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11. The Mechanism Which Has Brought About Direct Movement.
In the foregoing analysis of the developmental phases of ophiuroids it has been shown that the principle agent tending to produce direct development has been an increasing yolk-mass. One striking fact is the uniformity of the changes produced in diverse groups of ophiuroids—ranging from one of the most primitive species such as Ophiomyxa brevirima to the more specialised types. The modification of the gastrulation process, the suppression of the larva, and the increase in mesenchyme in which a schizocoel comes to form are some of the salient features of the yolky-egged forms.
The truly remarkable parallels which are observable in the embryonic stages of these various unrelated ophiuroids which have been influenced by accumulating yolk suggest very strongly that there exists some common principle which has operated in a common way upon these diverse forms. Before suggesting a possible answer to the problem of what this principle may be, it will be convenient to summarise the main stages in the sequence of forms showing reduction and loss of the larval stage.
We see in this sequence a progressive reduction in the size and number of the larval arms followed by reduction and final loss of the arm skeleton. Then comes reduction of the eoelomic pouches on either side, with a corresponding increase in the mesenchyme mass in which the coelom alternatively develops by splitting. Finally there is complete loss of bilateral symmetry, loss of the larval stage, loss of the vestigial enterocoels, and both general coelom and hydrocoel develop by splitting in the mesenchyme mass, which becomes increasingly important.
Now, in this progressive retreat inwards of the bilaterally disposed elements of the larval body with simultaneous recession of the radial symmetry into the earlier stages, we have a process suggesting alterations in the axial gradients of the embryo, these alterations being progressive along the series. Considering the modifications of development in this light, the following explanation may be put forward to explain the nature of the mechanism involved.
If we suppose that in the course of the evolutionary history of these larval forms changes began to take place in the metabolism along the axial gradients on either side of the body of the larva,
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the changes being in the nature of inhibitions, then a reduction and final loss of the organs on either side would occur—larval arms, skeletal rods, coelomic pouches; in other words, the bilateral symmetry would become gradually submerged, and the point at which radial symmetry was assumed would undergo a recession towards the gastrula stage. This is precisely the sequence of events which, it has already been suggested, has actually occurred. Thus, an inhibitory influence acting upon the earlier stages of development is sufficient to bring about entire loss of the larval stage.
Here, as in all theories, a useful test is the inductive one. What evidence have we that such a process can take place? Is there experimental evidence? The answer is, I believe, supplied already by the work of Child (1916). In the course of experimental work upon the plutei and earlier embryos of echinoids this worker was able to show that a wide range of simple chemical substances could cause an inhibitory effect very similar to that which has been postulated above. He was able to produce sea-urchin plutei showing successive degrees of reduction and obliteration of antero-posterior, medio-lateral and apico-posterior differences through inhibitions of the axial metabolic gradients. In extreme cases his larvae resembled somewhat the peculiar armless larvae already referred to in this paper. Child drew from his work the deduction that the larval forms of echinoderms may have been evolved by increases in the metabolism along the metabolic gradients, producing the outgrowths of the arms, etc. Combining his results with the sequence of forms described in this paper, we can similarly add that the larval forms of echinoderms may have been secondarily lost again through inhibitions in the same axial gradients. The inhibitory substance may have been developed in close association with the production of the yolk material, or it may be that the yolk material itself provided the inhibitory influence.
A further parallel remains. Child found that the inhibitory effects were differential—acting to a variable degree on various tissues. Mesenchyme he found was comparatively unaffected, and consequently underwent a great increase in his larvae at the expense of the bilateral organs which became reduced. Now this is also closely similar to the effect noted in the transition from indirect development to the direct type. With reduction and disappearance of the larva there has been a corresponding increase in the amount and importance of the mesenchyme.
Therefore, in the view of the writer, there is good reason to believe that direct development in echinoderms has been produced by an inhibitory influence upon the axial metabolic gradients of the larva. The inhibitory influence is associated with an increasing yolk-mass, and has manifested itself through a steady recession of the metamorphosis towards the gastrula stage.