80 
PACIFIC SCIENCE, Vol. IV, April, 1950 
about twice the volume of the preceding 
(Fig. It). 
By the following noon, July 10, the micro- 
meres in many of the embryos had increased 
to 16 in number and, in some instances, they 
were arranged in orderly rows of four at the 
animal pole. These cells did not differ much 
in size; the larger ones were at the outer 
angles and were, on an average, a little 
smaller than the first four micromeres. This 
result had apparently come about through 
further divisions of the eight micromeres ob- 
served in the preceding stage (Fig. 1 j). 
At this time in some of the embryos an 
additional division of a number of the micro- 
meres had resulted in a crowding together 
and a piling up into a pointed dome of these 
small cells at the animal pole (Fig. Ik). 
At 10:00 P.M., the same day, a marked 
change had taken place in the appearance of 
the embryos. An equal division of the ma- 
cromeres had occurred, making them now 
eight in number; furthermore, the rounded 
surfaces of these cells, which had stood out 
so conspicuously, were now contracted and 
flattened, giving the whole mass a rather 
globular form again (Fig. 1/, in). 
Owing to the thickness of the egg capsule, 
it was not possible to see the minute struc- 
tures of the embryos without tearing open 
the capsule and examining the embryos in 
water. Being thus placed in unnatural envi- 
ronments, their life was usuaily short and 
their development retarded. For this reason, 
the time normally required for the successive 
steps of development cannot be correctly 
indicated. 
At the time the micromeres became con- 
tracted and rounded off peripherally, the 
micromeres also pulled toward the center, 
crowded together, and thus, aided by their 
increased number, helped to restore the em- 
bryo to its former globular form. Viewed 
from the equator the embryo now resembled 
a basket full of eggs. This stage was observed 
July 11, at 3:00 P.M. (Fig. In). 
Twenty-four hours later the embryo had 
become compressed at the poles and had lost 
its spherical form. In some, 12 macromeres 
could be seen, a division of the row distal 
of the vegetal pole having taken place. The 
micromeres, increasing in number, occupied 
a larger area of the surface and extended 
laterally so as to form an obtuse angle with 
the macromeres. The beginning of move- 
ment of the embryo takes place at this stage 
although cilia cannot yet be seen (Fig. lo) . 
The following morning, July 13, an elon- 
gated form of the embryo had succeeded the 
former compressed one, the elongation hav- 
ing occurred along the equatorial plane. The 
micromeres, by their greatly increased num- 
ber, now extended over almost the whole of 
the embryo, leaving but a small area of the 
macromeres uncovered. This was the blastula 
stage. The body was slightly tapered at the 
anterior end, where there was a small tuft 
of short cilia on each side. By means of these 
cilia the embryo was able to turn over and 
move backward and forward. This was the 
trochophore stage (Fig. Ip). 
Twenty-four hours later, July 14, a gas- 
trula was formed, partly by epiboly and partly 
by invagination of the macromeres. A con- 
striction near the point corresponding to the 
vegetal pole of the ovum indicated the posi- 
tion of the blastopore. The micromeres, which 
were transparent, had become flattened and 
formed a layer of nearly equal thickness 
around the macromeres, which appeared as 
an inner dark mass. The archenteron ap- 
peared as a narrow fissure extending through 
the clear ectodermal layer and into the large 
yolk-laden endodermal cells. At the inner 
end of the archenteron, against the endo 
dermal cells, a group of small, clear cells 
was seen which were probably mesodermal. 
The length of the embryo at this age was 
0.16 mm. (Fig. 1 q) . 
An important change took place in the 
next 24 hours. Two large, ciliated lateral 
lobes and a smaller median lobe with shorter 
