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4. Formation of the Gastrula.
As the ontogenetic process advances the modifications produced in the larger and more yolky forms become ever greater. We thus find that the classical concept of evagination in the echinoderms becomes inadequate; for the evagination doctrine presupposes a static organogeny, whereas in actual fact organogeny is as dynamic and susceptible to moulding influences as any other biological process. Evolution, in other words, can act upon early stages of development as upon later stages. Embryological processes can be altered in the same way as can the adult products of these processes. This theory of the ability of evolution to act upon embryonic forms—so convincingly set out in de Beer's essay on “Embryology and Evolution” (1930)—receives strong support from the whole of the evidence provided by the yolky-egged ophiuroids.
Thus it is that in considering together the various types of gastrulation process met with in the Ophiuroidea we are able to detect the operation of a unidirectional evolutionary force, the strength of the force operating in direct proportion to the size of the yolk-mass. whereas the doctrine of unchangeable recapitulation during development becomes meaningless in the light of such a process as is observable here, the view which envisages embryonic forms as essentially plastic and subject to a greater or less degree of modification—according to circumstance—provides, in the view of the writer, the only possible explanation of the facts observed.
The traditional concept of gastrulation in the echinodermata supposed that it takes place by invagination from one pole—-the vegetal one. This is indeed “true in the dwarf-egged group, such as Ophiothrix and Echinus. But, without any logical reason, it was then immediately assumed that such a process must also occur in all echinoderms.
The first voices raised against this rigid hypothesis were those of Apostolides (1882) and Russo (1891), who both claimed that the endoderm was formed by delamination in the embryo of Amphipholis (Amphiura) squamata. As it happens, their claim was based on a misinterpretation—as shown in the accompanying account of the development of that species—but the importance of their observation is that it demonstrated that gastrulation in Amphipholis certainly did not occur by means of simple invagination, for such a process when it occurs is unmistakable. However, their claim received no recognition, although it was never actually disproved. MacBride, in fact, dismisses the whole of Russo's account as “improbable in the highest degree,” and omits it from his account of the echinoderms
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in his “Text-book of Embryology” (1914). The very fact of such an attitude is itself a demonstration of the inadequacy of the Recapitulation Theory—which of course provided no explanation of the peculiar development of Amphipholis.
Grave (1900) gave reasons for believing that the endoderm in Ophiura arose not by the invagination of a hollow archenteron, but as a solid inpushing which later became hollowed out to form an archenteron. Certain stages were missing from Grave's material, however, and advantage was taken of this fact to throw doubts upon the accuracy of his work (Bather 1901, and MacBride 1907). In his later work, however, MacBride (1914) quotes Grave's account without comment. As is seen from the facts quoted below, Grave's account receives strong support from independent evidence derived from the study of Kirk's ophiuroid.
Figs 16–18.—Effect on gastrulation of increasing yolk mass. Fig. 16, gastrulation in Ophiothrix (after MacBride); Fig. 17, first stage in gastrulatiou of Kirk's ophiuroid. Fig. 18, second (epibolic) stage of gastrulation in Kirk's ophiuroid. Figs. 17 and 18 simplified from Fell (1941).
In the latter species gastrulation takes place by means of two processes. First, as shown in Fig. 17, there is a slight inpushing of the macromeres from the vegetal pole. This results in the complete obliteration of the small blastocoel. The macromeres which were pushed in remain a solid mass of cells without any cavity. So far the process is similar to that described by Grave in Ophiura A second process commences now and involves extensive epiboly
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of the micromeres, which migrate towards a central point on the vegetal hemisphere and then turn inwards (see Fig. 18). The point at which the inwandering of these micromeres takes place obviously is homologous with the blastopore, and a temporary small depression at that region is all that represents the archenteron. At a very much Jater stage a second cavity appears and extends as an excavation tip through the solid endoderm mass to form the definitive enteron.
To sum up, the effect of increasing yolk-mass upon the process of gastrulation has been, first, to modify invagination till it takes the form of a solid inpushing of cells. At a later stage an excavation in this mass produces the definitive enteron. This modification applies both to the moderately yolky type (Ophiura) and to the heavily yolked type (Kirk's ophiuroid). In the ease of the latter, however, the modification proceeds to a further stage, for the small blastoeoel makes it mechanically impossible to invaginate all the endoderm, and hence a secondary process of epibolic inwandering of the micromeres takes place, surrounding and enclosing the whole of the vegetal hemisphere.