Mr. C. P. Sigerfoos on the Pholadide. 235 
portions, the latter somewhat larger than the former. The 
left entoblast always divides first. The cilia appear at the 
stage with two entoblasts’ The two or three apical cilia are 
more than twice as long as the embryo. The pre-oral cilia 
are not arranged on circles of cells. Hach cell which bears 
them has three or more. The apical cilia turn spirally, 
causing the embryo to rotate on its long axis in the direction 
of the hands of a clock. 
Gastrulation is partly epibolic, partly by invagination. 
The entoblast-cells continue to divide. The spindles are 
always transverse to the long axis of the cells, so that the 
primitive gut comes to be formed of very high cells. Small 
cells are not separated from them into the cleavage-cavity. 
After gastrulation begins the ectoblast continues to grow 
forward slightly at the posterior border of the blastopore, so 
that it becomes crescent-shaped. It does not close, but 
persists as the mouth. 
Professor Brooks several years ago pointed out a transitory 
change in shape of the oyster embryo. This is present in all 
of the four species I studied, and I believe to be connected 
with the formation of a second mesoblastic element. In 
addition to the mesoblast as usually described there is an 
anterior mesoblastic element, formed at about the time that 
the entoblast consists of four cells. The embryo becomes 
flattened in its dorso-ventral axis, and while so a pair of 
ectoblast-cells m¢grate from the surface and are deposited on 
the summit of the entoblast. The point on the surface from 
which they migrate soon closes over and leaves no trace of 
their inwandering. ‘The embryo soon afterwards resumes its 
spherical shape. 
Summing up: The first two planes of cleavage are meridional, 
followed by an equatorial. Segmentation is such that bilateral 
symmetry is not established till after the formation of the 
germ-layers. The entoblast-cells divide into almost equal 
parts, and gastrulation is partly epibolic, partly by invagina- 
tion. ‘The blastopore persists as the mouth. The first cilia 
are irregularly arranged ; the apical cilia are very large; the 
embryo rotates on its long axis, in the direction of the hands 
of awatch. In addition to the mesoblast, as usually described, 
there is a second factor derived from the ectoblast after the 
embryo is free-swimming. 
Teredo norvegica. 
I am indebted to Professor Dall for the determination of 
this species. It is found abundantly at Beaufort, grows 
