68 
Yon Jehring’s attempt to show that the early stages in the- 
development of Gasteropods differ fundamentally according, 
as they belong to one or the other of the two phyla between 
which he proposes to distribute them, is thus shown to be ab 
sojutely without a basis of fact. 
In a foot-note to his paper Eabl says that this similarity of 
ground-plan does not extend to the Lamellibranchs, but the 
embryology of the oyster shows that it does. 
In the Lamellibranch with which he is most familiar, IJnio, 
the process of segmentation is greatly abridged, and its true 
significance is only seen when it is compared with that of the 
oyster. This comparison shows that the eggs of Lamelli- 
branchs have passed through a change which is exactly the 
reverse of the one which Eabl has traced, and the peculiar 
interest of his paper lies in the fact that while he was tracing 
the process by which the food-yolk was acquired, I was en- 
gaged in tracing the process by which it has been lost, and 
that the form with which his series ends is the one with which, 
my series begins. 
It hardly seems necessary for me to say that I do not wish 
to be understood to hold that the Lamellibranchs are the de- 
scendants of Hassa, but simply that their ancestors laid eggs 
like the eggs of ISTassa. 
THE FORMATION OF THE DIGESTIVE TRACT. 
At the stage shown in Figures 32 and 33, the primitive di- 
gestive tract opens by a wide blastopore, which is situated 
upon what is to become the dorsal surface; and the outline of' 
the body in front of it is rounded and carries the velum, 
while behind it the outline is angular and pointed. At the 
stage shown in Figure 36, the outline of the body is nearly 
the same, and the external changes are so slight that the side 
which is below in this figure is at once seen to be the same as 
that which is below in Figure 32. This surface is still marked 
by a transverse groove, but the blastopore has closed up com- 
pletely, and one valve of the shell has made its appearance at 
each end of the groove. Posterior to the groove the papilla,. 
