392 PROCEEDINGS OF THE ACADEMY OF [April, 
be seen to be rapidly spreading over the posterior region. As this 
is being accomplished it also grows greatly in size, producing the enor- 
mous posterior vesicle which in figs. 109 and 110 extends far behind 
the internal organs of the body. The importance of such an organ 
must be considerable and, taken in connection with the early decrease 
in size of the head vesicle, strongly suggests that its functional value 
is similar in kind to that usually ascribed to the anterior or head 
vesicle of other larve. 
In all older veligers figured the original polar axis has become com- 
pletely bent upon itself, a rotation of 180 degrees having occurred. 
With regard to the median plane of the future embryo, the first cleavage 
plane is obliquely transverse to this plane. When the mesoderm is 
formed it is thrown over toward this median plane, and from the first 
is approximately bilateral in position (figs. 24, 31, 34). The elements 
of both entoblast and ectoblast, which in late stages of cleavages lie 
on the median plane, appear to be derived from cells of the early 
cleavages which occupied similar positions. Little rotation, if any, 
is apparent other than a certain amount of irregularity found in all 
portions of eggs with equal or nearly equal cleavage. 
Conklin describes for Crepidula an entire rotation of the ectoblastic 
cap at the time when the anterior and lateral cells of the fourth quartet 
arise. Heymons shows a similar rotation in Umbrella. Such a change 
of axis in the germ layers does not occur in Fiona, nor is there necessity 
for it. The large macromeres of Crepidula and Umbrella are here 
represented by small cells, which do not modify the positions of the 
germ layers at the time of their origin nor necessitate supplementary 
rearrangement. 
ABSTRACT. 
Maturation begins at the time of laying. Two polar bodies are 
given off, the first of which may or may not divide. The un- 
segmented egg of Fiona is rich in yolk, the spherules being com- 
paratively small. In shape the egg is round, but slightly flattened 
in the direction of its polar axis. One to three eggs are found in a 
roomy egg capsule. 
The early cleavage is strictly spiral after the dextral sequence. The 
first quartet of micromeres are much smaller than the macromeres, 
but with succeeding divisions the cleavage becomes equal in character. 
After the four macromeres are formed they give rise to successive 
quartets of micromeres. ‘The first three quartets contain all the ecto- 
