53 
bryo renders this, in the oyster, as in most other molluscs, 
the most difficult stage to study. 
I give three surface views of it (Figures 32, 34 and 35), in 
order to show the characteristics of the various aspects. Figure 
- 32 is a view of the left side, with the anterior end to the 
right and the dorsal surface below. Figure 34 is an anterior 
view of the ventral surface, that is, a view of the upper right 
hand surface of Figure 32, and Figure 35 is a view of the 
dorsal surface. In both 34 and 35 the anterior end is below. 
In the embryo from which these figures were drawn the polar 
globule was not present, but in other embryos it occupied 
the centre of the tuft of cilia of the velum, as shown, at a 
later stage, in 36, so that there can be no doubt that the velum 
occupies that end of the embryo which is above in Figure 31, 
and at the right in most of the preceding figures. 
Hear the centre of the ventral surface — the top of Figure 
32 — there is a well marked and constant protuberance of the 
body wall, which occupies the region which, in most mol- 
luscan embryos, gives rise to the foot, and which may perhaps 
be regarded as a rudiment of that organ. In front of this 
protuberance the anterior end of the body is round, and is 
occupied by the long cilia of the velum, which form a com- 
plete closed circlet. In the centre of the dorsal surface the 
body is crossed by a deep crescent-shaped furrow, 32, 34 and 
• 35 </, which is transverse to the long axis of the body, and 
which is seen in an optical section, 33 g , to be prolonged into * 
the body as the primitive digestive cavity. 
Posterior to this the body terminates in a pointed protu- 
berance, 32, 33 and 35 a , which is of importance in deter- 
mining the relation between this and later stages, and which 
may be called the anal papilla. A comparison of Figure 
33 with Figure 31 indicates that the present form of the body 
has been brought about by the infolding of the edges of the 
disk-shaped embryo, Figure 30, towards the dorsal surface, 
r in such a way as to carry the endoderm into the centre of the 
body, thus giving rise to a primitive digestive cavity, with a 
dorsal blastopore situated in the centre of a crescent-shaped 
^transverse furrow. Pabl has figured a stage in the develop- 
