TWO ABNOEMAL PLUTEI OF ECHINUS. 
247 
floor of the amniotic cavity. Now, when we consider that an 
“ echinus-rudiment ” has been formed on the right side of 
our larva, almost precisely similar to that which is normally 
formed on the left side, and which is typically developed in 
our larva, we are driven to the conclusion that not only is 
the ectoderm on the right side of the larva capable of forming 
an amniotic invagination — a conclusion which we had already 
reached from the consideration of the other larva — but that 
the right posterior coelom is capable equally with the left 
of budding off calcigenous mesenchyme, and of giving rise to 
evaginations which form dental pockets, and thnt the right 
side of the stomach equally with the left is capable of giving 
rise to a protrusion forming an adult oesophagus. 
Under ordinary circumstances none of the tissues of 
the right side of the larva undergo this development, 
and the reason that they do so in this case must be due 
to some kind of influence emanating from the abnormally 
developed right hydroccele. If, then, the appearance of a right 
hydrocoelic vesicle can so totally change the development of 
tissues which normally would have had quite a different 
fate, it is reasonable to conclude that the development of 
organs on the left side of the larva is due to a similar 
influence radiating from the left hydroccele. The final con- 
clusion to which we are led is, therefore, that the larva con- 
sists of sheets of uniform tissues, and that particular areas of 
these sheets are acted on by localised stimuli so as to be 
transformed into adult organs; or putting the conclusion in 
another way, the particular course that development takes 
in an Echinoderm larva, is only one out of many possibilities, 
and by no means exhausts the potencies of the embryonic 
layers, and there is a certain rude analogy between the 
maintenance of a race by the preservation of a few indivi- 
duals out of many born, and the development of an individual 
through the realisation of a few of the many potencies of the 
embryo. As mentioned above, I have found enantiomorphous 
larvae in the case of Asterina gibbosa. In one of these, 
development has proceeded so far that not only do the lobes 
