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| ==Chapter VI The Germinal Layers==
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| By the germ-layers we understand certain groups of cells which
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| contain in themselves the materials for certain definite groups
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| of organs and tissues. These groups of cells are definitely
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| separated from one another at an early period of development,
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| and the process of their separation is spoken of as the formation
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| of the germinal layers.
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|
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| The germinal layers in a Vertebrate are three in number, the
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| ectoderm, the endoderm, and the mesoderm. The ectoderm is
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| that group of cells which contains within itself the material for
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| the formation of the epidermis and epidermal derivatives like
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| hair, feather, skin-glands, the enamel of the teeth, the nervous
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| system both central and peripheral, and the sense organs, and
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| further the stomodaeum and proctodaeum, or entrances to the
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| mouth and anus ; the endoderm contains the material for the
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| lining epithelium of the alimentary canal and its outgrowths,
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| such as gill-slits, thyroid, thymus, lungs, liver, pancreas, bladder ;
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| while from the mesoderm- with which we include the notochord
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| - skeleton and connective tissues, muscles, blood and vascular
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| system, coelom and urogenital organs will be derived.
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|
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| The germ-layers are thus definable by thek fate in development. They may also be defined with reference to their position
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| in the embryonic body when they have been definitely segregated
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| from one another, for then the ectoderm is the outside layer,
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| the endoderm the inside layer, while the mesoderm with the
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| notochord is in between. Prior to that moment, however, it is
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| difficult if not impossible, to give generaUy valid definitions of
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| these sets of cells by their position, since the method of theur
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| origin from the different cells into which the substance of the
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| ovum is divided by cleavage varies in the several groups.
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|
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| In a Vertebrate the germinal layers are segregated durmg
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| a process which is known as the formation and closure of the blastopore, or in an older terminology ' gastrulation the ' gastrula ' being the name bestowed on this stage in which a new
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| cavity, the ' archenteron ' or primitive gut, is formed and is in
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| communication with the exterior by an aperture, the blastopore.
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|
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| This opening, and -ndth it the germinal layers, is from the first
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| bilaterally symmetrical. This is true of all Vertebrates, but in
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| the method of its origin the phylum must be divided into two
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| great groups, those in which the blastopore arises at the edge
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| of the blastoderm - ^the Anamnia - and those in which it appears
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| inside the blastoderm - the Amniota. By the help of the Gymnophiona, however, the gap between the two may be bridged.
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|
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| Anamnia
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|
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| We shall begin with the Anamnia, in which the conditions are much simpler.
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|
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| As a type we shall take the common English frog {Eana
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| iemporaria) .
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|
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| The first sign of the formation of the germ-layers is given as
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| soon as segmentation is at an end by the appearance of the
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| structure known as the dorsal lip of the blastopore (Fig. 61).
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| This is a short, deeply-pigmented rim bounding a groove, placed
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| parallel to the equator, and a little below it (about 25°) at that
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| point in the boundary between the pigmented and unpigmented
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| regions of the egg where the latter area is most extensive. This
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| is the side on which the grey crescent was formed and the original
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| unpigmented area so increased. The plane which includes the
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| egg-axis and the dorsal lip will shortly become the median
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| longitudinal or sagittal plane of the embryo ; it coincides evidently with the plane of symmetry of the unsegmented ovum.
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|
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| The egg is still in the position into which it turned at the time
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| of insemination with its axis vertical, and the heavy white pole
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| below. The changes that now take place as seen from this
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| vegetative pole are as follows. The rim of the groove begins
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| to travel downwards over the surface of the egg towards the
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| vegetative pole, the area over which it passes becoming covered
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| by cells which are as deeply pigmented as those of the animal
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| portion of the egg. At the same time the rim elongates, becoming
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| crescentic ; in other words, the processes of rim formation and overgrowth are extended to the right and left along the margin
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| of the pigmented area, and the lateral lips of the blastopore come into being. As the dorsal lip (the middle region of the rim)
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| continues on its course towards the vegetative pole, and as
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| continually fresh parts are drawn into the process at the sides,
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| the blastoporic lip becomes first semicircular, and then three parts of a circle, until finally, when that part which is diametrically opposite to the dorsal lip, namely the ventral lip, also
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| begins to grow down, it attains the form of a circle enclosing
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| the still uncovered portion of the vegetative hemisphere, the
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| yolk-plug. The dorsal lip has now moved down to or a little
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| beyond the vegetative pole.
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|
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|
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|
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|
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|
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| Fig. 61. - Diagrams of the closure of the blastopore in the egg of the
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| common frog {R. temporaria). In a-d the egg is viewed from the vegetative pole, in E, F from below. The dorsal lip is at the top of the figures.
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| In D the ventral lip has just been formed and the blastopore is circular.
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| In E the rotation of the whole egg has begun, and in f is complete.
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| At this moment the whole egg begins to rotate about a horizontal axis in the opposite direction to that in which the dorsal
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| lip moved ; and this rotation continues - the circle of the blastopore becoming smaller all the time - until the dorsal lip has
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| returned, rather beyond the point from which it started, to the
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| new equator, or horizontal plane through the centre of the egg.
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| The end now occupied by the blastopore is posterior. The angle
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| subtended by the arc traversed by the dorsal lip - both before
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| and during the rotation - is 75°, and the angle through which
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| the whole egg rotates is 100°. It follows that the vertical line
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| now drawn through the centre of the egg, which will be the
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| dorso-ventral line of the embryo, makes the same angle of 100°
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| with the original egg-axis ; that the animal pole is situated below
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| what will be the anterior end of the embryo (Fig. 62, f), since
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| the blastopore is posterior ; and that the antero -ventral haK of
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| the embryo is developed over the animal, the postero -dorsal half
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| over the vegetative hemisphere of the egg. The dorsal and ventral
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| lips are now actually dorsal and ventral.
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|
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| It is clear that the lip of the blastopore which is thus formed
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| and closed arises along the whole of the boundary between small
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| pigmented and large yolk-cells, and that the process is bilateral,
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| taking place, as it does, first and most rapidly at the dorsal lip,
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| last and least rapidly at the ventral lip, and at an intermediate
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| rate at the lateral lips in between.
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|
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| The examination of sections (Fig. 62) will now show us that
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| the closure involves (1) a movement of the yolk-cells into the
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| segmentation cavity together with (2) an overgrowth and ingrowi^h of cells at the blastoporic lip, resulting in the formation
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| of a new cavity, the ' archenteron ' ; and that during the process
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| the material for the germinal layers is brought into position and
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| laid down.
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|
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| A sagittal section of the egg passing through the dorsal lip at its first appearance shows the groove placed about 25° below
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| the equator in the zone of intermediate cells. The radial disposition of the cells immediately about the groove marks the
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| beginning of a process of overgrowth and ingrowth which becomes
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| more obvious a little later, when it is seen that a fold of small
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| cells has grown over a certain area of yolk-cells. This fold consists naturally of two sheets, an outer and an inner. The cells
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| of the outer sheet resemble closely the small pigmented cells of
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| the animal hemisphere into which they are uninterruptedly continued ; like the latter, they are arranged in about four layers,
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| the outermost of which is epithelial. At the lip of the blastopore
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| the outer passes into the inner sheet, the cells in the outermost
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| layer of the former being gradually turned over into the innermost layer of the latter. This inner sheet also consists of several
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| layers of cells, the innermost of which is pigmented and epitheUal, the remainder being more irregularly disposed. The inner
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| sheet forms the outer, or, as it will be when the egg has rotated,
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| the upper wall of the slit-like cavity between itself and the yolksurface now covered up. This cavity is the archenteron and the
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| inner sheet of the fold is its roof ; the original vegetative surface
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| of the egg forms its floor.
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|
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| This overgrowth and ingrowth of cells, with consequent formation of an archenteric cavity, takes place in an exactly similar
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| fashion at the lateral (Fig. 63, a) and ventral lips. By the time
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| the latter has appeared the archenteric cavity is much enlarged,
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| first by its being extended in an anterior direction into the yolkcells that have meanwhile been pushed up into the segmentation
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| cavity on the dorsal side, and secondly in a lateral and finaUy
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| a ventral direction by a movement of the mass of yolk-cells
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| towards these regions of the egg also. The segmentation cavity
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| is thus first reduced to a small space upon the ventral side and
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| then obliterated altogether. In a small percentage of cases
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| however, the segmentation cavity communicates with the front
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| end of the archenteron, is surrounded by yolk-cells, and mcorTiorated in the front end of the gut.
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|
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| ' t the shifting of the heavy yolk-cells to the ventral s.d
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| tZ alters the eentre of gravity and so ca,.es the rotafou of
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| the egg until equilibrium is regained.
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|
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|
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| arch
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| mes
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| mes. Y.
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|
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| Fig, After rotation ^'*''^°P°'^'^- before rotation ; e, During rotation ;
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|
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| archenteron; y p! yolk fri^f/-'^ segmentation cavity; arcL,
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|
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| formed below th;;^! ^ ^'^"'^ = mesoderm
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|
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| pushed into the segment; Sty."""^"'" ''""^'^ '^'^"^ "^'^ ^-^k-cella
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|
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|
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| With the exception of the yolk-plug the outer surface of the
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| egg is now covered by a sheet of small cells, disposed in about
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| four layers, the outermost of which is epithelial and pigmented.
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| This sheet is the ectoderm. In part it comes from the original
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| animal cells which formed the roof of the segmentation cavi. r â– but
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| part of it is derived from the outer sheet of the blastoporic fold.
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|
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| Fig 63 - Transverse sections of the frog's egg. A, During the closure
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| of The blastopJie • B, After, mes. 2, mesoderm differentiated from the
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| yoM Tus^^^^^^^ segmentation cavity (in B these are seen to be
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|
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| Central); U., lateral lip of the blastopore; n.c/^., notoehord.
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|
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| The notochord and the dorsal mesoderm are differentiated out
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| of the roof of the archenteron (Figs. 63 B, 64). The latter sheet
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| of cells becomes split into (1) a thin layer next the cavity (this
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| will be the roof of the ahmentary canal) and (2) a layer next
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| the outside. This outer layer is divided into (a) a median strip
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| or rod, which is the notochord, and (6) two lateral shee s, the
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| dorsal mesoderm. The notochord is not separated miti after
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| the sheets of mesoderm have been detached. The separation o
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| both notochord and mesoderm begins at the anterior end and
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| proceeds backwards. At the lip of the blastopore there is thus
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| for a time an undiiferentiated mass of tissue in which ectoderm
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| notochord, mesoderm, and roof of the alimentary ^canal are all
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| continuous (Fig. 62, e). It will be remembered that the front
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| end of the archenteron arises by an extension of that cavity
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| into the yolk-cells ; here, therefore, yolk-cells form the roof, and
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| it is from them that the anterior portions of notochord and dorsal mesoderm are formed. The posterior portions, however,
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| arise in that part of the archenteric roof which comes into position
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| as the inner sheet of the blastoporic fold.
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|
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| The ventral mesoderm (Fig. 63, b) has a similar double origin.
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| In front the floor of the archenteron is formed of the yolk-cells
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| pushed into the segmentation cavity ; the cells next the ecto
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|
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|
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|
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|
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| Fig. 64.- Three stages in the differentiation of the roof of the archenteron
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| in the irog. arch, archenteron; n.ch., notochord ; mes., dorsal mesoderm.
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|
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| derm subdivide and become mesoderm. Behind mesoderm arises
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| from the inner sheet of the fold at the ventral lip. At the sides
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| of the embryo dorsal and ventral mesoderm pass continually
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| into one another. The middle layer, therefore, taken as a whole,
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| arises anteriorly and ventrally from the yolk-cells, posteriorly
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| and dorsally from the blastoporic overgrowth ; the former is in the
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| onginal animal, the latter in the original vegetative hemisphere.
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|
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| Smce mesoderm is formed also at the lateral lips, the two
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| sheets of this tissue which flank the notochord aje necessarily continuous, around the blastopore (Fig. 64*), with the mesoderm
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| at the ventral lip (Fig. 62, e) ; only at the dorsal lip, where the
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| notochord is formed, is there an interruption in the middle layer.
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| The endoderm or lining of the gut cavity is what is left of the
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| roof and floor of the archenteron, the roof of the gut being
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| the thin layer left when the notochord and mesoderm have been
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| detached, the floor the bulky mass of yolk-cells after the separation of the ventral mesoderm.
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|
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| It must be remembered that though the differentiation of
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| these germ-layers is only completed when the blastopore has
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| closed, it has in reality been in progress during the earlier stages.
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|
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| Fro 64* - Horizontal section of an older stage showing the sheets of
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| mesoderm passing back into the lateral Ups of the blastopore (b.j).).
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|
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| It still remains for us to discuss very briefly the origm of
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| the cells from which the blastoporic fold is derived, that is, the
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| origin of parts of each of the three germ-layers. The inner layer
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| of the fold is certainly derived neither wholly from the smaU
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| cells of the animal hemisphere, nor whoUy from the large cells
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| of the vegetative hemisphere, but from the region about the
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| egg-equator, in which the cells are of a character intermediate
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| between these two (Fig. 62, i, z.). The outer layer of the fold
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| comes from the same source, and from an extension of the roof
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| of the segmentation cavity. These intermediate cells divide
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| rapidly and give rise to the fold, which, as we have seen, contains ectodermal, endodermal, and mesodermal elements.
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|
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|
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| To sum up, the ectoderm of the frog comes partly from the
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| cells of the animal hemisphere, partly from the intermediate
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| cells ; the endoderm in part from the latter, in part from the
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| yolk-cells, while the mesoderm and notochord have a similar
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| double origin ; and the materials for these layers are brought
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| into their definitive positions during the bilateral closure of the
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| blastopore, which arises all along the line separating animal from
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| vegetative cells.
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|
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| We shall see that a similar statement may be made for the
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| remaining Anamnia.
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|
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| Fig. 65. - ^Formation of the germ-layers in Petrornyzon. (After Scott.)
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| A, Sagittal section ; b, c. Transverse sections of two stages ; arch., archenteron ; d.l., dorsal lip of the blastopore ; n.ch., notochord ; d.m., dorsal
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| mesoderm ; v.m., ventral mesoderm ; m.t., medxillary tube (here a solid
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| wedge of cells).
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|
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|
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|
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| ==Cyclostomata==
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|
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| In Petrornyzon (Fig. 65) the formation and closure of the
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| blastopore, the origin and extension of the archenteron, resemble
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| the same processes in the frog, with the exception that a ventral
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| lip is never developed. The ventral mesoderm is diflferentiated
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| from the yolk-cells pushed into the segmentation cavity, as in
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| the frog, and these latter cells form the floor of the gut. They
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| give rise, however, to much more than that, since the roof of
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| the archenteron is converted wholly into the notochord and the
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| gut is then completed by the upgrowt.h of yolk-cells from the
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| sides and underneath the notochord. The dorsal mesoderm arisea
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| m connexion with the overgrowth at the lip of the blastopore
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|
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|
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| In the Myxinoids (Fig. 66) segmentation produces a blastoderm at one end of the elHpsoid egg. At one point in the edge
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| of this blastoderm a dorsal blastoporic lip appears, and the
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| material for the germ-layers of the embryo is laid down during
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| the bilateral overgrowth and ingrowth of cells in this region.
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| The yolk is not wholly covered by this process, but as soon as
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|
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|
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|
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|
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| Fig, 66- Bdellostoma. Overgrowth of the posterior edge or dorsal lip
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| of the blastoderm over the yolk, d.l, dorsal lip (posterior edge) ; v.l,
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| ventral lip (anterior edge) ; op.r., operculum of the shell. (After Bashford
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| Dean.)
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|
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|
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|
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| the body of the embryo is formed all parts of the edge of the
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| blastoderm grow down and the blastopore eventually closes at
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| the vegetative pole.
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|
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|
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|
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| Elasm obe anohh
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| Germ-layer formation begins with the appearance at one point
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| in the edge of the blastoderm of a fold or overturning of cells
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| of the superficial layer. This point is, as will appear, in the
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| middle line and at the posterior end. The fold, the rim of which
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| is the dorsal lip of the blastopore, is slightly raised and covers
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| over a space- the beginning of the archenteron- between itself
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| and the yolk (Figs. 67, 68). By the continued backward growth
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| of the fold and by the ingrowth of its under layer the archenteron
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| attains a considerable length. The floor of the archenteron is formed of yolk, into which yolk-nuclei subsequently make their
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| way ; its roof consists of a columnar epithelium derived in
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| part from the overturning of cells at the lip of the blastopore,
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| in part possibly from the posterior marginal cells of the lower
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| layer.
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|
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| Fig. 67. - Overgrowth of the lip of the blastopore and formation of the
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| embryo in Elasmobranchs. (a-c after Riickert, d-f after Ziegler.) c.s.,
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| caudal swelling ; l.L, lateral lip. In r the formation of a lip has extended
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| almost to the anterior edge. In d, e, the medullary folds are still open, in
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| r they are closed.
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|
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|
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|
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| But while this process is taking place at the dorsal lip, that is,
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| at the median posterior margin of the blastoderm's edge, it is
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| also being extended, though in a far less degree, to the neighbouring regions, the lateral lips, on the right and on the left.
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| The archenteron thus comes to assume a crescentic shape, with
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| a median anterior prolongation ; the latter underlies the embryonic portion of the blastoderm, while the crescentic part is
| |
| wholly extra-embryonic, and remains very shallow, though it is
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| subsequently prolonged to the right and left round the edges of
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| the blastoderm until a slight overgrowth is formed even at the
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| anterior margin.
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|
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| With the overgrowth at the lips of the blastopore the material
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| for the germinal layers is laid down (Fig. 69). The superficial
| |
| layer is now the ectoderm. The mesoderm consists of two
| |
| parts : (1) two sheets of cells lying one on each side of the
| |
| middle line over the embryonic portion of the archenteron ;
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| posteriorly these sheets pass into the caudal swellings- two
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| thickenings at the edge of the blastoderm, one on each side of
| |
| the middle line - where they are continuous with the roof of the
| |
| archenteron, out of which they have been differentiated ; (2) the
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| formation of mesoderm is, however, not limited to the parts
| |
| immediately adjacent to the dorsal lip, but is carried on at the
| |
| lateral lips, and, as these extend forwards round the whole edge
| |
| of the blastoderm, at the anterior edge as well. This extraembryonic mesoderm is naturally continuous in the caudal
| |
| swellings with the embryonic mesoderm first described ; it takes
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| part only in the formation of the area vasculosa.
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|
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| The notochord is formed from a median strip of cells which
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| is cut out of the roof of the archenteron ; the process, like the
| |
| differentiation of the mesoderm, takes place from before backwards. With the separation of the notochord and mesoderm
| |
| the remainder of the archenteric roof is endoderm, and gives rise
| |
| to the alimentary canal, the front end and sides bending dovm
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|
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|
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|
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| VJ
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|
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|
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|
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| THE GERMINAL LAYERS
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|
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|
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| 123
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|
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|
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|
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| Fig. 69.- Five successive transverse sections through the hinder
| |
| (embryonic) portion of the blastoderm of the dog-fish during tlie formation
| |
| ot the germinal layers a is posterior, cutting the two caudal sweUings ;
| |
| E, Anterior through the head of the embryo, arch., archenteron; mes
| |
| mesoderm; e.m., embryonic mesoderm; ex.m., extra-embryonic mesoderm ; w.c^., notochord ; lateral lip of the blastopore
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|
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|
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|
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| 124
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|
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|
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|
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| THE GERMINAL LAYERS
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|
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|
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| VI
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|
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|
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|
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| and meeting to form the ventral wall. The yolk in the floor of
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| the archenteron plays no part in this process (Fig. 70).
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|
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| Up to the present it is the posterior edge or dorsal lip which
| |
| has been principally active, but now the anterior and lateral
| |
| margins of the blastoderm become exceedingly vigorous and
| |
| begin to grow over the yolk, the overgrowth being accompanied,
| |
| as stated above, by a slight marginal invagination ; and eventu
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|
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|
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| y.n.
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|
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| Fig. 70.- Two stages in the formation of the gut of the dog-fish by the
| |
| bending down and fusion of the edges of the roof of the archenteron.
| |
| y.n., yolk-nuclei.
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|
| |
| ally the anterior edge makes the whole ckcuit of the yolk, passmg
| |
| round the vegetative pole and reappearmg behind the embryo
| |
| as the ventral lip of the small ' yolk-blastopore ' (Fig. 71). At
| |
| the dorsal hp backgrowth of the caudal sweUings is responsible
| |
| for the posterior elongation of the body of the embryo alone,
| |
| the body being raised above the surface of the yolk. Where the
| |
| body passes into the hinder edge of the blastoderm growth of
| |
| the latter ceases, but the lateral edges immediately adjacent
| |
| to this point swing backwards untU they bound a narrow
| |
| median strip of yolk by which alone the aperture at the dorsal
| |
| lip now communicates with the rest of the blastopore.
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|
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| as
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| B
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| a.e = i/.l.
| |
|
| |
| C. D.
| |
|
| |
| f M^*^" Extension of the blastoderm over the yolk after formation and
| |
| loldmg ofiE of the embryo in an Elasmobranch. a, The lateral lips liave
| |
| swung back parallel to one another behind the dorsal lip, so enclosino- a
| |
| narrow strip of yolk. B, Side view of the same, c, The anterior ed-o
| |
| («.e.) has passed beyond the vegetative pole, and in d it appears behind the
| |
| embryo as the ventral lip (yi.) ; y.b., yolk- blastopore.
| |
|
| |
|
| |
|
| |
| P. 124
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 125
| |
|
| |
|
| |
|
| |
| Teleostei
| |
|
| |
| The processes are essentially the same as in the Elasmobranchs.
| |
| Blastopore formation begins at the posterior edge, where the
| |
|
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 72.- Growth of the blastoderm over the yolk after the formation of
| |
| the material for the embryo in the Teleostean fish Serramis. (After Wilson )
| |
| d.L, dorsal hp of the blastopore (posterior edge of the blastoderm) â– a e
| |
| anterior edge of the blastoderm or ventral lip {v.l.) of the blastopore •
| |
| s.c, segmentation cavity ; o.g., oil-globulc. '
| |
|
| |
| backward growth of the dorsal lip with concomitant development of an archenteric cavity gives rise to the body of the
| |
| embryo, but the process is extended to the lateral and anterior
| |
| edges, where there is a slight invagination. By the growth of
| |
|
| |
|
| |
|
| |
| Fig. 73.- Sagittal sections through the blastoderm of Serranm during
| |
| the formation of the germinal layers. (After Wilson.)
| |
|
| |
| A, Beginning of overgrowth at dorsal lip {d.l.).
| |
|
| |
| B, Overgrowth at anterior edge.
| |
|
| |
| c, Later stage of posterior edge. t u i
| |
|
| |
| D, The anterior edge has become theventraUip {v.l.); n.cA., notochord ;
| |
|
| |
| end., endodorm ; m.p., medullary plate ; par., parablast (periblast) ; y.f.,
| |
|
| |
| yolk-plug ; K.v., Kuppfer's vesicle.
| |
|
| |
|
| |
|
| |
|
| |
| Fig. li.- Serra^ms. Transverse sections showing differentiation of the
| |
| roof of the archenteron into notochord
| |
| (n.ch.), mesoderm (mes.), and cndodprm
| |
| {end.); j^ar., parablast (periblast). (After
| |
| Wilson.)
| |
|
| |
|
| |
|
| |
| aic. 3
| |
|
| |
| Fig. 75. - Serranus. Formation of the gut (a/.c.) by the
| |
| bending down of tlie sides of the
| |
| roof of the archenteron. s.n.ch.,
| |
| sub-notocliordal rod; aid., cndoderm. (After Wilson.)
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 127
| |
|
| |
|
| |
|
| |
| these extra-embryonic edges the yolk is finally enclosed and the
| |
| anterior margin is then the ventral lip (Fig. 72). Notochord
| |
| and mesoderm are differentiated in the roof of the embryonic
| |
| part of the archenteron, the rest of this layer giving rise to
| |
| the alimentary canal, as in Elasmobranchs. Extra-embryonic
| |
| mesoderm arises at the remaining edges of the blastoderm
| |
| (Figs. 73-75).
| |
|
| |
|
| |
|
| |
|
| |
| v.l.
| |
|
| |
|
| |
|
| |
| Fig. 76.- Formation of the germ-layers in Ganoid fishes, a, b, in the
| |
| Sturgeon {Acipenser) (after Bashford Dean) ; c, d, in Amia (after Sobotta) ;
| |
| arch.^ archenteron ; d.l, dorsal lip ; v.l., ventral lip ; n.ch., notochord ;
| |
| mes., mesoderm.
| |
|
| |
| Ganoidei
| |
|
| |
| Our knowledge of the differentiation of the germinal layers is
| |
| very slight, but it is known that the closure of the blastopore
| |
| is bilateral, and that mesoderm is formed at its lips, the notochord in the middle dorsal line (Fig. 76).
| |
|
| |
|
| |
|
| |
| 128 THE GERMINAL LAYERS VI
| |
|
| |
|
| |
|
| |
| DrPNOi
| |
|
| |
| The holoblastic egg of Ceralodus resembles that of the frog
| |
| very closely in the development of its archenteron. The roof
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 77. - Formation of the germ-layers in Dipnoi. A, B, in Ceralodus
| |
| (after Semon) ; c, D, in Lepidosiren (after Graham Kerr), arch., archenteron-; d.l, dorsal lip ; n.ch., notochord ; ines., dorsal mesoderm.
| |
|
| |
| of this cavity, however, takes no part in the formation of the gut,
| |
| but is differentiated simply into median notochord and lateral
| |
| plates of mesoderm. The yolk-cells then grow up to complete
| |
| the dorsal wall of the aUmeutarx, canal (Fig. 77, A, u).
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 129
| |
|
| |
|
| |
|
| |
| Lepidosiren resembles the frog in all respects, except that the
| |
| yolk is more vokimmous and that a ventral lip is never developed
| |
| (Fig. 77, c, D).
| |
|
| |
| Urobelotjs Amphibia
| |
|
| |
| The method of germ-layer separation is here practically identical with that which is observed in the frog, except in one
| |
| important respect. In the bilateral closure of the blastopore,
| |
| the presence of a ventral as well as of a dorsal lip (Fig. 78, A)
| |
| and the formation of the mesoderm from a double source, the
| |
| two groups closely resemble one another ; but while in the frog
| |
| the under layer of the roof of the archenteron persists as the
| |
| dorsal lining of the alimentary tract, in the Urodeles the roof
| |
| of the archenteron becomes wholly converted into the notochord,
| |
| as in Petromyzon, and the gut must be completed dorsally by
| |
| an ingrowth of yolk-cells from the sides (Fig. 78, b, c).
| |
|
| |
| The Anurous Amphibia, such as the toad, generally resemble
| |
| the frog in this matter, but in one case the notochord is described
| |
| as being formed from the middle streak of the whole thickness
| |
| of the roof, and even in the frog such a procedure may be
| |
| experimentally instigated by subjecting the embryos to the
| |
| influence of cane sugar and other substances.
| |
|
| |
| A comparison of these processes in the small-yolked and the
| |
| large-yolked types shows that :
| |
|
| |
| 1. The blastoderm of the large-yolked corresponds to the
| |
| animal region of the small-yolked egg, the yolk to the vegetative
| |
| part, and that the edge of the blastoderm in the former is equivalent to the boundary between animal and yolk cells in the latter.
| |
|
| |
| 2. In both this bounding line becomes in its entirety the lip
| |
| of the blastopore (except where the ventral lip is absent), the
| |
| posterior point of the edge in the large-yolked being equivalent
| |
| to the dorsal lip of the small-yolked, the anterior point to the
| |
| ventral lip.
| |
|
| |
| 3. In both the germinal layers are laid down during the
| |
| bilateral closure of this blastopore, the notochord stretching in
| |
| front of the dorsal lip, the mesoderm springing from the lateral
| |
| lips in two sheets which are continuous with one another behind
| |
| the ventral lip.
| |
|
| |
| 1355 T
| |
|
| |
|
| |
|
| |
| 130
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| 4. The principal points of difference are two. First, the closure
| |
| of the blastopore in Elasmobranchs, Myxinoids, and Teleostei,
| |
| is effected in two periods ; during the first the overgrowth is
| |
| almost confined to the dorsal lip and produces the material for
| |
|
| |
|
| |
|
| |
|
| |
| -piQ. 78 - Formation of the germ-layers in the Axolotl.
| |
|
| |
| A, Sagittal section after completion of the blastopore and rotation of
| |
| the egg.
| |
|
| |
| B, Transverse section of the same stage. i. u a
| |
| c Dorsal part of a transverse section of a later stage, n.ch., notochord;
| |
|
| |
| d.l.', dorsal hp ; v.l, ventral lip ; mes.v., mesoderm formed ^Wentral hp ;
| |
| mei.l, dorsal mesoderm ; mes.2, ventral mesoderm (from the yolk-cellh
| |
| pushed into the segmentation cavity) ; end., endoderm.
| |
|
| |
| the formation of the embryo ; in the second the yolk is gradually
| |
| covered by an extension of the blastoderm in which the lateral
| |
| and anterior margins are alone concerned. Secondly, in these
| |
| cases a part only of the blastoporic hp is involved in the formation of the embryo, the lateral and ventral lips remaining wholly
| |
| extra-embryonic.
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 131
| |
|
| |
|
| |
|
| |
| Gymnophiona
| |
|
| |
| In this group the egg is so laden with yolk that in it segmentation nearly approaches the meroblastic type and results in a
| |
| blastoderm lying on a partially divided yolk. This blastoderm
| |
| consists of a superficial epithelium of columnar cells, covering
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 79.- Formation and closure of the blastopore in the Gymnophiona.
| |
| A-D, Surface views of the blastoderm of Hypogeophis. The lateral lips are
| |
| seen to meet behind, and so form the ventral lip ; y.p., yolk-plug (after
| |
| Brauer). e. Embryo of Ichthyophis lying on the partially segmented yolk
| |
| which is still uncovered by the blastoderm. (After the brothers Sarasin.)
| |
|
| |
| several irregular layers of scattered cells which are more abundantly supplied with yolk. The cavities between these cells are
| |
| equivalent to the ordinary segmentation cavity. Below these
| |
| again is the yolk, divided at its surface into cells, and containing
| |
| nuclei scattered through its substance. Immediately round the
| |
| blastoderm the surface of the yolk is also partially segmented.
| |
| At one point- the posterior middle point- of the edge of this
| |
|
| |
| I 2
| |
|
| |
|
| |
|
| |
| 132
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| blastoderm the dorsal lip appears (Fig. 80) ; it exhibits the
| |
| usual radiate arrangement of cells. The lip quickly grows back
| |
| and so produces a long archenteron which comes to open into
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| 80.- Formation of the germ-layers in Hyi)0!7eoi)te. (After Br auer.)
| |
| A-c Sagittal sections of three successive stages, d, Transverse section
| |
| Lough^he blastopore and yolk-plug {y-v.) ; s.c, ^^tlX /f
| |
| into which in B and o the archenteron {aroh.) opens ; U., dorsal hp , l.L,
| |
| lateral lip ; and vX., ventral Up.
| |
|
| |
| the segmentation cavity in front. The roof of the archenteron
| |
| which seems to be derived entirely from the superficial layer of
| |
| the blastoderm, consists of a plate of columnar cells, its floor of
| |
| the partially segmented yolk.
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 133
| |
|
| |
|
| |
|
| |
| The process of overgrowth is not limited to the dorsal lip,
| |
| but extends to the immediate right and left. Surface views
| |
| (Fig. 79, a-d) show that the transversely placed li]p soon becomes
| |
| crescentic, and that the horns of the crescent then grow not
| |
| only backwards, but towards the middle line as well, approaching
| |
| one another until they meet and so form what is the ventral
| |
| lip of the now circular blastopore. In section it is seen that
| |
| there is a slight ingrowth at the lateral and at the ventral lips
| |
| of a plate of cells continuous with the similarly formed jolate
| |
|
| |
|
| |
|
| |
|
| |
| ¥iG. 81.- Transverse sections of HypogeopMs showing the differentiation
| |
| ot the root of the archenteron into notochorcl (nxh.) and mesoderm, and
| |
|
| |
| Z fXf (Iftttrarxg"' ^"'^-^
| |
|
| |
| which forms the roof of the archenteron in front ; beneath the
| |
| plate is a slit-like space, also, of course, archenteric ; in the midst
| |
| of the blastopore is the projecting typically Amphibian yolk-plug.
| |
|
| |
| But m spite of this resemblance there is a very serious difference between the ventral lip of the Gymnopliiona and that of
| |
| ail other Anamnia. For while in the latter the whole of the edge of
| |
| the blastoderm or small-celled area is converted into a blastoporic
| |
| hp, the posterior point being the dorsal, the diametrically
| |
| opposite anterior point becoming sooner (in small-yolked eggs)
| |
| or la er (m large-yolked eggs) the ventral lip, and while consequently the whole of the vegetative surface of the egg is covered
| |
| up when the blastopore closes, in the former the anterior and
| |
|
| |
|
| |
|
| |
| 134
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| a large part of the two lateral edges take no iiart in this process,
| |
| which is confined to the posterior and immediately adjacent
| |
| portions of the edge ; this small portion gives rise to the dorsal
| |
| and two lateral lips, which latter by their fusion produce the â–
| |
| remarkable similitude of the ventral lip of other forms. As
| |
| a result the vegetative hemisphere is still uncovered when the
| |
| blastopore has become circular (Fig. 79, d, e). The importance
| |
| of this fact for the correct understanding of the relations of the
| |
| blastopore to the blastoderm in the Amniota cannot possibly be
| |
| over-emphasized.
| |
|
| |
| To return to the germinal layers. The superficial layer is now
| |
| the ectoderm. The roof of the archenteron becomes divided
| |
| into a median strip- the notochord, and two lateral sheets- the
| |
|
| |
| mesoderm which are continuous with one another behind the
| |
|
| |
| yolk-plug by means of the cell-plate invaginated at the lateral
| |
| and ventral hps (Fig. 81). The mesoderm has in fact precisely
| |
| the same relations as in other Anamnia at this stage. The
| |
| notochord passes back into the dorsal lip. No additions are
| |
| made to either notochord or mesoderm from any other source.
| |
| The roof of the gut (endoderm) is completed by upgrowth and
| |
| ingrowth of vegetative cells underneath the midcUe layer.
| |
|
| |
| Amniota
| |
|
| |
| Whereas in the Anamnia the blastoporic lip appears at the
| |
| edge of the blastoderm, in the Amniota it hes wholly within the
| |
| latter. The blastopore leads into an archenteron, and with the
| |
| formation of these structures the materials for the germinal
| |
| layers are laid down. Only in the more primitive forms is the
| |
| archenteric cavity well developed ; usually it is much reduced
| |
| and represented only by the ' neurenteric ' passage or ' chordacanal In primitive forms the upper and lower layers are still
| |
| united at the point where the blastopore and archenteron arise,
| |
| and both layers may perhaps be said to share in their formation ;
| |
| but in most cases all these parts are derived from the upper
| |
| layer of the blastoderm alone, the subsequent fusion with the
| |
| lower layer being purely secondary. The edge of the blastoderm,
| |
| which is entirely independent of the blastopore, grows steadily over
| |
| the surface of the yolk, finally enclosing it at the vegetative pole.
| |
|
| |
|
| |
|
| |
| i
| |
|
| |
|
| |
|
| |
|
| |
| IPiG 82 - Three stages in the formation of the blastopore at the hinder
| |
| end of the embryonic shield of a Reptile {PlaUjdacUjlus). Sm'face views.
| |
| (After Will.)
| |
|
| |
|
| |
|
| |
| P. 1S5
| |
|
| |
|
| |
|
| |
| II
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 136
| |
|
| |
|
| |
|
| |
| The Reptiles will be considered first as the whole process is
| |
| far clearer in them than in the other two groups.
| |
|
| |
| Rbptilia
| |
|
| |
| There is distinguishable in the blastoderm at the close of
| |
| segmentation a circular or oval area placed excentrically towards
| |
| the posterior end ; this area is the embryonic shield. The upper
| |
| layer of the blastoderm consists of cyHndrical cells in the embryonic shield, of flat cells in the surrounding region ; below it
| |
| is the segmentation cavity. The lower layer is an irregular sheet
| |
| of scattered rounded cells, not arranged at present in an epithelium, and is constantly being reinforced by the addition of
| |
| cells from the nucleated yolk beneath. Between the lower layer
| |
| and the yolk is a shallow cavity, the subgerminal cavity. In
| |
| some forms, such as Platydactylus and Lacerta, there is one point
| |
| in the margin of the embryonic shield where upper and lower
| |
| layers are continuous ; this is the primitive plate, and it is
| |
| situate at what will be the hinder end (Fig. 83, a). The lower
| |
| layer cells before long arrange themselves in a flat epithelium.
| |
| Meanwhile a depression has appeared in the primitive plate ;
| |
| this is the beginning of the archenteron, and its anterior margin
| |
| is the dorsal lip of the blastopore. Seen from the surface (Fig. 82)
| |
| the dorsal lip presents the appearance of a transverse rim bounding a groove at the hinder edge of the embryonic shield. The
| |
| rim rapidly becomes crescentic, the horns of the crescent turn
| |
| back, meet, and fuse behind the primitive plate which now
| |
| corresponds exactly to the Gymnophionan yolk-plug.
| |
|
| |
| During the backgrowth of the horns of the crescent, which
| |
| are the lateral blastoporic lips, the cavity of the archenteron
| |
| has rapidly extended until it reaches the anterior end of the
| |
| embryonic shield (Fig. 83) ; the cavity is broad. The roof consists of a layer of columnar cells which at the dorsal lip turn
| |
| over in the ordinary way into the cells of the upper layer. The
| |
| floor is in front distinct from the lower layer, and here it consists of a single layer of cubical cells ; behind the dorsal lip - in
| |
| the primitive plate- it is much thickened, and from this thickenmg there proceeds backwards a narrow tongue of cells between
| |
| the upper and the lower layers.
| |
|
| |
|
| |
|
| |
| 136
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| A transverse sectign (Figs. 84, a ; 86) through the blastopore
| |
| shows the mass of cells of the primitive plate flanked on each
| |
| side by a projecting blastoporic lip and sending out between
| |
| the upper and lower layers two lateral sheets of cells.
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 83. - Sagittal sections of the blastopore and archenteron in the
| |
| Gecko Platydadylus. (After Will.) a-e, Successive stages ; jj.^p., primitive
| |
| plate ; pd., lower layer or paraderm ; s.g.c, subgerminal cavity ; arch.,
| |
| archenteron ; d.l, dorsal lip ; y.p., yolk-plug ; mes.v., mesoderm formed
| |
| at the ventral lip.
| |
|
| |
| The resemblance between these structures and those in the
| |
| Amphibian, and particularly the Gymnophionan egg when the
| |
| blastopore has become circular, is sufficiently obvious. The
| |
| dorsal and lateral lips (there is no ventral lip in the Reptiles)
| |
| clearly correspond in the two cases ; the mass of cells in the
| |
| primitive plate embraced by these lips is the yolk-plug ; the
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| Fio. 84. - Four successive transverse sections through the blastopore
| |
| and archenteron of Plalydactylus. (After Will. )
| |
|
| |
| A, Posterior section through the yolk-plug {y.f.) ; l.l, lateral lip ; 2^(1. ,
| |
| lower layer ; mes., mesoderm springing from the lateral lips.
| |
|
| |
| B is more anterior, just behind the dorsal lip.
| |
|
| |
| c is just in front of the dorsal lip, where the floor of the archenteron
| |
| {arch.) is still intact, and
| |
|
| |
| D more anterior, where the archenteron communicates with the subgerminal cavity.
| |
|
| |
|
| |
|
| |
| c
| |
|
| |
| arc?-..
| |
|
| |
|
| |
|
| |
|
| |
| c
| |
|
| |
|
| |
|
| |
|
| |
| 138
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| cavity of invagination is the archenteron in which floor corresponds to floor and roof to roof ; lastly, the sheets of cells
| |
| projecting beneath the upper layer at the sides of and behind
| |
| the blastopore are the equivalents of the mesoderm formed at
| |
| the lateral and ventral lips in the Amphibia.
| |
|
| |
| From this comparison it follows of course that cells which are
| |
| the morphological equivalents of the yolk-cells of the Amphibia
| |
| are to be found in the upper layer of the Reptihan blastoderm.
| |
| That layer, therefore, cannot be termed the ectoderm until the
| |
| process of invagination is complete.
| |
|
| |
| The floor of the archenteron now fuses throughout with the
| |
| lower layer, and as soon as the fusion is completed perforations
| |
|
| |
|
| |
|
| |
|
| |
| -PTP 86 - Transverse section of the blastopore and yolk-plug (y.p.) of
| |
| thTS^itoise (Trionyx). (After Mitsukuri.) U., lateral lip ; > m/so^erm
| |
| produeed at the lateral lips ; pel., lower layer not yet detaehed from the
| |
| yolk (stippled).
| |
|
| |
| begin to appear in tte fused layers (Figs. 83, E ; 84, e). They
| |
| seem to be unable to keep pace with the general gro^vth of the
| |
| blastoderm and to become first stretched and then fenestrated.
| |
| But to whatever causes the perforation may be due, the floor
| |
| of the archenteron with the underlying lower layer completely
| |
| disappears, and the archenteron then communicates freely with
| |
| the subgerminal cavity. The roof of the archenteron is now
| |
| inserted by its edges into the surrounding lower layer.
| |
|
| |
| The median strip of the roof next thickens to form the notochord (Fig 85), and separates from the two lateral portions which
| |
| then become the mesoderm. The notochord passes posteriorly
| |
| into the dorsal lip. the plates of mesoderm into the latei^hps
| |
| of the blastopore, and here the latter are perfectly contmuous
| |
| with the mesoderm produced at the sides of and behind the
| |
|
| |
|
| |
|
| |
| I
| |
|
| |
|
| |
|
| |
| Fig 87*.- Area pellucida of the lien's egg. a, After 12 hours , b, After
| |
| 18 hours' incubation, as seen by transmitted light. J5r.£/., prnnitive groove ;
| |
| n.ch., notochord ; pr.am., pro-amnion.
| |
|
| |
|
| |
|
| |
| P. 139
| |
|
| |
|
| |
|
| |
| VI THE GERMINAL LAYERS 139
| |
|
| |
| blastopore (Figs. 84, a, b ; 86). The mesoderm thus exhibits
| |
| all the relations which it has in the Anamnia.
| |
|
| |
| The Uning epithelium of the alimentary canal (endoderm) is
| |
| derived from the lower layer, which grows in from the sides
| |
| below the mesoderm and notochord (Fig. 85, c, d). From this
| |
| layer the gut is subsequently folded off, the remainder being
| |
| yolk-sac epithelium. In several cases the lip of the blastopore
| |
| is not the only source of origin of notochord and mesoderm,
| |
| both receiving additions in front, and the mesoderm at the sides
| |
| also, from the lower layer.
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 87. - ^Formation of the primitive streak and groove of the chick by
| |
| proliferation of cells of the upper layer. Transverse sections.
| |
|
| |
| A, At 10 hours. There is at present no sign of the primitive groove ;
| |
| the lower layer {'pd.) takes no part in the proliferation.
| |
|
| |
| B, At 15 hoiu's. The primitive groove has appeared. It is occupied
| |
| by a projecting mass of cells, tlie yolk-plug {y.'p.), and bounded by the
| |
| lateral hps {U.). The proliferated cells spread out on cacli side as the
| |
| lateral sheets of mesoderm {mes.).
| |
|
| |
|
| |
| The conditions observed in the Birds are very readily derived
| |
| from and very easily understood in the light of those which
| |
| obtain in the Reptiles.
| |
|
| |
| There appears in the posterior region of the blastoderm a proliferation of cells in the upper layer (Fig. 87, a) ; this rapidly
| |
| extends in the median line, and along it there appears a narrow
| |
| groove. The cell proliferation is the ' primitive streak ', the groove the "primitive groove" (Fig. 87*).
| |
|
| |
|
| |
|
| |
| Fig. 90. - ^Anterior (a) and posterior (b) halves of a sagittal section
| |
| through the primitive streak and associated structures of the sparrow.
| |
| (After Schauinsland.) There is a sUght cavity, archenteron, below the
| |
| dorsal Hp (d.i.), and a well-marked ventral lip {vL). n.ch., notochord ;
| |
| p.s., primitive streak ; 7nes.v., mesoderm behind the ventral lip ; p.a.,
| |
| lower layer.
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 91. - Transverse section of the anterior end of the blastoderm of the chick "at 15 hours showing the formation of anterior notochord (n.ch.) and
| |
| mesoderm (mes.) directly from the lower layer {end.) ; ec., ectoderm.
| |
|
| |
| This primitive groove is simply an elongated and laterally
| |
| compressed blastopore. In front of the anterior end - the dorsal
| |
| lip - the notochord is produced (Figs. 88, 89) ; to right and left
| |
| of the notochord are the sheets of mesoderm which, springing
| |
| from the sides - the lateral lips - of the groove (Fig. 87, b), are
| |
| continued into one another behind its posterior end, where there
| |
| may be an actual ventral lip (Fig. 90). The archenteric cavity has, however, in most cases disappeared, though a vestige of it is
| |
| sometimes to be seen (Fig. 90) . Between the sides of the groove-
| |
| which still exhibit the structure characteristic of blastoporic hps, is
| |
| merely a mass of cells- representative of the yolk-plug (Fig. 87, b)
| |
| - fused with the lower layer. The so-called ' neurenteric canal ' ,
| |
| which appears later, is the sole remnant of the archenteron
| |
| together with the communication which we have seen to become
| |
| established between it and the subgerminal cavity in Reptiles.
| |
| The primitive streak and groove invariably originate in the upper Icayer, fusion with the lower layer being merely secondary ;
| |
| only after the germ-layers have been formed can the upper layer
| |
| be described as ectoderm.
| |
|
| |
| The notochord and mesoderm receive increments in front from
| |
| the lower layer (Fig. 91).
| |
|
| |
| The gut (endoderm) is formed as in Reptiles.
| |
|
| |
| Mammaiaa
| |
|
| |
| In the Monotremata there is a long archenteron with a much
| |
| reduced lumen produced from the upper layer. The blastopore
| |
| is an elongated ' primitive groove '. The notochord and mesoderm have the usual relations to these structures. The interpretation put by Wilson and Hill on their observations - namely,
| |
| that the dorsal lip and archenteron are derived from the ' primitive plate ' while the primitive streak and groove are of distinct
| |
| origin - is probably erroneous. We may accept Assheton's
| |
| explanation that the ' primitive plate ' of the authors is simply
| |
| the point of final enclosure of the yolk by the blastoderm,
| |
| a precociously rapid process in this form, and that archenteron
| |
| and primitive groove are, as everywhere else, parts of one and
| |
| the same structure (Fig. 92).
| |
|
| |
| We are still in ignorance of the formation of the germinal layers
| |
| in Marsupials, though we may hazard the conjecture that the embryonic area of the blastocyst wall will be found to behave like the
| |
| embryonic shield in Reptilia, that a blastopore and archenteron will
| |
| be developed near its posterior edge in connexion with which the
| |
| notochord and mesoderm will arise in the usual way, that the
| |
| archenteron will break through into the subgerminal cavity below
| |
| the lower layer, and that this layer will give rise to the gut.
| |
|
| |
| This indeed is what occiurs in the Placental Mammals, the
| |
| only diflference being that here the embryonic area is from the
| |
| first enclosed in the sac of the trophoblast as part of the embryonic knob. This knob, as we have already seen, is, together
| |
| with the lower layer, differentiated from the original inner mass.
| |
|
| |
| The embryonic area (Fig. 92*), derived from the embryonic
| |
| knob, behaves precisely as the embryonic shield of the upper layer
| |
| in Reptiles, giving rise to an archenteron and blastopore ; this event
| |
| is, however, postponed until after the amnion has been formed.
| |
|
| |
|
| |
|
| |
| trek a
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 132. Diagiain of the egg of Ornithorhynchus after formation of the germinal layers. (After Assheton's modification of Wilson and Hill.) x, the
| |
| point at which the blastoderm has finally enclosed the yolk ; here the
| |
| upper layer (double line) and lower layer (broken line) are continuous with
| |
| one another and with the yolk. This is the ' primitive plate ' of Wilson and
| |
| Hill, a to p, primitive streak ; a, anterior end (dorsal lip) ; p., posterior
| |
| end. In front of a. is the archenteron (arch.), behind p. the mesoderm of
| |
| the ventral lip {mes.v.).
| |
|
| |
|
| |
|
| |
| Fig. 92.- Embryonic shield of the dog. (After Bomiet.) In the embryonic shield, where the cells are columnar, the nuclei are more closely packed
| |
| than in the surrounding trophoblast, where the cells are flat. At tne
| |
| posterior end is a notch, the blastopore (lower end in the fagure).
| |
|
| |
|
| |
|
| |
|
| |
| When the archenteron has been developed it behaves in the
| |
| manner we are already acquainted with. Its floor fuses with
| |
| the lower layer, and then the two break away so that the archenteron comes to communicate with the subgerminal or yolk-sac
| |
| cavity (Fig. 93). The notochord is differentiated out of its
| |
| roof, the mesodermal sheets pass into the lateral lips and are
| |
|
| |
|
| |
|
| |
| Fig. 93. - a. Longitudinal section of the embryonic shield and blastopore
| |
| of the bat, VesperlUio. (After Van Beneden.) The archenteron (arch.) has
| |
| broken through into the subgerminal cavity [s.g.c.) or cavity of the blastocyst. Below tlie dorsal lip (d.l.) is the blastopore (so-called neurcnteric
| |
| canal), and behind this the yolk- plug {ij.j}.). (With this should be compared
| |
| Fig. 138, which shows a human embryo in the same stage.)
| |
|
| |
| B, Transverse section showing the origin of the notochord [n.cli.) from
| |
| the roof of the rudimentary archenteron in the mouse. The floor of the
| |
| archenteron has already disappeared, mes., mesoderm ; 'pd., lower layer.
| |
| Above is the ectoderm of the medullar}^ plate.
| |
|
| |
| continuous with one another behind the blastopore. Accessory
| |
| notochordal and mesoblastic material is proliferated in front from
| |
| the lower layer. After this the lower layer is endoderm, and
| |
| gives rise to the gut and yolk-sac, after growing in from the
| |
| sides underneath the notochord.
| |
|
| |
| The archenteron may be well developed (as in VesperUlio), but
| |
| more usually is reduced to a narrow canal, the ' chorda-canal '
| |
| or, so called, ' ncurenteric ' passage.^
| |
|
| |
| ^ Neurenteric passage means properly the communication between the
| |
| medullary tube and the hmd end of the archenteron. See below, chap. vii.
| |
|
| |
|
| |
|
| |
| The Relation between the Amniote and the Anamnian Blastopore
| |
|
| |
| The facts we have now reviewed will have made it evident
| |
| that there are certain features common to the separation of the
| |
| germinal layers in all Vertebrates.
| |
|
| |
| Thus in all cases the material for the germ-layers is laid down
| |
| during an overgrowth and ingrowth of cells which takes place
| |
| at the lip of the blastopore during the formation and closure
| |
| of the latter. This closure is always bilaterally symmetrical,
| |
| beginning at the dorsal lip and taking place most actively there,
| |
| less actively at the lateral lips, and least of all at the ventral
| |
| lip. It leads to the formation of a bilateral archenteron, the
| |
| extent of which is greatest anteriorly, least posteriorly. The
| |
| layer that now remains outside is the ectoderm. The notochord
| |
| is differentiated out of the roof of the archenteron in the middle
| |
| line in front of the dorsal lip, while the mesoderm sheets which
| |
| flank the notochord pass back to the lateral lips and are confluent with one another behind the ventral lip.
| |
|
| |
| A, 1-3, The closure of the blastopore in such a form as the frog ; 1, 2,
| |
| before, 3, after rotation of the egg. The blastoderm, or small-celled area,
| |
| is heavily stippled. Its whole edge, which becomes the lip of the blastopore, is represented by a thick continuous line, d.l., dorsal, v.l., ventral lip.
| |
|
| |
| B, 1-3, Three similar stages in such a form as Lepidosiren, where the
| |
| ventral lip is absent. Only that part of the edge of the blastoderm which
| |
| becomes converted into a blastoporic lip - namely, the posterior and immediately adjacent parts - is indicated by the thick continuous line, d.l,
| |
| dorsal lip. '
| |
|
| |
| c, 1, 2, The condition seen in the Gymnophiona, where still less of the
| |
| edge of the blastoderm- only a small part at the posterior end, represented
| |
| by the thick line- becomes the lip of the blastopore, but the lateral lips
| |
| swing back, meet, and fuse to form the ventral lip, v.l. Thus the yolk
| |
| (white) remains uncovered.
| |
|
| |
| D, 1, 2, The Amniote blastopore. The heavily stippled area is the
| |
| embryomc shield, the central portion only of the Amniote blastoderm
| |
| but the equivalent of the whole blastoderm of the Anamnia. From the
| |
| posterior part of its margin a blastoporic lip is formed (d.l, dorsal lip)
| |
| and by the bending back and union of the lateral lips a ventral lip (v I )
| |
| as m the Gymnophiona. i' v
| |
|
| |
| The lightly stippled area outside this represents the extra- embryonic
| |
| Fâ„¢,"? blastoderm; which is equivalent to the yolk-cells immediately
| |
| surrounding the blastoderm of the Gymnophiona ^
| |
| xlr^l^^-^l the unsegmented yolk (white). Thus the blastopore of the
| |
| f^t lTfC formed inside its blastoderm, but at the edge of what is equiva
| |
|
| |
| Th« blastoderm, namely, the embryonic shield. ^
| |
|
| |
| lUe yolk is finally covered later on by the growth of the blastoderm
| |
|
| |
| k2
| |
|
| |
|
| |
|
| |
| 148
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| So far there is general agreement. There is, however, a very
| |
| serious difference between the two great groups of Vertebrates
| |
| in respect of the reh^.tion of the blastoporic lip to the blastoderm
| |
| - the cap of cells produced at the end of segmentation in a largeyolked egg or the area of small cells in a small-yolked egg - for
| |
| in the Anamnia the blastopore arises from the edge of this
| |
| blastoderm (Fig. 94, a), while in the Amniota it arises inside it
| |
| (Fig 94, d). By the help of the Gymnophiona, however, the
| |
| second condition may without difficulty be derived from the first.
| |
|
| |
| In the Gymnophiona (Fig. 94, c) (1) the blastoderm is an
| |
| oval area of columnar cells resting upon and surrounded by
| |
| a partially segmented yolk. (2) Only a part of the edge of the
| |
| blastoderm is converted into a blastoporic lip, namely, a small
| |
| region at the posterior end. Here a dorsal lip is formed and
| |
| lateral lips quickly follow ; the lateral lips then turn back,
| |
| encircling a small area of the yolk, behind which they meet and
| |
| fuse to form a ventral lip to the now circular blastopore. In
| |
| this process the anterior margin of the blastoderm is wholly
| |
| unconcerned. (3) The archenteron opens into the segmentation
| |
| cavity, notochord and mesoderm are derived from its roof, the
| |
| endoderm from the yolk-cells which lie in its floor. The notochord stretches in front of the dorsal lip ; the mesoderm sheets
| |
| springing from the lateral lips are continuous with one another
| |
| behind the ventral lip.
| |
|
| |
| As a result of this peculiarity in the formation of the ventral
| |
| lip the yolk remains uncovered. In all other Anamnia, however,
| |
| where the ventral lip is developed from the anterior edge of the
| |
| blastoderm, the yolk is necessarily covered up by the closure of
| |
| the blastopore.
| |
|
| |
| We turn now to the Amniota, to the Reptiles for instance,
| |
| and find (1) that the embryonic shield is a circular or oval
| |
| area of columnar cells resting upon a lower layer, and surrounded
| |
| by a zone of flattened cells. (2) At the posterior margin of this
| |
| embryonic shield upper and lower layers are continuous. Here
| |
| a dorsal lip is formed and lateral lips quickly follow ; the lateral
| |
| lips turn back encircling a small area of the outer zone of cells-
| |
| where these are continuous ^\'ith the lower layer- behind which
| |
| they meet and fuse to form (a virtual, in some cases an actual)
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| 149
| |
|
| |
|
| |
|
| |
| ventral lip to the now circular blastopore. In this process the
| |
| anterior margiji of the embryonic shield is wholly unconcerned.
| |
| (3) The archenteron opens into the subgerminal cavity, notochord
| |
| and mesoderm are derived from its roof, the endoderm from the
| |
| lower layer. The notochord stretches in front of the dorsal lip,
| |
| the sheets of mesoderm springing from the lateral lips are continuous with one another behind the ventral lip.
| |
|
| |
| It seems clear, then, that the embryonic shield of the Amniota
| |
| is the representative of the blastoderm of the Gymnophiona
| |
| (and of all Anamnia), while the marginal zone of the upper
| |
| layer, together with the lower layer with which it is at one
| |
| point - ^the primitive plate - still united, represents the yolk-cells
| |
| or nucleated yolk.
| |
|
| |
| In passing from the Gjrmnophiona to the higher Vertebrates
| |
| we have therefore to suppose that with the further increase of
| |
| yolk segmentation has become restricted not to the blastoderm
| |
| alone (as in Fishes), but to the blastoderm and those circumjacent and subjacent cells which in the Gymnophiona are partially
| |
| segmented from the yolk. In the most primitive Reptiles the
| |
| lower layer cells are still crowded with yolk and still retain
| |
| a connexion, in the primitive plate, with the marginal cells of
| |
| the upper layer. In other Reptiles, in Birds, and in Mammals this
| |
| primitive connexion is lost, and it is only secondarily, after the
| |
| formation of the primitive groove and streak, that the upper
| |
| fuses with the lower layer.
| |
|
| |
| The Gymnophionan condition must in turn be derived from
| |
| some Anamnian blastopore in the formation of which the anterior
| |
| edge takes no part, in which consequently no ventral lip is formed.
| |
| Such a form may be found in Lepidosiren (Fig. 94, b), in which
| |
| the yolk is less abundant than in the Gymnophiona, but more
| |
| abundant than in the typical smaU-yolked egg. Here the formation of a blastopore is restricted to the dorsal and lateral Jips.
| |
| The absence of a ventral lip may be a very primitive feature,
| |
| smce none is found in Petromyzon.
| |
|
| |
| It may also be noticed that the union of segmentation cavity
| |
| with archenteron occurs here and there in various Anamnia,
| |
| sometimes in Eana, and in Petromyzon, thus foreshadowing the
| |
| condition seen in Gymnophiona and the Amniota.
| |
|
| |
|
| |
|
| |
| 150
| |
|
| |
|
| |
|
| |
| THE GERMINAL LAYERS
| |
|
| |
|
| |
|
| |
| VI
| |
|
| |
|
| |
|
| |
| In the Anamnia, indeed, the archenteron has a direct relation
| |
| to the endoderm in that, after notochord and mesoderm have
| |
| been differentiated, the aUmentary canal is formed from its roof,
| |
| or floor, or both. But as we pass up the series the archenteric
| |
| cavity loses this significance, its lumen dwindles and finally
| |
| disappears, and its function is reduced to the differentiation of
| |
| notochord and mesoderm alone. The endoderm is then derived
| |
| from the lower layer cells - ^representative of yolk-cells - ^which
| |
| line the segmentation cavity.
| |
|
| |
| The same lower layer cells may contribute to the notochord
| |
| and mesoderm anteriorly, and this, as we have seen, is of constant occurrence in such small-yolked Anamnian types as the
| |
| Amphibia, and Petromyzon ; not, however, in the large-yolked
| |
| eggs of Fishes.
| |
|
| |
| The Significance of the Gebminal Layers
| |
| It will have been repeatedly noticed that the same elementary
| |
| organ or germ-layer may come into being by different processes.
| |
| This is true of the front end of the notochord and mesoderm,
| |
| and still more obviously of the endoderm, for the lining epithehum
| |
| of the alimentary canal may be derived from the roof only of
| |
| the archenteron (Elasmobranchs and Teleostei), from the floor
| |
| only {Petromyzon, Urodela, Ceratodus), from both roof and floor
| |
| {Rana, Lepidosiren), from the yolk-cells in the floor and from
| |
| those in the segmentation cavity (Gymnophiona, occasionally
| |
| Rana), or from the lower layer (yolk-) cells of the segmentation
| |
| cavity alone (Amniota).
| |
|
| |
| In considering such discrepancies in the mode of origin of
| |
| homologous structures- and discrepancies of this kind are of
| |
| common occurrence, not only in development from the egg but
| |
| also in budding and regeneration- it must be borne in mind
| |
| that experiment has shown the formation of the embryonic
| |
| organs- such as the germ-layers- to be dependent on the
| |
| presence of certain stuffs in the cytoplasm of the ovum, but
| |
| that these stuffs are not necessarily deposited in the situations
| |
| which will eventually be occupied by the organs to which they
| |
| give rise, nor even in the same position in the ova of animals
| |
| belonging to the same group. Thus they may occupy dissimilar
| |
|
| |
|
| |
|
| |
| VI THE GERMINAL LAYERS 151
| |
|
| |
| positions also in the segmented ovum, and again in the later
| |
| stage which we speak of as gastrulation or the closure of the
| |
| blastopore. The necessary materials - now cut up into cells -
| |
| have then to move into their definite positions, and thus we
| |
| witness the roof of the gut being formed by an upgrowth of
| |
| yolk-cells, or its floor by a bending down of the roof of the
| |
| archenteron.
| |
|
| |
| The way in which an organ is developed is not, therefore,
| |
| necessarily a criterion of its homologies. Homologous structures,
| |
| that is, those derived, like the alimentary tract of the Vertebrate, from some common ancestral structure, may differ in their
| |
| origin during individual development. The stuffs on which their
| |
| differentiation depends are doubtless comparable, but the paths
| |
| by which that differentiation is achieved may be diverse.
| |
|
| |
|
| |
|
| |
| LITERATURE
| |
|
| |
| R. AsSHETON. Professor Hubrecht's paper on the early ontogenetic
| |
| phenomena in Mammals. Quart. Jotirn. Micr. Sci., 1909.
| |
|
| |
| E. VAN Beneden. Untersuchungen iiber die Blatterbildung, den
| |
| Chordakanal und die Gastrulation bei Siiugetieren. Anat. Am. iii, 1888.
| |
|
| |
| R. Bonnet. Beitrage zur Embryologie des Hundes. Anat. Hefte,
| |
| Abt. ix, 1897.
| |
|
| |
| A. Beaueh. Beitrage zur Entwickelungsgeschichte der Gymnophionen.
| |
| Zool. Jahrb. x, 1897.
| |
|
| |
| Bashfoed Dean. The early development of gar-pike and sturgeon.
| |
| Journ. Morph. xi, 1895.
| |
|
| |
| Bashfoed Dean. On the embryology of Bdellostoma stouti. Festschr.
| |
| f. C. von Kupffer, Jena, 1899.
| |
|
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| L. F. Hennequy. Embryog6nie de la truite. Journ. de VAnat. et de la
| |
| Phys. xxiv, 1888.
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|
| |
| J. W. Jenkinson. Remarks on the germinal layers of Vertebrates and
| |
| on the significance of germinal layers in general. Mem,. Manchester Lit.
| |
| and Phil. Soc. I, 1906.
| |
|
| |
| J. Geaham Keeb. The development of Lepidosiren paradoxa. Quart.
| |
| Journ. Micr. Sci. xlv, 1901.
| |
|
| |
| K. MiTSUKUEi and C. IsmKAWA. On the formation of the germinal layers
| |
| in Chelonia. Quart. Journ. Micr. Sci. xxvii, 1886.
| |
|
| |
| J. Ruckeet. Die erste Entwickelung des Eies der Elasmobranchier.
| |
| Festschr. f. C. von Kupffer, Jena, 1899.
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|
| |
| H. ScHAUiNSLAND. Studien zur Entwickelungsgeschichte der Sauropsiden. Zoologica, xvi, 1903.
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|
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|
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|
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| 162
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|
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|
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|
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| THE GERMINAL LAYERS
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|
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|
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|
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| VI
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|
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|
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|
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| 11. Semon. Die Furchung und Entwickelung dcr Keimbliitter bei
| |
| Ceralodiis forstcri. Zool. Forschiivgsreise in Anslralien, 1901.
| |
|
| |
| A. E. Shipley. The development of Pctromyzon jluvialilis. Quart.
| |
| Joum. Micr. Sci. xxvii, 1887.
| |
|
| |
| J. SoBOTTA. Die Gastrulation von .4»n{aaZw. VerJtaiidl. Anat. Geaellsch.
| |
| Berlin, 1896.
| |
|
| |
| C. O. Whitman and A. C. Eycleshymer. The egg of Aviia and its
| |
| cleavage. Journ. Mor]}h. xii, 1897.
| |
|
| |
| L. Will. Die Entwickelungsgescliichte der Ileptilien. Zool. Jahrh. vi, ix.
| |
|
| |
| H. V. Wilson. The embryology of the sea- bass {Scrramts alrarius).
| |
| Bull. U. S. Fish Commission, ix, 1889.
| |
|
| |
| J. T. Wilson and J. P. Hill. Observations on the development of
| |
| Ornilhorhynchus. Phil. Trails. Roy. Soc, Series B, cxcix, 1907.
| |
|
| |
| H. E. ZiEGLEE. Beitrage zur Entwickelungsgeschichte von Torpedo.
| |
| Arch. mikr. -Anal, xxxix, 1892.
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|
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|
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|
| |
| i
| |
|
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|
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|
| |
| Fig. 95. - External features of the development of the tadpole of the
| |
| Frog.
| |
|
| |
| a. Medullary plate, anterior end : the three divisions of the brain are
| |
| apparent.
| |
|
| |
| h. The same embryo from the posterior end : the sides of the medullary
| |
| plate pass back on either side of the blastopore. The blastopore is now
| |
| reduced to a narrow slit by the approximation of the lateral lips ; at the
| |
| dorsal and ventral lips the aperture is rather wider.
| |
|
| |
| c. Medullary folds and groove, anterior end : the three divisions of the
| |
| brain are readily seen, and the anterior part of what will be the spinal
| |
| cord. External to the inner medullary folds are the outer, and these pass
| |
| in front into the broad gill-plates, in front of which again are the senseplates.
| |
|
| |
| d. Closure of medullary folds, but the suture is still visible : the gill-plate
| |
| is divided on each side into two, and in front of it is the sense-plate ; behind
| |
| the gill-plate is a slight constriction.
| |
|
| |
| e. Anterior view of the same embryo : the medullary folds have not
| |
| quite closed in front. Beneath their anterior end is a depression, the
| |
| stomodaeum, and on either side of this the sense-plates ; the gill-plates can
| |
| just be seen behind these.
| |
|
| |
| f. Posterior view of the same embryo : the medullary folds have closed
| |
| over the dorsal division of the blastopore (neurenteric canal) while the
| |
| ventral remains as the proctodaeum. The middle region of the blastopore
| |
| is marked by a very narrow suture.
| |
|
| |
| g. Later embryo from below showing the stomodaeum, in front of the
| |
| V-shaped sucker, and posteriorly the proctodaeum at the base of the tailstump.
| |
|
| |
| h. Older embryo from the right side. The tail is rather longer, the
| |
| proctodaeum at its base : the stomodaeum can be seen in front between
| |
| the two halves of the sucker. At the side of the head in front is the
| |
| nostril, behind the gill-slits.
| |
|
| |
| i. Older embryo (ready to hatch) with well-developed tail and external
| |
| gills.
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|
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|
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|
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| Betweenl52 andlG")
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