61 
Pulmonates, are imaginary and unlike anything in nature, 
and I therefore take pleasure in stating here that my own 
work upon the oyster tends to show the perfect accuracy of' 
the observations in the present paper, not only so far as the 
early stages are concerned, but also as regards the later his- 
tory of the embryo. 
figure 7, Plate X, of the egg of Unio is clearly the same 
as my Figure 13 ; Figure 10 is almost identical with 19 of 
the oyster; Figure 11-14 are very similar to 20-23 of the 
oyster ; Figure 15 differs from 26 of the oyster only in the 
presence of a segmentation cavity ; 17 and 18 are the same as 
27 and 28 of the oyster, except that they are not flattened 
vertically, and his figures 28 and 30 are essentially the same 
as my 32 and 38. 
I have already shown that the stage 13 of the oyster egg, 
which is usually reached by passage through a number of 
intermediate stages, and by the formation and obliteration of 
a third spherule, may be reached by a more direct process, 
which is exceptional in the case of the oyster. It is interest- 
ing to notice that Flemming and Pabl agree that the indirect 
form of segmentation which is normal in the oyster, is want- 
ing in Anodonta and Unio, and that this stage is reached 
directly in a manner which is only occasionally met wdth in- 
the oyster. 
There can be no doubt, that in Anodonta at least, the trefoil 
stage is really wanting, and has not simply been overlooked,, 
for Flemming actually watched and has figured the change 
of the spherical unsegniented egg into the form shown in his 
Figure 6. 
In addition to the obseryations above referred to, we have 
a number of papers which deal with the development of 
various species of Cycladidse, and contain some observations 
upon the early stages, but no one has succeeded in getting 
anything like a complete series of observations, and those 
which are recorded are not at all in harmony with each other. 
In his “ Contributions to the Developmental History of the 
Mollusca, No. I, The Early Development of Pisidium pusil- 
lum,” (Phil. Trans. 1875, vol. 165, part I), Lankester gives a 
