16 
W. H. CALDWELL. 
Development of Phoronis. 
The details of the segmentation of the ovum are not required 
in this paper. A planula slightly oval in form is the final term 
of the process. The long axis of this planula coincides with 
the future long axis of the gastrula. One half of the cells are 
large (endoblast), the other half are small (ectoblast). 
Gastrulation. 
The gastrula is formed by invagination. The first sign of 
this is the flattening of the endoblast-half of the oval planula. 
The sides begin to grow over the endoblast, and this takes place 
in such a way that the saucer-shaped structure is deepened 
towards the future anterior end (PI. II, fig. 1). The anterior 
end also grows rapidly over the endoblast, thus early indicating 
the future prseoral lobe. The bilateral symmetry is thus 
clearly marked. Posteriorly the sides fold over so as to meet in 
the middle line (PI. II, fig. 9). The cavity of the archenteron 
is now sufficiently large to form lips to the blastopore. Quite 
posteriorly the lips completely fuse, so that during the gastru- 
lation the extreme posterior portion of the archeuteric cavity is 
obliterated (PI. II, fig. 10). It is represented by a fused solid 
mass of cells (r). The lips of the blastopore continue to 
approach the middle line, and as they touch fuse with each 
other. This fusing proceeds from behind forwards. The 
blastopore has in this way become divided into two parts 
exactly comparable to the parts long known in some vertebrates 
(PI. II, fig. 2). I shall speak therefore of the posterior portion 
as the primitive streak, and the groove along the line of closure 
as the primitive groove. The invagination has now produced 
a gastrula with an opening situated in the anterior portion of 
the blastopore into a large archenteric cavity (PI. I, fig. 11). I 
shall now use the terms dorsal and ventral as defined by the 
non-blastoporic and the blastoporic regions respectively. The 
ventral surface now begins to grow very much more rapidly 
than the dorsal. The growth results in the posterior point of 
the primitive streak becoming terminal. The exact behaviour of 
