shown reason for the
| |
| conclusion that the non-formative region of the Marsupial
| |
| blastocyst is the homologue of the extra-embryonal ectoderm
| |
| of the Monotreme and Reptile, and if that conclusion be
| |
| accepted it follows that the outer enveloping layer of the
| |
| Eutherian blastocyst, the so-called trophoblast of Hubrecht,
| |
| is none other than extra-embryonal ectoderm, as maintained
| |
| by Van Beneden, Keibel, Bonnet, Jenkinson, Lee, MacBride
| |
| and others, the homologue of that of Reptilia.
| |
| | |
| I am therefore wholly unable to accept the highly speculative conclusions of Hubrecht, set forth with such brilliancy
| |
| in a comparatively recent number of this Journal ('08), as
| |
| to the significance and phylogeny of this layer. These conclusions, on the basis of which he has proceeded to formulate
| |
| such far-reaching and, indeed, revolutionary ideas not only
| |
| on questions embryological, but on those pertaining to the
| |
| phylogeny and classification of vertebrates, have already
| |
| been critically considered by Assheton ('09) and MacBride
| |
| ('09), also in the pages of this Journal, and found wanting,
| |
| and they are, to my mind, quite irreconcilable with the facts
| |
| I have brought to light in regard to the early development
| |
| of Marsupials. I yield to no one in my admiration for the
| |
| epoch-making work of Hubrecht on the early ontogeny and
| |
| placentation of the Mammalia, and I heartily associate
| |
| myself with the eulogium thereanent so admirably expressed
| |
| by Assheton in the cx'itique just referred to (p. 274), but
| |
| I am bound to confess that as concerns his views on the
| |
| phylogeny of this layer, which he has termed the “ trophoblast,†he seems to me to have forsaken the fertile field of
| |
| legitimate hypothesis for the barren waste of unprofitable
| |
| speculation, and to have erected therein an imposing edifice on
| |
| the very slenderest of foundations.
| |
| | |
| Before I proceed to justify this, my estimate of Hubrecht's
| |
| views on the phylogeny of the trophoblast, let me first set
| |
| forth his conception so far as I understand it. He starts
| |
| with the assumption that the vertebrates (with the exception
| |
| | |
| | |
| THE EARLY EEVELOrMENT OF THE MARSUPIALIA. 105
| |
| | |
| | |
| of Ainpliioxus, the CyclostoineSj and the Elasraobi'anclif!) are
| |
| descended from “vermiform predecessors of coelenterate
| |
| pedigree†possessed of free-swimming larvte, in which there
| |
| was present a complete larval membi'ane of ectodermal derivation, and of the same order of differentiation “as the outer
| |
| larval layer which in certain Nemertines, Gephyreans, and other
| |
| worms often serves as a temporaiy envelope that is stripped
| |
| off when the animal attains to a certain stage of development.â€
| |
| When, for oviparity and larval development, viviparity and
| |
| embryonic development became established in the Protetrapodous successors of the ancestral vermiform stock, the
| |
| larval membrane did not disappear. On the contrary, it is
| |
| assumed that it merely changed “its protective or locomotor
| |
| function into an adhesive one,†and so, development now
| |
| taking place in utero, it is quite easy to understand how tlie
| |
| larval membrane could gradually become transformed into
| |
| a trophic vesicle, containing the embryo as before, and
| |
| functional in the reception of nutriment from the walls of
| |
| the maternal uterus. The final stages in the evolution of
| |
| this trophic vesicle constituted by the old larval membrane
| |
| are met with amongst the mammals, since in them it
| |
| became vascularised so as to constitute a “yet more
| |
| thorough system of nourishment at the expense of the
| |
| maternal circulatory system.†Such, then, is the phylogeny
| |
| of the trophoblast according to Hubrecht. The Eutheriau
| |
| mammals, which it is held trace their descent straight back to
| |
| some very early Protetrapodous stock, viviparous in habit and
| |
| with small yolk-poor, holoblastic eggs, exhibit the trophoblast in its most perfect condition. Hubrecht therefore starts
| |
| with them, and attempts to demonsti'ate the existence of a
| |
| larval membrane, or remnants of such, externally to the
| |
| embryonal ectoderm in all vertebrates with the exceptions
| |
| already mentioned. There is no question of its existence in
| |
| the Meta- and Eutherian mammals. “We may,†writes
| |
| Hubrecht ('08, p. 12), . . . “insist upon the fact that
| |
| | |
| . . . all Didelphia and Monodelphia hitherto investi
| |
| gated show at a very early moment the didermic stage out of
| |
| | |
| | |
| 106
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| which the embryo will be built up enclosed in a cellular
| |
| vesicle (the troplioblast), of which no pai‘t ever enters into
| |
| the embryonic organisation.†The common possession by the
| |
| Metatheria and Eutheria of a larval membi'ane is after all
| |
| only what might be expected, “since after Hill's ('97)
| |
| investigations, we must assume that the didelphian mammals
| |
| are not descended from Ornithodelphia but from monodelphian
| |
| placental ancestors.†As concerns the Prototheria, although
| |
| they cannot in any sense be regarded as directly ancestral to
| |
| the other mammals, we nevertheless find the trophoblastic
| |
| vesicle “ compax'atively distinct.†“In many reptiles and
| |
| birds,†however, it is “.distinguished with great diflSculty
| |
| from the embryonic shield,†and this is explained bv the
| |
| fact that the Sauropsida which are assumed to have taken
| |
| their origin from the same Protetrapodous stock as the
| |
| mammals but along an entirely independent line, have
| |
| secondarily acquired, like the Prototheria, the oviparous
| |
| habit, with its concomitants, a yolk-laden egg and a shell, and
| |
| this latter acquisition has naturally tended “to relegate any
| |
| outer larval layer to the pension list†('09, p. 5). “Concerning the yolk accumulation in the Sauropsidan egg, there
| |
| is no trouble at all to suppose that the vesicular blastocyst
| |
| of an early vivipai-ous ancestor had gradually become yolkladen. The contrary assumption, found in the handbooks,
| |
| that the mammalian egg, while totally losing its yolk, has
| |
| yet preserved the identical developmental featui-es as the
| |
| Sauropsid, is in ideality much more difiicult to reconcile with
| |
| sound evolutionary principles†('09, p. 5).
| |
| | |
| Amongst the lower Vertebrates the larval membrane is
| |
| clearly enough recognisable in the so-called Deckschicht of
| |
| the Teleostomes, Dipnoans, and Amphibians. It is frankly
| |
| admitted that Amphioxus, the Cyclostomes, and the Elasmobranchs “ show in their early development no traces of a
| |
| Deckschicht†(larval layer, troiDhoblast), but there is no
| |
| difficulty about this, since it is easy enough to suppose, in
| |
| view of other characters, that “ the Selachians may very well
| |
| have descended from ancestors without any outer larval layer â€
| |
| | |
| | |
| THE EAKLY HEVELOrMENT OL<' THE MAESOPIALIA. 107
| |
| | |
| {'08, p. 151), and ‘'for Cyclostomes tlie same reasoning holds
| |
| good†(p. 152).
| |
| | |
| The trophoblast, then, is conceived of by Hubrecht as a
| |
| larval membrane of ectodermal derivation, which invests the
| |
| embryonal ahlage in all Vertebrates with the exceptions
| |
| mentioned, 'which is subject to secondary reduction, and which
| |
| is homologous throughout the series. As I understand the
| |
| conception, what is ordinarily called extra-embryonal ectoderm in the Sauropsida is not trophoblast, otherwise Hubrecht
| |
| could hardly write - “in reptiles and birds traces of the
| |
| larval layer have in late years been unmistakably noticedâ€
| |
| ('09, p. 5) ; nevertheless what other writers have termed
| |
| embryonal and extra-embryonal ectoderm in the Prototheria
| |
| is claimed by Hubrecht as trophoblast (at all events that is
| |
| my interpretation of his statement that a trophoblastic vesicle
| |
| is present in these forms), and yet some years ago Hubrecht
| |
| ('04, p. 10) found it diflBcult “ to understand that the name
| |
| has been misunderstood both by embryologists and gynecologists.†My own feeling is that the more recent developments in his views have tended to obscure rather than to
| |
| clarify our ideas as to the trophoblast, especially if we must
| |
| now hold that the chorion or serosa of the Sauropsida is not
| |
| homologous with that of the Prototheria, which necessarily
| |
| follows if the extra-embi'yonal ectoderm of the Sauropsidan is
| |
| not the same thing as that of the Monotreme.
| |
| | |
| Assuming that we have formed a correct conception of the
| |
| trophoblast as a larval membrane, and bearing in mind that it
| |
| is best developed in the Metatheria and Eutheria, since these
| |
| alone amongst higher Vertebrates have retained unaltered
| |
| the viviparous habits of their Protetrapodous ancestors, let us
| |
| see what basis in fact there is for the statement of Hubrecht
| |
| ('08, p. 68) that “before the ectoderm and the entoderm
| |
| have become differentiated from each other there is in
| |
| mammals a distinct larval cell-layer surrounding (as soon as
| |
| cleavage of the egg has attained the morula stage) the
| |
| mother-cells of the embryonic tissues.†Now that statement
| |
| as it stands, I have no hesitation in characterising as entirely
| |
| | |
| | |
| 108
| |
| | |
| | |
| .T. P. HlIiL.
| |
| | |
| | |
| misleading, inasmuch as it is applicable not to the Mammalia
| |
| as a whole, but, so far as it refers to matters of undisputed
| |
| fact, to one only of the three mammalian subclasses, viz. the
| |
| Eutheria. So far as the latter ai'e concerned, practically all
| |
| observers, as we have seen, are agreed that there is present
| |
| during at least the early stages of development a complete
| |
| outer layer of cells which encloses the embryonal anlage
| |
| or inner cell-mass (that portion of it immediately overlying
| |
| the latter being termed the “ Deckschicht †or “Rauber's
| |
| layerâ€). It is, of course, this envelojDing layer or trophoblast which Hubrecht interprets as a larval membrane.
| |
| It fulfils the conditions, and were the Eutheria the only
| |
| Vertebrates known to us, the idea might be plausible
| |
| enough.
| |
| | |
| Turning now to the Metatheria, and I'emembering that these,
| |
| according to Hubrecht, are descended from the Eutheria, we
| |
| should naturally expect to find the supposed larval membrane
| |
| fully developed, with all its ancestral relations ; and so we do
| |
| if we are content to accept Hubrecht's interpretation of
| |
| Selenka's results and figures in the case of Didelphys. The
| |
| “ urentodermzelle †of Selenka is for Hubrecht “ undoubtedly
| |
| the mother-cell of the embryonic knob,†the ectoderm of
| |
| Selenka is manifestly the trophoblast - a complete larval
| |
| layer. It is no doubt unfortunate that Hubrecht had to rely
| |
| on the work of Selenka as his source of information on the
| |
| early development of Marsupials, but it must be remembered
| |
| that he reads his own views into Selenka's figures. On the
| |
| basis of my own observations on the early ontogeny of Marsupials, I have no hesitation in affirming that a larval membrane, in the sense of Hubrecht, does not exist in any of the
| |
| forms (Dasyurus, Perameles, Macropus) studied by me. The
| |
| observations recorded in the preceding pages of this paper
| |
| demonstrate, in the case of Dasyurus without the possibility
| |
| of doubt, the entire absence of any cellular layer external
| |
| to the formative region of the blastocyst, i.e. in a position
| |
| corresponding to that occupied by Rauber's layer in Eutheria,
| |
| whilst in the case of Perameles and Macropus, they yield not
| |
| | |
| | |
| THE EAULY DEVELOPMENT OP THE MAESUPIALTA. 109
| |
| | |
| | |
| the slightest evidence for the existence of any such layer.
| |
| The formative region of the Marsupial blastocyst, which is
| |
| undoubtedly the homologue of the inner cell mass of the
| |
| Eutheria, forms from the first part of the unilarninar blastocyst wall, and is freely exposed. The remainder of the latter
| |
| is constituted by a layer of non-formative cells, the destiny
| |
| of which is the same as that of the so-called trophoblast of
| |
| the Eutheria. I have therefore ventui'ed to suggest that they
| |
| are one and the same. If, then, the trophoblast is really a
| |
| larval membrane, we must assume, in the case of the Marsupial, either that its “ Deckschicht '' portion has been completely suppressed (but why it should have been I fail to
| |
| understand, unless, perhaps, it is a result of the secondary
| |
| acquisition by the Marsupials of a shell-membrane, these
| |
| mammals being even now on the, way to secondarily assume
| |
| the oviparous habit !), or that the non-formative region of the
| |
| Marsupials is not the homologue of the trophoblast, in which
| |
| case the Marsupials must be held to have entirely lost the larval
| |
| membrane, since there is no other layer present which could
| |
| possibly represent it. These considerations may well give us
| |
| pause before we calmly accept Hubrecht's conception of the
| |
| trophoblast as a larval membrane present in all mammals
| |
| without exception.
| |
| | |
| Coming now to the Prototheria, we find, according to
| |
| Hubrecht, the trophoblastic vesicle . . . yet compara
| |
| tively distinct,†and so it is if we accept the interpretation of
| |
| Hubrecht of the observations and figures of Semon, Wilson
| |
| and Hill. The unilarninar blastoderm of these authors is
| |
| unmistakably the trophoblast. The cells situated internally
| |
| to that in the region of the white yolk-bed are not entodertnal, as suggested by Semon, but constitute for Hubrecht
| |
| “ the mother cells of the embryonic knob.†I need only quote
| |
| again the opinion of Assheton thereanent and express my
| |
| agreement therewith; he writes (^09, p. 233) : For this view
| |
| | |
| I can see no reason derivable from actual specimens described
| |
| and figured by those four authors†(Caldwell, Semon, Wilson
| |
| and Hill). It would appear, then, that the assumption of
| |
| | |
| | |
| 110
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| Hubreclit of the presence of a larval membrane of the nature
| |
| postulated in the Prototheria and Metatheria is devoid of
| |
| foundation in fact, so that there but remains the question of
| |
| the significance of the outer enveloping layer of the Eutherian
| |
| blastocyst. As regards that, I venture to think that the
| |
| alternative interpretation of E. van Beneden and other
| |
| investigators, which I have attempted to develop in the
| |
| pages of this paper, affords a simpler and more satisfying
| |
| explanation of its significance and phylogeny than that
| |
| advocated by Prof. Hubrecht, an interpretation, moreover,
| |
| which is more in accordance, not only with all the known
| |
| facts, but ''with sound evolutionary principles and with the
| |
| conclusions arrived at by the great majority of comparative
| |
| anatomists and palaeontologists as to the origin and intei-relationships of the Mammalia.
| |
| | |
| And I also venture to think that what has just been said
| |
| holds true with reference to the views advocated by Mr.
| |
| Assheton. These views owed their origin to certain appearances which he found in some segmenting ova of the sheep
| |
| (but, be it noted, not in all those he examined), and he has
| |
| attempted to re-intei pret not only his own earlier observations,
| |
| but those of other workers on the early ontogeny of the Eutheria
| |
| in the light of his newer faith, and not only so, he holds that it
| |
| is also possible to apply that in the interpretation of the early
| |
| ontogeny of Marsupials (v. '08, p. 235, and '09, p. 229). He
| |
| maintains that the inner cell-mass of Eutheria is purely ectodermal, aud that the enveloping trophoblast layer of the blastocyst arises in common with the entodermal lining of the same
| |
| and is therefore also entodei'mal. " On the theory I advocate,â€
| |
| he writes ('09, p. 235), " the trophoblast is of Eutherian
| |
| mammalian origin only and is not homologous to any form of
| |
| envelope outside the group of Eutherian mammals.†These
| |
| views of Assheton are not only at variance with those of all
| |
| other investigators who have worked at the early ontogeny of
| |
| Eutheria, but they are quite irreconcilable with my observations on the development of Dasyurus herein recorded. I claim
| |
| to have shown in that Marsupial that the formative region, the
| |
| | |
| | |
| THE EAELY DEVELOPMENT OP THE MAllSUPIALIA. Ill
| |
| | |
| | |
| homologneof the inner cell-mass, gives origin not only to the
| |
| embryonal ectoderm, but to the entire entoderm, whilst tlie
| |
| non-formative region, whose homology to the trophoblast of
| |
| Eutheria is admitted by Assheton, arises quite independently
| |
| of the entoderm and a long time before the latter inakes its
| |
| appearance. There is, then, in Dasyurus no question of a
| |
| common origin of the entoderm and the non-forrnative or
| |
| trophoblastic region of the blastocyst wall. And exception
| |
| inay be taken to Assheton's views on quite other grounds
| |
| (e. g. the question of the homologies of the foetal membranes
| |
| in the series of the Amniota), as he himself is well awai'e, and
| |
| as Jenkinson ('00) has also emphasised. I feel, however, I can
| |
| leave further discussion of Assheton's views until such time
| |
| as my observations on Dasyurus are shown to be erroneous or
| |
| inapplicable to other Marsupials.
| |
| | |
| 3. The Entypic Condition of the Eutherian
| |
| | |
| Blastocyst.
| |
| | |
| If, now, on the basis of the homologies I have ventm-ed to
| |
| advocate in the preceding pages, we proceed to compare the
| |
| Metatherian with the Eutherian blastocyst, we have to note
| |
| that, whereas in the latter the extra-embryonal or trophoblastic ectoderm alone forms the blastocyst wall in early
| |
| stages and completely encloses the embryonal knot, in the
| |
| former, the homologous parts, viz. the non-formative or exti'aembryonal and the formative or embryonal regions, both
| |
| enter into the constitution of the unilaminar blastocyst
| |
| wall, there being no such enclosure of the one by the
| |
| other as occurs in the Eutherian blastocy.st (Text-fig. 2, p. 98).
| |
| It is characteristic of the Marsupial as of the Monotreme that
| |
| the embryonal region is from the first superficial and freely
| |
| exposed. It is spread out as a cellular layer and simply
| |
| forms part of the blastocyst wall or blastoderm. It is equally
| |
| characteristic of the Eutherian that the homologous part, the
| |
| embryonal knot, has at first the form of a compact mass,
| |
| which is completely enclosed by the trophoblastic ectoderm.
| |
| | |
| | |
| 112
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| The latter alone constitutes the unilaminar wall of the
| |
| blastocyst and has the embryonal knot adherent at one spot
| |
| to its inner surface. The formative cells which compose
| |
| the knot thus take at first no part in the constitution of
| |
| the outei wall of the blastocyst^ and may or may not
| |
| do so in later stages according as the covering layer of the
| |
| trophoblast (the Deckschicht or Rauber's layer) is transitory or permanent. This peculiar developmental condition, characterised by the internal position of the formative
| |
| or embryonal cells within the blastocyst cavity, has been
| |
| termed by Selenka (TO) “entypy†(Entypie des Keimfeldes).^ It is a phenomenon exclusively found in the
| |
| Eutheria and characteristic of them alone, amongst the
| |
| mammals. In the Marsupial, as in the Monotreme, the
| |
| formative cells are freely exposed, and constitute from the first
| |
| part of the blastocyst wall just as those of the Sauropsida form
| |
| a part of the general blastoderm. Limited as entypy thus
| |
| appears to be to the higher mammals, the probability is that
| |
| we have to do here with a purely secondary, adaptive feature.
| |
| | |
| If we proceed to inquire what is the significance of this
| |
| remarkable difference in the early developmental phenomena
| |
| of the lower and higher mammals, it seems to me that we have
| |
| to take account, in the first place, of the differences in the
| |
| structure of their respective eggs, and especially we have to
| |
| bear in mind that the Eutherian ovum is considerably more
| |
| specialised than even the Metatherian. It is on the average
| |
| smaller than the latter, i.e. it has suffered in the course of
| |
| phytogeny still further reduction in size, and has lost, to an
| |
| even greater extent than the Marsupial ovum, the store of foodyolk ancestrally present in it. Moreover, it has suffered a still
| |
| further i-eduction in respect of its secondary egg-membranes.
| |
| The Metatherian ovum still retains in its shell-membrane a
| |
| | |
| ^ “ Unter Entypie des Keimfeldes mdcbte ich dalier verstanden
| |
| wissen : Die nicht dm-cli Bildung typischer Anmionfalten geschehende,
| |
| sondern durcli eine schon wiihrend der Gastrulation erfolgende Absclinurung des Keimfeldes ins Innere der Eiblasenbnlle (Oborion) †('00,
| |
| p. 203).
| |
| | |
| | |
| THE EARLY DEVELOILMENT OP THE MARSUriAl.IA, 113
| |
| | |
| | |
| vestigial representative of the shell of the presumed oviparous
| |
| common ancestor of the Metatheria and Eutheria. The
| |
| Eutherian ovum, on the other hand, has lost all trace of the
| |
| shell in correlation with its more complete adaptation to the conditions of intra-nterine development. The albumen layer is
| |
| variable in its occurrence, being present in some (e.g. rabbit)
| |
| and absent in others (e.g. pig, Assheton), whilst the zona
| |
| itself, though always present, is variable both as to its thickness and the length of time it persists.
| |
| | |
| Strangely enough, although the prevaling opinion amongst
| |
| mammalian embryologists is that the Eutherian ovum has
| |
| been derived phylogenetically from an egg of the same telolecithal and shell-bearing type as is found in the Monotremes,
| |
| no one, so far as I am aware, has ever taken the shell into
| |
| account, and ventured to consider in what way its total disappearance from an ovum already greatly reduced in size,
| |
| might affect the course of the early developmental phenomena.
| |
| That is what I propose to do here, for iu my view it is just in
| |
| the complete loss of the shell by the Eutherian ovum that we
| |
| find the key to the explanation of those remarkable differences
| |
| which are observable between the early ontogeny of the
| |
| Eutheria and Metatheria, and which culminate in the entypic
| |
| condition so distinctive of the former. The acquisition of a
| |
| shell by the Proamniota conditioned the appearance of the
| |
| amnion. The loss of the shell in the Eutheria conditioned the
| |
| occui'rence in their ontogeny of entypy.
| |
| | |
| As we have seen, the mammalian ovum, already in the
| |
| Monotremes greatly reduced iu size as compared with that of
| |
| reptiles, and quite minute in the Metatheria and Eutheria,
| |
| contains within itself neither the cubic capacity nor the food
| |
| material necessary for the production of an embryo on the
| |
| ancestral reptilian lines. We accordingly find that the
| |
| primary object of the first developmeutal processes in the
| |
| mammals has come to be the formation of a vesicle with a
| |
| complete cellular wall, capable of absorbing nutrient fluid from
| |
| the maternal uterus and of growing I'apidly, so as to provide
| |
| the space necessary for embryonal differentiation.
| |
| | |
| VOL. 56, PART 1. NEW SERIES.
| |
| | |
| | |
| 8
| |
| | |
| | |
| 114
| |
| | |
| | |
| ,T. r. HILL.
| |
| | |
| | |
| In the Monotremes this vesiculai' stage is rapidly and
| |
| directly attained as the result, firstly, of the rearrangement
| |
| of the blastomeres of the cleavage-disc to form a unilaminar
| |
| blastodermic membi'ane overlying.tbe solid yolk, and, secondly,
| |
| of the rapid extension of the peripheral (extra-embryonal)
| |
| region of the same, in contact with the inner surface of the
| |
| firm sphere furnished by the egg-envelopes. During the
| |
| completion of the blastocyst embryonal differentiation remains
| |
| in abeyance, and practically does not start until after growth
| |
| of the blastocyst is well initiated.
| |
| | |
| In the Marsupial, notwithstanding the fact that the ovum
| |
| has become secondarily holoblastic, the mode of formation
| |
| of the blastocyst is essentially that of the Monotreme.
| |
| Cleavage is of the radial type, and owing to the persistence
| |
| of the shell, wliicb with the zona forms a firm resistant
| |
| sphere enclosing the egg, the radially arranged blastomeres ai'e able to assume the form of an open ring and to
| |
| proceed directly to the formation of the unilaminar wall of
| |
| the blastocyst. The enclosing sphere provides the necessary
| |
| firm surface over which the products of division of the upper
| |
| and lower cell-rings of the 16-celled stage can respectively
| |
| spread towards opposite poles, so as to directly constitute the
| |
| formative and non-formative regions of the blastocyst wall.
| |
| In my opinion it is the persistence of the resistant shellmembrane round the ovum which conditions the occurrence
| |
| in the Marsupial of this direct method of blastocyst formation.
| |
| As in the Monotreme, so here also embryonal differentiation
| |
| commences only after the blastocyst has gi'ovvn considerably
| |
| in size.
| |
| | |
| ^ In the Eutheria, on the other hand, in the absence of the
| |
| shell-membrane, not only is the mode of formation of the
| |
| blastocyst quite different to that in the Marsupial, but
| |
| the relations of the constituent parts of the completed
| |
| structure also differ markedly from those of the homogenous parts in the latter. The cleavage process here leads
| |
| only indirectly to the formation of the blastocyst, and must be
| |
| held to be csenogeneticaily modified as compared with that of
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MAESUPIALIA. 115
| |
| | |
| | |
| lower mammals. In the cross-shaped arrangement of the
| |
| blastomeres in the 4-celled stage, in the occurrence of a
| |
| definite morula-stage and of the entypic condition, we have
| |
| features in which the early ontogeny of the Eutheria differs
| |
| fundamentally from that of the Metatheria. They are intimately correlated the one with the other, and are met "with in
| |
| all Eutheria, so far as known, but do not occur either in the
| |
| Prototheria or the Metatheria, so that we must regard them
| |
| as secondary features which were acquired by the primitive
| |
| Eutheria under the influence of some common causal factor
| |
| or factoi's, subsequent to their divergence from the ancestral
| |
| stock common to them and to the Metatheria. Now the crossshaped 4-celled stage and the morula-stage are undoubtedly
| |
| to be looked upon simply as cleavage adaptations of prospective
| |
| significance in regard to the entypic condition, so that the
| |
| problem reduces itself to this - how came these adaptations
| |
| to be induced in the first instance ? In view of the facts that
| |
| in the Metatheria, in the presence of the shell-membrane, the
| |
| formation of the blastocyst is the direct outcome of the cleavage
| |
| process, and is effected along the old ancestral lines without
| |
| any enclosure of the formative cells by the non-formative,
| |
| whilst in the Eutheria, in the absence of the shell-membrane, blastocyst formation results only indirectly from the
| |
| cleavage-process, is effected in a way quite different from
| |
| that characteristic of the Metatheria, and involves the
| |
| complete enclosure of the formative by the non-formative
| |
| cells, I venture to suggest that the cleavage adaptations
| |
| which I'esult in the entypic condition were acquired in the first
| |
| instance as the direct outcome of the total loss by the already
| |
| greatly reduced Eutlierian ovum of the shell-membrane.^
| |
| This view necessarily implies that the presence of a thick
| |
| zona such as occurs round the ovum in certain Eutheria is
| |
| secondary, and what we know of this membrane in existing
| |
| Eutheria is at all events not adverse to that conclusion.
| |
| | |
| This suggestion I first put foi'ward in a course of lectures on the
| |
| early ontogeny and placentation of the Mammalia delivered at the
| |
| University of Sydney in 1904.
| |
| | |
| | |
| 116
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| Amongst tlie Marsupials the zona is quite thin (about -00] 6
| |
| imn. in Dasyurus), presumptive evidence that it was also thin
| |
| in the ancestral stock from which the Meta- and Eutheria
| |
| diverged, whilst amongst the Eutheria themselves the zona,
| |
| as Robinson ('03) has pointed out, is not only of very varying
| |
| thickness, but persists round the ovum for a very varying
| |
| period iu different species. It appears to be thinnest in the
| |
| mouse ('001 mm.), in most Eutheria it is considerably thicker
| |
| (•01 mm., bat, dog, rabbit, deer), whilst in Cavia it reaches
| |
| a thickness of as much as -02 mm. In those forms in which
| |
| the blastocyst early becomes embedded in, or attached to, the
| |
| mucosa, the zona naturally disappears early. In the rat,
| |
| mouse and guinea-pig it disappears before the blastocyst is
| |
| formed. Hubrecht failed to find it in the 2-celled egg of
| |
| Tupaia, and it was already absent in the 4-celled stage of
| |
| Macacus nemestrinus, discovered by Selenka and described by Hubrecht. On the other hand, it may persist for
| |
| a much longer period, up to the time of appearance of the
| |
| primitive streak (rabbit, dog, ferret). These facts sufficiently demonstrate the variability of the zona in the Eutherian
| |
| series, and its early disappearance in certain forms before the
| |
| completion of the blastocyst stage shows that it can have no
| |
| supporting function in i-egard to that.
| |
| | |
| Postulating, then, the disappearance of the shell-membrane
| |
| and the presence of a relatively thin, non-resistant zona (with
| |
| perhaps a layer of albumen) round the minute yolk-poor ovum
| |
| of the primitive Eutherian, and remembering that the ovum
| |
| starts with certain inherited tendencies, the most immediate
| |
| and pressing of which is to produce a blastocyst comprising
| |
| two differentiated groups of cells, the problem is how, in the
| |
| absence of the old supporting sphere constituted by the eggenvelopes, can such a vesicular stage be most easily and
| |
| most expeditiously attained ? The Eutherian solution as we see
| |
| it in operation to-day is really a very simple one, and withal a
| |
| noteworthy instance of adaptation in cleavage (Lillie, '99).
| |
| In the absence of any firm supporting membrane round the
| |
| egg, and the consequent impossibility of the blastomeres pro
| |
| | |
| THE EARLV DEVELOPMENT OF THE MARSUPIALIA. 117
| |
| | |
| ceecling- at once to forna the blastocyst wall, they are under
| |
| the necessity of keeping together, and to this end cleavage
| |
| has become adapted. For the ancestral radial arrangement
| |
| of the blastomeres in the 4-celled stage, characteristic of the
| |
| Monotreme and Marsupial, there has been substituted a
| |
| cross-shaped grouping into two pairs, and, as the outcome of
| |
| this adaptive alteration in the cleavage planes, there results
| |
| from the subsequent divisions, not an open cell-ring, as in tbe
| |
| Marsupial, but a compact cell-group or morula. In this we
| |
| again encounter precisely the same differentiation of the
| |
| blastomeres into two categories, respectively formative
| |
| (embryonal) and non-formative (trophoblastic) insignificance,
| |
| as is found in the 16-celled stage of the Marsupial, but, since
| |
| the two groups of cells are here massed together, and in the
| |
| absence of any firm enclosing sphere, cannot spread independently so as to form directly the wall of the blastocyst,
| |
| there has arisen the necessity for yet other adaptive modifications. Attention has already been directed to the tardiness
| |
| of differentiation in the embryonal region of the Monotreme
| |
| and Marsupial blastocyst, and here in the minute Eutherian
| |
| morula we find what is, perhaps, to be looked upon as a
| |
| further adaptive exaggeration of this same feature in the
| |
| inertness which is at tirst displayed by the formative cells,
| |
| and which is in marked contrast with the activity shown by
| |
| the non-formative ectodermal cells.^ It is these latter, it
| |
| | |
| * The inertness of the formative cell-mass is accounted for by Assheton
| |
| ('98, p. 251) as follows : “ Now, as the epiblast plays the more prominent
| |
| part in the formation of the l^nlk of the embi-yo dui-ing the earliest
| |
| stages, it clearly would be useless for tlie embryonic part to exhibit
| |
| much energy of growth until the old conditions [in particular sufficient
| |
| room for embryonal differentiation] were to a certain extent regained ;
| |
| hence the lethargy exhibited by the embryonic epiblast in mammals
| |
| during the first week of develoxunent. No feature of the early stages of
| |
| the mammalian embryo is more striking than this inertness of the
| |
| embryonic eiriblast - or, as I should nowjrrefer to call it, simply epiblast
| |
| - during the first few days.†Assheton, it should be remembered, holds
| |
| that the inner cell-mass of Eutheria furnishes only the embryonal
| |
| ectoderm.
| |
| | |
| | |
| 118
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| should be recollected, which exhibit the greatest growthenergy during the formation of the blastocyst in the Monotreme and Marsupial, and so their greater activity in the
| |
| Eutherian tnoi'ula is only what might be expected. Dividing
| |
| more rapidly than the formative cells, they gradually grow
| |
| round the latter, and eventually form a complete outer layer
| |
| enveloping the inert formative cell-group. This process oFovergrowth or epiboly is entirely comparable in its effect with the
| |
| spreading of the extra-embryonal region of the unilamiiiar
| |
| blastodermic membrane in the Monotreme to enclose the yolkmass, and with that of the non-formative cells in the Marsupial
| |
| to complete the lower hemisphere of the blastocyst, growlh
| |
| round an inert central cell-mass being here substituted for
| |
| growth over the inner surface of a I'esistant sphere constituted
| |
| by the egg-envelopes, such as occurs during the formation of
| |
| the blastocyst in the Monotreme and Marsupial. .Just as the
| |
| first objective of the cleavage process in the latter is to effect
| |
| the completion of the cellular wall of the blastocyst, so hei*e
| |
| the same objective recurs, and is attained in the simplest
| |
| possible way in the new circumstances, viz. by the I'apid envelopment of the formative by the, non-formative cells. Thus
| |
| at the end of the cleavage process in the EutheiJan we have
| |
| formed a solid entypic morula in which an inner mass of
| |
| formative cells is completely surrounded by an outer enveloping layer of non-formative or ti'opho-ectodermal cells, homogenous with the extra-embryonal ectoderm of the Sauropsidan
| |
| and Monotreme and the non-formative region of the unilaminar blastocyst of the Marsupial. Conversion of the solid
| |
| morula into a hollow blastocyst capable of imbibing fluid
| |
| from the uterus and of growing rapidly now follows. Intraor intercellular vacuoles appear below the inner cell-mass, by
| |
| the confluence of which the blastocyst cavity is established,
| |
| and the inner cell-mass becomes separated from the enveloping layer of tropho-ectoderm, except over a small area where
| |
| the two remain in contact.
| |
| | |
| The complete enclosure of the formative cells of the inner
| |
| cell-mass by the non-formative ectodermal cells of the
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 119
| |
| | |
| | |
| enveloping layer which produces this peculiar entypic condition in the Eutherian blastocyst, I would interpret, then, as
| |
| a purely adaptive phenomenon, which in the given circumstances effects in the simplest possible way the early completion
| |
| of the blastocyst wall, and whose origin is to be traced to
| |
| that reduction in size and in its envelopes which the Eutherian
| |
| ovum has suffered in the course of phylogeny, in adaptation
| |
| to the conditions of intra-uterine development. In particular,
| |
| starting with a shell-bearing ovum, already minute and
| |
| undergoing its development in utero, I see in the loss of
| |
| the shell such as has occurred in the Eutheria an intelligible
| |
| explanation of the first origin of those adaptations which
| |
| culminate in the condition of entypy. I am therefore wholly
| |
| unable to accept the view of Hubrecht (^08, p. 78), that " what
| |
| Selenka has designated by the name of Entypie is - from
| |
| our point of view - no secondary phenomenon, but one
| |
| which repeats very primitive featui*es of separation between
| |
| embryonic ectoderm and larval envelope in invertebrate
| |
| ancestors.â€
| |
| | |
| I see no reason for supposing that the intimate relationship
| |
| which is early established in many Eutheria between the
| |
| trophoblastic ectoderm and the uterine mucosa has had anything to do with the origination of the entypic condition. In
| |
| ray view such intimate relationship involving the complete
| |
| enclosui'e of the blastocyst in the mucosa only came to be
| |
| established secondarily, after entypy had become the rule.
| |
| On the other hand, the peculiar modifications of the entypic
| |
| condition met with in rodents with “^inversion†(e.g. i-at,
| |
| mouse, guinea-pig) are undoubtedly to be correlated, as Van
| |
| Beneden also believed ('99, p. 332), with the remarkably early
| |
| and complete enclosure or implantation of the germ in the
| |
| mucosa such as occurs in these and other Eutheria. Similar
| |
| views are expressed by Selenka in one of his last contributions
| |
| to mammalian embryology. He writes ('00, p. 205) - “Dass
| |
| die Entypie des Keimfeldes und die Blattinversion begiinstigt
| |
| wil'd durch die friihzeitige Yerwachsung der Eiblase mit dem
| |
| Uterus, ist nicht in Abrede zu stellen. Aber da dieser
| |
| | |
| | |
| 120
| |
| | |
| | |
| J. P, HILL.
| |
| | |
| | |
| Prozess auch in solclieu Eiblasen dei- Saugetiere vorkommen
| |
| kanii, die iiberhaupt nichb, odei- erst spiiter mifc dem Uterus
| |
| verwachsen, so kaiiu die Keimfeld-Entypie zwar durch die
| |
| frube Verwacbsung veraiilasst, aber nicht ausscldiesslich
| |
| liervorgerufeii werclen.†He goes on to remark that - “Die
| |
| Vorbedingimgeti zur Eutypie miissen in der Struktur der verwachseuden Eiblase gesucht werden/^ and expi-esses his
| |
| agreement with the views of Van Beneden as to tlie significance to be attributed to the early cleaviige phenomena in
| |
| Eutheria.
| |
| | |
| The attitude of the illustrious Belgian embryologist whose
| |
| loss ws have so recently to deplore, towards this problem is
| |
| clearly set forth in the last memoir which issued from his
| |
| hand. “Je suis de ceux,^' he wrote (T9, p. 332), “qui pensent
| |
| que toute Pembryologie des Mammiferes placentaires temoigue
| |
| quTls derivent d'animaux qui, comme les Sauropsides et les
| |
| Mouotremes, produisaieut des oeufs meroblastiques. Je ne
| |
| puis a aucun point de vue me rallier aux idees contraires formulees eb defendues par Hubrecht. L^hypothese de Hubrecht
| |
| se heurte a des difiicultes morpliologiques et physiologiques
| |
| insurmontables : elle laisse inexpliquee Pexistence, chez les
| |
| Mammiferes placentaires, d'une vesicule ombilicale et dTne
| |
| foule de caracteres commnns a tons les Amniotes et distiuctifs
| |
| de ces auimaux.'^ Holding this view of tlie origin of the
| |
| Eutheria, Van Beneden based his interpretation of their early
| |
| ontogenetic phenomena on the belief that “ la reduction progressive du volume de Poeuf d'une part, le fait de son
| |
| developpement iutrauterin de hautre ont dii avoir une influence preponderante sur les premiers processus evolutifs.â€
| |
| | |
| Balfour, in his classical treatise, had already some eighteen
| |
| years earlier expressed precisely the same view. “The
| |
| features of the development of the placental Mammalia,^' he
| |
| wrote (‘Mem. Edn.,^ vol. iii, p. 289), “receive their most
| |
| satisfactory explanation on the hypothesis that their ancestors
| |
| were provided with a large-yolked ovum like that of Sauropsida. The food-yolk must be supposed to have ceased to be
| |
| developed on the establishment of a maternal nutrition through
| |
| | |
| | |
| THE EAELY DEVELOPMENT UE THE MAKSUPJALIA. 121
| |
| | |
| | |
| the uterus. . . . The embryonic evidence of the common
| |
| | |
| origin of Mammalia and Sauropsida, both as concerns the
| |
| formation of the layers and of the embryonic membranes is
| |
| as clear as it can be.'''
| |
| | |
| That view of tlie derivation of the Mammalia receives, I
| |
| venture to think, striking confirmation from the observations
| |
| and conclusions set forth in the preceding pages of this
| |
| memoir, and from it as a basis all attempts at a phylogenetic
| |
| interpretation of the early ontogenetic phenomena in the
| |
| Mammalia must, I am convinced, take their origin. Such an
| |
| attempt I have essayed in the foregoing pages, with what
| |
| success the reader must judge.
| |
| | |
| | |
| Addendum.
| |
| | |
| The memoir of Prof. 0. Van der Stricht, entitled “La structure de I'cBuf des Mammiferes (Chauve-souris, Vesperugo
| |
| noctula) : Troisieme Partie†(‘Mem. de PAcad. roy. de
| |
| Belgique,' 2nd ser., t. ii, 1909), came into my hands only
| |
| after my own paper had readied its final form, and therefore
| |
| too late for notice in the body of the text. In this extremely
| |
| valuable contribution, Van der Stricht gives a detailed
| |
| account of the growth, maturation, fertilisation, and early
| |
| cleavage-stages of the ovum of Vesperugo, illustrated by a
| |
| superb series of drawings and photo-micrographs. All I can
| |
| do here, however, is to direct attention to that section of the
| |
| paper entitled “ Phenomeues de deutoplasmolyse an pole
| |
| vegetatif de I'ceuf†(pp. 92 - 96), in which the author describes
| |
| the occurrence in the bat's ovum of just such a process of
| |
| elimination of surplus deutoplasmic material as I have
| |
| recorded for Dasyurus. Van der Stricht's interpretation of
| |
| this phenomenon agrees, I am glad to find, with my own.
| |
| He writes (pp. 92-93): “ Ce deutoplasme rudimentaire, i\
| |
| peine ebauche dans I'ovule des Mammiferes, parait etre
| |
| encore trop abundant dans I'oeuf de Chauve-souris, car ces
| |
| materiaux de reserve, en partie inutiles, sont partiellement
| |
| elimines, expulses de la cellule.â€
| |
| | |
| | |
| 122
| |
| | |
| | |
| .T. P. HILL.
| |
| | |
| To this pi'ocess of elimination of surplus deutoplasm he
| |
| applies the name deutoplasmolyse,†and states that Ce
| |
| phenomene consiste dans I'apparition de lobules vitellins
| |
| multiples, en nombre tres variable, a la surface du vitellus au
| |
| niveau du pole vegetatif. Ces bourgeons a peu pres tous de
| |
| meme grandeur, les uns etant cependant un peu plus volumineux que les autres, apparaissent dans le voisinage des globules
| |
| polaires et presentent la structure du deutophisme. 11s sont
| |
| formes de vacuoles claires, a I'interieur desquelles on aper^oit
| |
| parfois de petits grains vitellins, dont il a ete question plus
| |
| haut. . . . Ce processus de deutoplasmolyse devient
| |
| | |
| manifeste surtout apres I'expulsion du second globule polaire,
| |
| pendant la periode de la fecondation. 11 pent etre tres
| |
| accentue, au stade du premier fuseau de segmentation et au
| |
| debut de la segmentation de I'oeuf, notamment sur des ovules
| |
| divises en deux et en quatre (figs. 59, 61, 62, d).†It would
| |
| therefore appear that, whilst in Dasyurus the surplus deutoplasm is eliminated always prior to the completion of the
| |
| first cleavage and in the form of a single relatively large
| |
| spherical mass, in Vesperugo it is cast off generally, though
| |
| not invariably, before cleavage begins, and in the form of a
| |
| number of small separate lobules.
| |
| | |
| | |
| List op References.
| |
| | |
| '94. Assheton, R. - “ A Re-investigation into the Early Stages of the
| |
| Development of the Rabbit,†‘ Quart. Journ. Micr. Sci.,' vol. 34.
| |
| | |
| '98. “ The Development of the Pig during the Pirst Ten Days,â€
| |
| | |
| ‘ Quart. Journ. Micr. Sci.,' vol. 41.
| |
| | |
| '98. “ The Segmentation of the Ovum of the Sheep, with Obser
| |
| vations on the Hypothesis of a Hypoblastic Origin for the
| |
| Trophoblast,†‘ Quart. Journ. Micr. Sci.,' vol. 41.
| |
| | |
| '08. “ The Blastocyst of Capra, with Some Remarks upon the
| |
| | |
| Homologies of the Germinal Layers of Mammals,†‘Guy's
| |
| Hospital Reports,' vol. Ixii.
| |
| | |
| '09. “Professor Hubrecht's Paper on the Early Ontogenetic
| |
| | |
| Phenomena in Mammals ; An Appreciation and Criticism,â€
| |
| ‘ Quart. Journ. Micr. Sci.,' vol. 54.
| |
| | |
| | |
| 123
| |
| | |
| | |
| THE EAELY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| '97. Bonnet. R. - “ Beitriige zur Embvyologie des Himdes,†‘ Anatomische Hefte,' Bd. ix.
| |
| | |
| '01. “ Erste Fortsetzimg,†‘ Anatomisclie Hefte,' Bd. xvi.
| |
| | |
| '87. Caldwell, W. H. - “ The Erabiyology of Monotremata and Marsnpialia,†Part I, ‘ Phil. Trans. Roy. Soc.,' vol. clxxviii B.
| |
| | |
| '95. Duval, M. - “Etudes sur I'embryologie des Oliciropteres,†‘ Joura.
| |
| de I'Anat. et de la Pliysiol.,' t. xxxi.
| |
| | |
| '86. Heape, W. - “ The Development of the Mole (Talpa Europea), the
| |
| Ovarian Ovum, and Segmentation of the Ovum,†‘Quart. Joum.
| |
| Micr. Sci.,' vol. 26.
| |
| | |
| '97. Hill, J. P. - “ The Placentation of Perameles,†‘ Quart. Journ.
| |
| Micr. Sci.,' vol. 40.
| |
| | |
| '00. “ On the Foetal Membranes, Placentation and Parturition of
| |
| | |
| theNative Cat(Dasyurus viverrinus),†‘Anat. Anz.,'Bd.xviii.
| |
| | |
| '88. Hubrecht, A. A. W. - “ Keimbliitterbildung und Placentation des
| |
| Igels,†‘ Anat. Anz.,' Bd. iii.
| |
| | |
| '89. “ Studies in Mammalian Embryology : (1) The Placentation
| |
| | |
| of Erinaceus europaeus, with Remarks on the Physiology of
| |
| the Placenta,†‘ Quart. Joura. Micr. Sci.,' vol. 30.
| |
| | |
| '95. “ Die Phylogenese des Amnions und die Bedeutung des
| |
| | |
| Trophoblastes,†‘ Verhand. Kon. Akad. v. Wetensch. Amsterdam,'
| |
| vol. iv.
| |
| | |
| '02. “ Fiirchung und Keimblattbildung bei Tarsius Spectrum,â€
| |
| | |
| ‘ Yerhand. Kon. Akad. v. Wetensch. Amsterdam,' vol. viii.
| |
| | |
| '04. “ The Ti'ophoblast,†‘ Anat. Anz.,' Bd. xxv.
| |
| | |
| '08. “ Early Ontogenetic Phenomena in Mammals, and their
| |
| | |
| Bearing on oim Intei'pretation of the Phylogeny of the Vertebrates,†‘ Quart. Joura. Micr. Sci.,' vol. 53. .
| |
| | |
| '09. “The Foetal Membranes of the Vertebrates,†‘ Proc.
| |
| | |
| Seventh Interaational Congress, Boston Meeting,' August 19th
| |
| to 24th, 1907.
| |
| | |
| '00. Jenkinson, J. W. - “A Re-investigation of the Early Stages of the
| |
| Development of the Mouse,†‘ Quart. Journ. Micr. Sci.,' vol. 43.
| |
| | |
| '06. “ Remarks on the Germinal Layers of Vertebrates and on
| |
| | |
| the Significance of Germinal Layers in General,†‘ Mem. and
| |
| Proc. Manchester Lit. and Philos. Soc.,' vol. 1.
| |
| | |
| '01. Keibel, F. - “Die Gastrulation und die Keimblattbildung der
| |
| Wirbeltiere,†‘ Ergebnisse der Anatomie und Entwickelungsgeschichte ' (Merkel u. Bonnet), Bd. x.
| |
| | |
| “ Die Entwickelung der Rehes bis zui* Anlage des Meso
| |
| blast,†‘ Arch, fiir Anat. u. Physiol. Anat. Abth.'
| |
| | |
| | |
| ' 02 .
| |
| | |
| | |
| 124
| |
| | |
| | |
| J. r. Hii,L.
| |
| | |
| | |
| 0/. Lams, H., and Doonne, J. - “ Nouv^elles recheivhes sur la Maturation et la Fecondation de I'cenf des Maminiferes,†‘ Arch de Biol.,'
| |
| t. xxiii.
| |
| | |
| 03. Lee, T. Gr. ‘Implantation of the Ovum in Sf)ermoi)hilus
| |
| tridecemlineatus, Mitcli.,†‘ Mark Anniv. Vol.,' Art. 21.
| |
| | |
| '99. Lillie, F. R. - ‘ Adaptation in Cleavage,†‘Biol. Lect. Wood's
| |
| Holl.,' 1897 - 98 (Ginn & Co., Boston).
| |
| | |
| '09. MacBride, E. W. - “ The Formation of the Layers in Amphioxus
| |
| and its bearing on the Interjiretation of the Eai'ly Ontogenetic
| |
| Processes in other Vertebrates,†‘ Quart. Journ. Micr. Sci.,' vol. 54.
| |
| | |
| 03. Robinson, A. Lectures on the Early Stages in the Development
| |
| of Mammalian Ova and on the Formation of the Placenta in
| |
| Different Groups of Mammals,†‘ Journ. of Anat. and Physiol.,'
| |
| vol. xxxviii.
| |
| | |
| 86. Selenka, E. ‘ Studien iiber Entwickelungsgeschichte der Thiere,'
| |
| IV (1 and 2), “ Das Opossum (Didelphys virginianaj,†Wiesbaden.
| |
| | |
| '91. ‘‘ Beutelfuchs und Kiinguruhratte ; zur Entsteliungs
| |
| geschichte der Amnion der Kantjil (Tragulus javanicus) ;
| |
| Att'en Ost-Indiens,†‘ Studien fiber Entw. der Tiere,' H. 5, Erste
| |
| Hiilfte.
| |
| | |
| '00. ‘ Studien hber Entw. der Tiere,' H. 8, Menschenaffen.
| |
| | |
| “ III, Entwickelung des Gibbon (Hylobates und Sianianga),â€
| |
| Wiesbaden : 0. W. Kreidel.
| |
| | |
| '94. Semon, R. - “Zur Entwickelungsgeschichte der Monotremen,â€
| |
| ‘ Zool. Forschungsreisen iin Australien, etc.,' Bd. ii. Lief 1.
| |
| | |
| '95. Sobotta, J. “ Die Befruchtung und Furchung des Eies der Mans,â€
| |
| ‘ Arch, fiir Mikr. Anat.,' Bd. xlv.
| |
| | |
| '75. Van Beneden, E. - †La Maturation de I'cEuf, la fecondation et les
| |
| Iiremieres phases du develoiipement embryonnaire des Mammiferes d'apres les recherches faites sur Je Lapin,†‘ Bull, de I'Acad.
| |
| roy. des sciences, des lettres, et des beauxaits de Belgique,' t. xl.
| |
| | |
| '80 “ Recherches sur I'emliryologie des Maminiferes, la forma
| |
| tion des feuillets chez le Lapin,†‘ Arch, de Biologie,' t. i.
| |
| | |
| '99 “ Recherches sur les premiers Stades du developpement du
| |
| | |
| Murin (Vespertilio murinus),†‘Anat. Anz.,' Bd. xvi.
| |
| | |
| '03 Van der Stricht, O. - ‘‘La Structure et la Polarite de I'ceuf de
| |
| Chauve-Souris (V. noctula),†‘ Comptes rendus de I'Association
| |
| des Anatomistes, V“ Session, Liege.'
| |
| | |
| “ La Structure de I'ceuf des Maminiferes. Premiere partie,
| |
| | |
| L'oocyte an stade de I'accroissement,†“Arch, de Biologic,'
| |
| t. xxi.
| |
| | |
| | |
| '04
| |
| | |
| | |
| THE EARLY DEVELOEMENT OF THE MARSTJriALIA. 125
| |
| | |
| | |
| '05 Van cler Stvidit, O. - “ La Stvuctnre de I'ceuf des MammifOTes.
| |
| Denxieine partie, Structure de I'ceuf ovarique de la femme,†‘ Bull,
| |
| de I'Acad. Roy. de Medicine de Belgique,' Seance du 24 J uin, 1905.
| |
| | |
| '97 Wilson, J. T., and Hill, J. P. - “ Observations upon the Development and Succession ot the Teeth in Perameles; togethei with
| |
| a Contribution to the Discussion of the Homologies of the Teeth
| |
| in Marsupial Animals,†‘ Quart. Journ. Micr. Sci., vol. xxxix.
| |
| | |
| '03 “ Primitive Knot and Early Gastrnlation Cavity co
| |
| existing with independent Primitive Streak in Ornithorhynchus,â€
| |
| ‘ Proc. Roy. Soc.,' vol. Ixxi.
| |
| | |
| '07 “ Observations on the Development of Ornithorhyn
| |
| chus,†‘ Phil. Trans. Roy. Soc.,' Series B, vol cxcix.
| |
| | |
| | |
| EXPLANATION OF PLATES 1-9,
| |
| | |
| Illustrating Prof. J. P. Hill's paper on “ The Early Development of the Marsupialia, with Special Reference to the
| |
| Native Cat (Dasyurus vi verrinus).â€
| |
| | |
| [All figures are from specimens of Dasyurus, unless otherwise indicated. Drawings were executed with the aid of Zeiss's camera lucida,
| |
| except figs. 61-63, which were drawn from photographs.]
| |
| | |
| List of Common Reference Letters.
| |
| | |
| Ab7i. Abnormal blastomei'e, fig. 37. alh. Albumen, eg. Coagulum.
| |
| d. p. Discus proligerus. d. z. Deutoplasmic zone. emb. a. Embryonal
| |
| area. emb. ect. Embiyonal ectoderm, ent. Entoderm. /. ep. Follicular
| |
| epithelium. /. a. Formative area of blastocyst wall. /. c. Formative
| |
| cell. /. z. Formative zone. i. c. Internal cell, fig. 34. Z. eat. Limit of
| |
| extension of entoderm. Z. p. Incomjilete ai'ea of blastocyst wall at lower
| |
| pole. p. b'. First polar body. p. b'. s. First polar spindle, p. V. s.
| |
| Second polar spindle, p. s. Perivitelline space, s. m. Shell-membrane.
| |
| sp. Sperm in albumen. Zr. ect. Non-formative or trophoblastic ectoderm (tropho-ectoderm). y.b. Yolk-body. z. p. Zona.
| |
| | |
| PLATE 1.
| |
| | |
| Fig. 1. - Photo-micrograph (x 150 diameters) of the full-grown
| |
| ovarian ovum, '27 X ‘26 mm. diameter. The central deutoplasmic
| |
| zone (cZ. z.) and the peripheral formative zone (/. z.), in which the
| |
| | |
| | |
| 126
| |
| | |
| | |
| J. 1>. HITiL.
| |
| | |
| | |
| vesicular nucleus ('QS X '03 mni. diameter) is situated, are clearly distinguishable. The zona (z. p.) measures •0021-'0025 mm. in thickness.
| |
| Outside it are the follicular epithelial cells of the discus proligerus
| |
| (d.p.), which is thickened on the upper side of the figure, where it
| |
| becomes continuous with the membrana granulosa. (D. v i v., 21 . vii .
| |
| '04, Hermann's fluid and iron-hsematoxylin.)
| |
| | |
| Fig. 2. - Photo-micrograph ( X. 150) of ripe ovarian ovum (in which
| |
| first polar body is separated and second polar spindle is present, though
| |
| neither is visible in figure), '29 X '23 mm. maximum diametei'. FoUicle
| |
| 1'4 X IT mm. diameter. The ovum exhibits an obvious polarity.
| |
| Deutoplasmic zone {d. z.) in upper hemisphere ; formative zone (/. z.)
| |
| foi-ming lower. (D. v i v., 14, 26 . vii . '02, Flemming's fluid and
| |
| iron-haematoxylin.)
| |
| | |
| Fig. 3. - Photo-microgi'aph ( x 150) of ripe ovarian ovum ('28 x '24
| |
| mm. diameter) with first polar body (p. bK) and second polar spindle.
| |
| First polar body, •026-‘03 x '01 mm. Second polar spindle, '013 mm.
| |
| in length. (D. v i v., 14, 26 . vii . '02, Flemming's fli;id and ironhaematoxylin.)
| |
| | |
| Fig. 4. - Photo-micrograph (x 256) of ovarian ovum in process of
| |
| growth (“pseudo-alveolar†stage). Ovum, ‘26 X '20 mm. diameter.
| |
| Zona, •0017-‘002 mm. in thickness. (D. v i v., 14, 26 . vii . '02,
| |
| Hermann, iron-haematoxylin.)
| |
| | |
| Fig. 5. - Photo-microgi-aph (X 1250) of peripheral i-egion of ripe
| |
| ovarian ovum ('28 X T26 mm. diameter) with first polar spindle ('015
| |
| X '013 mm.). (D. v i v., 23 . vii . '02, Ohlmaicher's fluid, iron-haema
| |
| toxylin.)
| |
| | |
| Fig. 6. - Photo-micrograph (x 1250) of peripheral region of ripe
| |
| ovarian ovum ('26 X T8 mm.), showing first polar body (p. b'.) ('03 X
| |
| •006 mm.). (D. v i v., 14, 26 . vii . '02, Flemming, iron-hfematoxylin.)
| |
| | |
| Fig. 7. - Photomicrograph ( X 1250) of periplieral region of ovum, fig.
| |
| 3, showing portion of first polar body (p. 5'.), and the second polar
| |
| spindle. The dark body lying between p. 5'. and the surface of the
| |
| ovum is a displaced red blood-corpuscle.
| |
| | |
| Figs. 8 and 9. - Photo-micrographs ( X about 84) of unsegmented ova,
| |
| respectively '33 mm. and '35 mm. in diameter, from the uterus, taken
| |
| immediately after their transference to the fixing fluid (picro-nitroosmic acid), showing the shell-membrane (s. m.), laminated albumen
| |
| {alb.), with sperms (sp.), the zona (z. p.), perivitelline space {p. s.), and
| |
| the body of the ovum, with its formative (/. z.), and deutoplasmic {d. z.)
| |
| zones. (D. v i v., 15, 19 . vii . '01.)
| |
| | |
| Fig. 10. - Photo-micrograph ( X 150) of section of imsegmented ovum
| |
| almost immediately after its passage into the uterus, showing the veiy
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 127
| |
| | |
| | |
| thin sliell-inembvane externally (s. m.) (about '0016 mm. in thickness),
| |
| the albumen {alb.), zona (z-i?.), and the deutoplasmic {d. z.) and formative
| |
| (/. z.) zones of its cytoplasmic body. The male pronucleus is visible in
| |
| the formative zone. Diameter of entire egg about '29 mm. (D. viv.,
| |
| 15, 19 . vii . '01, Picro-nitro-osmic and iron-hffimatoxylin.)
| |
| | |
| Fig. 11. - Photo-micrograph ( X 150) of section of unsegmented ovum
| |
| from the uterus, slightly older than that of fig. 10. Diameter of entire
| |
| egg in fresh state •34-'35 mm., of the ovum proper '3 X ‘28 mm. ; thickness of shell, -0024 mm. In the figure the female pronucleus is visible
| |
| near the centre of the formative zone (/. z.), and the male pronucleus
| |
| lies a little above it and to the right. The perivitelline space (jJ.s.)
| |
| is pai-tiaUy occupied by coagulum. (D . viv., 21 . v . '03, f. Hermann,
| |
| iron-hsematoxylin.)
| |
| | |
| PLATE 2. •
| |
| | |
| Fig. 12. - Photo-micrograph ( X 150) of an unsegmented ovum from
| |
| the irterus, of the same batch as that of fig. 11, and '34 mm. in diameter.
| |
| The two pronuclei are visible in the central region of the formative
| |
| zone.
| |
| | |
| Fig. 13. - Photo-microgi-aph ( X 330) of uterine ovum. Stage of first
| |
| cleavage spindle. Diameter, '315 mm. (D. viv., 1, 15 . vii . '01, f.
| |
| Picro-nitro-osmic, iron-hiematoxylin.)
| |
| | |
| Fig. 14. - Photo-micrograph ( X about 78) of egg in the 2-celled stage,
| |
| taken immediately after its transference to the fixing fluid. Lateral
| |
| view. y. b. Yolk body. Diameter of entire egg about "34 mm. (D . viv.,
| |
| 1, 15 . vii . '01. Picro-nitro-osmic.)
| |
| | |
| Fig. 15. - Photo-microgi'aph (x about 78) of another 2-celled egg,
| |
| seen from lower pole. Diameter, '35 mm. (D. viv., 4 B, 23 . vi . '02.
| |
| Perenyi's fluid.)
| |
| | |
| Fig. 16. - Photo-micrograph (x about 78) of another 2-celled egg,
| |
| of the same batch as preceding. End view, showing one of the two
| |
| blastomeres and the yolk -body (y. b.).
| |
| | |
| Fig. 17. - Photo-micrograph (x 150) of vertical section of 2-celled
| |
| egg, "34 mm. in diameter, showing the shell-membrane ('0064 mm. thick),
| |
| traces only of the albumen, the zona (z.p.), and the two blastomeres (the
| |
| left one measuring, from the sections, T6 x T8 x TO mm., its nucleus
| |
| ‘031 X ‘027 mm. ; the right one, T6 x T9 X "09 mm., its nucleus, '03 x
| |
| •028 mm.). Note the differentiation in their cytoplasmic bodies.
| |
| (D . viv., 6, 21 . vii . '01, Picro-nitro -osmic and iron-hsematoxylin.)
| |
| | |
| Fig. 18. - Photo-micrograph (x 150) of vertical section of 2-celled
| |
| egg, '32 mm. in diameter, with shell-membrane '005 mm. thick, showing
| |
| the two blastomeres, and enclosed between their upper ends the yolk
| |
| | |
| 128
| |
| | |
| | |
| J. r. Hii,L.
| |
| | |
| | |
| body {y. b.). (D . viv., 1, 15 . vii . '01, f. Picro-nitro-osmic, iron-htematoxylin.)
| |
| | |
| Figs. 19 and 20. - Photo-micrographs ( x about 70) of 4-eelled eggs
| |
| taken immediately after transference to Perenyi's fluid. Fig. 19, side
| |
| view, showing yolk-body (y. h.) ; fig. 20, polar view. Diameter of entire
| |
| egg about -35 mm. (D . viv., 14 b, 18 . vi . '02. Perenyi.)
| |
| | |
| Fig. 21. - Photo- micrograph (x about 70) of another 4-celled egg,
| |
| from the same batch as the preceding, seen from lower pole.
| |
| | |
| Fig. 22. - Photo-micrograph (x 150) of section of 4-ceUed egg of
| |
| same batch as those of figs. 19 and 20. The two right and the two
| |
| left blastomeres respectively form pairs, so that the plane of the first
| |
| cleavage is parallel with the sides of tlie plate, that of the second with
| |
| the top and bottom of the same. The two left blastomeres are still
| |
| connected by a narrow cytoplasmic bridge. Thickness of shell,
| |
| •0072 mm.
| |
| | |
| Fig. 23. - Photo-micrograph ( x 150) of a vertical section through
| |
| a 4-celled egg. ‘35 mm. in diameter, showing two of the blastomeres
| |
| and a small portion of the yolk-body {y. b.). Note, as in fig. 22, the
| |
| marked diflierentiation in the cytoplasm of the blastomeres. (D. viv.,
| |
| 4, 27 . vi . '01. Picro-nitro-osmic, iron-hsematoxylin.)
| |
| | |
| Figs. 24 and 25. - Photo-micrographs ( x 140) of horizontal sections
| |
| through a 16-celled egg, '38 mm. diameter, fig. 24 showing the eight
| |
| larger, more yolk-rich cells of the lower (non-formative) ring, and fig. 25
| |
| the eight smaller, less yolk-rich cells of the upper (formative) ring.
| |
| Shell ‘0075 mm. in thickness, yolk-body (not included in the figures)
| |
| 'll X TO mm. in diameter. (D. viv., 3 b, 26 . vi . '01; 15, f and |.
| |
| Picro-nitro-osmic and iron-hsematoxylin.)
| |
| | |
| Fig. 26. - Photo-micrograph (x 140) of a vertical section of an egg
| |
| of the same batch and size as that represented in figs. 24 and 25, but
| |
| with seventeen cells - formative = 9 (6 + [1 X 2] + 1) in division ;
| |
| non-formative = 8. Two of the formative cells (/. c.) of the upper ring
| |
| are seen enclosing between them the faintly mai'ked yolk-body {y. b.),
| |
| and below them two of the much more opaque non-formative cells
| |
| {tr. ect.) of the lower ring.
| |
| | |
| | |
| PLATE 3.
| |
| | |
| Fig. 27. - Photo-micrograph (x about 76) of the just completed
| |
| blastocyst, '39 mm. in diameter. From a spirit specimen. The dark
| |
| spherical mass (eg.) in the blastocyst cavity is simply coagulum, produced by the action of the fixative (picro-nitro-osmic acid) on the
| |
| albuminous fluid which fills the blastocyst cavity. (D. viv., 2 b,
| |
| 16 . vii . '01.)
| |
| | |
| | |
| THE EARLY DEVELOPMENT OE THE MARSUPIALIA. 129
| |
| | |
| Fig. 28. - Plioto-anicrogi-apli ( X about 76) of a blastocyst of the same
| |
| batch as the preceding, •45 mm. in diameter. From a spirit specimen.
| |
| eg. Coagulum.
| |
| | |
| Fig. 29. - Photo-micrograph (x about 75) of another blastocyst,
| |
| •45 mm. diameter, of the same batch as the preceding, but taken
| |
| immediately after transference to the fixative. Viewed from the upper
| |
| pole. y. b. Tolk-body seen through the unilaminar wall.
| |
| | |
| Fig. 30. - Photo-micrograph ( X about 75) of a blastocyst of the same
| |
| batch as the preceding, about '39 mm. in diameter, in which the cellular
| |
| wall has not yet been completed over the lower polar region.
| |
| | |
| Fig. 31. - Photo-micrograph ( X 140) of a section of a blastocyst,
| |
| •39 mm. diameter, of the same batch as the preceding and at precisely
| |
| the same developmental stage, the cellular wall having yet to be completed over the lower polar region (l.p.). In the blastocyst cavity is
| |
| seen the yolk-body (y. b.) partially surroixnded by a mass of coagulum
| |
| (eg.). (D. viv., 2 B, 16 . vii . '01, m. = '39, Picro-nitro-osmic and
| |
| iron-hsematoxylin.)
| |
| | |
| Fig. 32. - Photo-micrograph ( X 140) of another blastocyst, ^41 mm.
| |
| in diameter, of the same batch as the preceding, also 'with the cellular
| |
| wall still absent over the lower polar region. Shell-membrane ‘0075 mm.
| |
| in thickness, y. b. Tolk-body. c. g. Coagulum. The cellular wall
| |
| comprises about 130 cells.
| |
| | |
| Fig. 33. - Photo-micrograph ( X 140) of a blastocyst of the same batch
| |
| as the preceding, with a complete unilaminar cellular wall. y. b. Yolkbody, in contact with inner surface of wall, in the region of the upper
| |
| pole.
| |
| | |
| Fig. 34. - Photo-micrograph (x 100) of a section of a blastocyst
| |
| •57 mm. in diameter, i. c. Internal ceU. (D . vi v., 29 . vi . '04, y . Pici^onitro-osmic.)
| |
| | |
| Fig. 35. - Photo-micrograph (x 100) of a section of a blastocyst, '73
| |
| mm. diameter, of the same batch as the pi^eceding, shell, ^0045 mm.
| |
| thick.
| |
| | |
| Fig. 36. - Photo-micrograph (x 100) of a section of a blastocyst -66
| |
| mm. diameter, of the same batch as the pi-eceding. Lower hemisphere
| |
| opposite yolk-body {y. b.) formed of larger cells than upper. Hermann
| |
| fixation.
| |
| | |
| Fig. 37. - Photo-micrograph (x 140) of section of an abnormal
| |
| vesicle, 397 mm. diameter of the same batch as the normal vesicles
| |
| represented in figs. 27-33. abn. large binucleate cell, regarded as a
| |
| blastomere of the lower hemisphex^e which has failed to divide in noi^mal
| |
| fashion, cf . text, p. 42.
| |
| | |
| VOL. 5(5, PART 1. NEW SERIES.
| |
| | |
| | |
| 9
| |
| | |
| | |
| 130
| |
| | |
| | |
| .1. P. HILL.
| |
| | |
| | |
| PLATE 4.
| |
| | |
| Fig. 38 - Photo-micrograpli ( x 10) of entire blastocyst 4'5 mm. diameter to show the junctional line {j. 1.) between formative and nonformative regions. From a spirit specimen. (D . viv., /3, 25 . vii . '01.
| |
| Picro-nitro-osmic.)
| |
| | |
| Fig. 39. - Photo-microgi-aph ( x about 10) of an entire blastocyst,
| |
| 4'5 mm. diameter with distinct embryonal area {emh. a.). (D. viv., 5,
| |
| 18 . vii . '01.)
| |
| | |
| Fig. 40. - Photo-micrograph { X 10) of entire blastocyst about 5 mm.
| |
| diameter showing embryonal area' {emh. a.), peripheral limit of entoderm (1. ent.), and the still unilaminar region of the wall {tr. ect.). (D.
| |
| viv., 8 . vi . '01.)
| |
| | |
| Fig. 41. - Photo-micrograph ( x 150) of an in toto preparation of the
| |
| wall of a blastocyst of 3'5 mm. diameter. (D . viv., 16, 21 . vii . '01.)
| |
| | |
| Fig. 42. - Photo-micrograph (x 150) of an in toto preparation of the
| |
| wall of a blastocyst of 3'25 mm. diameter, j. 1. Junctional line between
| |
| the formative (/. a.) and non-formative {tr. ect.) regions of the wall.
| |
| (D. viv., 24 . vii . '01.)
| |
| | |
| Figs. 43 and 44. - Photo-micrographs (x 150) of in toto preparations
| |
| of the wall of 4'5 mm. blastocyst showing the jimctional line between
| |
| the formative (/. a.) and non-formative {tr. ect.) regions. (D. viv.,
| |
| P, 25 . vii . '01. Picro-nitro-osmic and Ehrlich's hsematoxylin )
| |
| | |
| Fig. 45. - Photo-micrograph ( x 150) of a corresponding preparation
| |
| of the wall of a more advanced 4'5 mm. blastocyst ('99 stage), in which
| |
| the two regions of the wall are now clearly distinguishable. (D. viv.,
| |
| 8.7. '99. Picro-nitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| Fig. 46. - Photo -micrograph ( x 150) of a corresponding preparation
| |
| of a slightly more advanced blastocyst ('04 stage). (D. viv., 6 . 7 . '04.
| |
| Picro-nitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| PLATE 5.
| |
| | |
| Fig. 47. - Photo-micrograph (x 150) of an in toto preparation of the
| |
| formative region of a 6 . 7 . '04 blastocyst, showing the proliferation
| |
| of spherical interaal cells refeiTed to in the text, p. 53.
| |
| | |
| Fig. 48. - Photo-micrograph ( X 150) of an in toto preparation of the
| |
| wall of a vesicle of the same batch as that represented in fig. 39, in
| |
| which a small part of the junctional line between the embryonal ectodenn and the extra-embryonal {tr. ect.) is visible, the free edge of the
| |
| entoderm {ent.) not having reached it. (D. viv., 5, 18 . vii . '01. Picronitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| | |
| THE EARr.Y DEVELOPMENT OF THE MARSdPIALIA. 131
| |
| | |
| | |
| Fig. 49. - Photo-micrograpli ( X 150) of a con-esponding preparation
| |
| of a vesicle of the same batch as the preceding, in which the wavy and
| |
| irregularly thickened free edge of the entoderm {ent.) practically
| |
| coincides with the junctional line and so conceals it from view.
| |
| | |
| Fig. 50. - Photo-micrograph (x 150) of an in to to preparation of a
| |
| vesicle (8 . vi . '01 batch) viewed from the inner surface as in the corresponding preceding figures. The entoderm in the region of the embryonal
| |
| ax-ea has been removed, so that one sees the inner surface of the embryonal
| |
| ectoderm [emh. ect.) ; it is still in situ, though not in a quite intact condition over the adjoining portion of extra-embryonal ectoderm. The
| |
| entoderm has not yet extended over the region indicated by the reference
| |
| line to tr. ect., so that here the extra-embryonal ectoderm is cleai-ly
| |
| visible. The jimctional line is apparent. (D. viv., 8 . vi . '01. Picronitro-osmic. Ehrlich's hsematoxylin.)
| |
| | |
| Fig. 51 (Plate 3). - Photo-microgi-aph ( X 310) of a section of a 30celled egg of Perameles obesula; egg b, '24 X '23 mm. diameter,
| |
| showing the xinilaniinar layer formed by the blastomeres.
| |
| | |
| Fig. 52 (Plate 3). - Photo-micrograph (x 240) of a section of a
| |
| blastocyst of P. nasuta '29 X •26 mm. diameter, showing the shellmembrane {s.vi.), zona (z.p.), and the unilaminar celhxlar waU. The
| |
| portion of the latter adjacent to the reference lines is composed of
| |
| smaller but thicker cells than the remainder.
| |
| | |
| PLATE 6.
| |
| | |
| Figs. 53 and 54. - Drawings ( X 84) of a 6-celled egg '34 mm. diameter,
| |
| fig. 53 showing a side view and fig. 54 a view from the lower pole.
| |
| Observe the characteristic I'ing-shaped arrangement of the blastomeres.
| |
| y. b. Yolk -body, the shell-membrane, albumen layer with sperms included, and the zona are readily distinguishable. Outlines drawn with
| |
| the aid of the camera lucida immediately after transference of the egg
| |
| to the fixing fluid. (D . viv., 22, 16 . vii . '01.)
| |
| | |
| Figs. 55 and 56. - Drawings ( X about 88) of a 16-ceUed egg (about ‘37
| |
| mm. diameter) as seen fx'om the side and lower pole respectively, from
| |
| the same batch as the eggs represented in figs. 24, 25, and 26. The charactei'istic aii'angement of the blastomex'es in two sxxpex'imposed, open
| |
| x'ings (each of eight cells) and the diffex'ence in size between the cells of
| |
| the two riixgs are evident. The ix'x-egxxlar body (c.g.) seen ixx the cleavage
| |
| cavity in fig. 56 is a mass of coagxxluixx. Dx'aunx from a spix'it specimen.
| |
| The albumen layer as represented in fig. 56 is too thick. (D. viv.,
| |
| 3 B, 26 . vi . '01.)
| |
| | |
| Figs. 57 and 58. - Drawings (x about 85) of a 12-celled egg (-38 xixm.
| |
| diameter) as seen from the side axxd lower pole respectively. Four of
| |
| | |
| | |
| 132
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| the blastomeres of the 8-ceHed stage have already divided (4 + 4x2)
| |
| = 12. From a spirit specimen and from same batch as preceding.
| |
| | |
| Fig. 59. - Drawing ( x about 88) of a 31-celled egg ('375 mm. diameter)
| |
| as seen from the lower pole. From a spirit specimen and fi-om the same
| |
| batch as the preceding. The irregular body in the blastocyst cavity is
| |
| formed by coagulnm. Formative cells = 16; non- formative = 14 + 1
| |
| in division.
| |
| | |
| Fig. 60. - Drawing ( X about 88) of another 31-celled egg ('375 diameter)
| |
| from the same batch as the preceding. Side view.
| |
| | |
| Fig. 61. - Drawing (x 100) of an entire blastocyst (‘39 mm. diameter)
| |
| from the same batch as those shown in figs. 27-29.
| |
| | |
| Fig. 62. - Drawing ( x about 80) of an entire blastocyst (‘4 mm.
| |
| diameter) from the same batch as the preceding.
| |
| | |
| Fig. 63. - Drawing (x 80 of an entire blastocyst ('6 mm. diameter)
| |
| made from a photogi'aph taken directly after transference of the specimen to the fixing fluid. Cells of lower hemisphere with imich more
| |
| marked perinuclear areas of dense cytoplasm than those of the upper.
| |
| D. viv., 2, 11 . vii . '01.)
| |
| | |
| Fig. 64. - Section of the wall of a blastocyst, 2'4 mm. diameter
| |
| (x 630). (D. viv., 7 . vi . '01.)
| |
| | |
| Figs. 65, 66, 67. - Drawings (x 630) of small portions of in toto
| |
| preparations of the formative region of 6 . 7 . '04 blastocysts to demonstrate the mode of origin of the primitive entodermal cells {ent., fig. 67).
| |
| Fig. 65 shows a dividing entodermal mother-cell in position in the
| |
| unilaminar wall, siuTounded by larger lighter staining cells (prospective
| |
| embryonal ectodermal cells). In fig. 66 is seen a corresponding cell, a
| |
| poi-tion of whose cell-body has extended inwards so as to underlie
| |
| (overlie in figure) one of the ectodermal cells of the wall. . In fig. 67
| |
| are seen two entodermal cells, evidently sister-cells, the products of the
| |
| division of such a cell as is seen in figs. 65 or 66. One of them (the
| |
| upper) is still a constituent of the unilaminar wall, the other {ent.) is a
| |
| primitive entodermal cell, definitely internal. (D . viv ., 6 . 7 . '04. Picronitro-osmic, Ehrlich's haematoxylin.)
| |
| | |
| | |
| PLATE 7.
| |
| | |
| Figs. 68, 69, 70. - Drawings (x 630) of portions of preparations
| |
| similar to the above. For description see text. (D. viv., 6, 7, '04.)
| |
| | |
| Fig; 71. - Drawing (x about 630) of a portion of an in toto preparation of the formative region of an '01 blastocyst showing two
| |
| primitive entodermal cells, one of them in division. (D. viv., (3,
| |
| 25 . vii . '01. Picro-nitro-osmic and Ehrlich.)
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSQPIALIA. 133
| |
| | |
| | |
| Fig. 72. - D rawing (x 630) corresponding to the above, from the
| |
| formative region of a 6 . 7 . '04 blastocyst, also showing two primitive
| |
| entodermal cells, evidently sister-cells.
| |
| | |
| | |
| PLATE 8.
| |
| | |
| Figs. 73, 74, 76. - Sections of the formative region of 6.7. '04 blastocysts, showing the attenuated shell-membrane, the unilaminar waU, and
| |
| in close contact with the inner surface of the latter, the primitive entodermal cells {ent.) ( X 630).
| |
| | |
| Fig. 75. - Section corresponding to the above, showing an entodermal
| |
| mother-cell {ent.), part of whose cell-body nndei'lies the adjacent ectodermal cell of the wall. The spheroidal inwardly projecting cell on the
| |
| left is probably also an entodermal mother-cell (x 630).
| |
| | |
| Fig. 77. - Section ( x 630) of the non-formative I'egion of a 6 . 7 . '04
| |
| blastocyst.
| |
| | |
| Fig. 78. - Section ( X 630) of the embryonal ai'ea, and the adjoining
| |
| portion of the still imilaminar extra-embryonal region of a blastocyst of
| |
| the 5 . '01 stage, emb. ect. Embryonal ectoderm, ent. Entoderm, tr.
| |
| ect. Extra-embryonal ectoderm (tropho-ectoderm). The position of the
| |
| junctional line is readily recognisable. (D . vi v. , 5, 18 . vii . '01. Picronitro-osmic and Delafield's hsematoxylin.)
| |
| | |
| Fig. 79. - Section (x 630) through the corresponding regions in an
| |
| 8 . vi . '01 blastocyst. Note the thickening of the embryonal ectoderm
| |
| {emb. ect.), and the peripheral extension of the entoderm {ent.) below
| |
| the tropho-ectoderm. (D. viv., 8 . vi . '01. Picro-nitro-osmic and
| |
| Lelafield.)
| |
| | |
| Fig. 80. - Section (x 600) through the formative (embryonal) region
| |
| of a blastocyst of P. nasuta, 1‘3 mm. in diameter. It is thicker than
| |
| that of the Dasyure blastocyst at the corresponding stage of development ; the primitive entodermal cells are well mai-ked.
| |
| | |
| Fig. 81. - Section ( x 600) corresponding to the above from another
| |
| 1-3 mm. blastocyst of P. nasuta, of the same batch as the preceding,
| |
| but apparently very slightly earlier, the entodermal cells being stiU in
| |
| process of separating from the unilaminar wall. ent. Entoderm, tr. ect.
| |
| Tropho-ectoderm.
| |
| | |
| | |
| PLATE 9.
| |
| | |
| Fig. 82. Section (x about 430) of a section of a blastocyst of M.
| |
| ruficollis -35 mm. in diameter, showing the major portion of the
| |
| formative region (/. a.) and a small portion of the non-formative {tr. ect.).
| |
| | |
| | |
| 134
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| The shell-membrane varies in thickness in the sections from (J05 min.
| |
| over the former region to '003 mm. over the latter.
| |
| | |
| Figs. 83, 84, 85. - Drawings ( X 630) of small portions of the formative
| |
| (and in fig. 83 of the adjoining portion of the non-formative) region of
| |
| the above blastocyst of M. ruficollis more highly magnified, ent.
| |
| Primitive entodermal cells. Note in fig. 83 a cell of the wall in division,
| |
| the axis of the spindle being oblique to the surface.
| |
| | |
| | |
| | |
| | |
| J. P. Hill, Photo.
| |
| | |
| Watbslow & Sows LiMlTiiD, Collotype.
| |