804 
parallel to the second cleavage plane. A strongly marked bilateral 
arrangement of the cleavage spheres results. At the sixteen-cell-stage 
the blastoderm again becomes circular and retains this shape through- 
out the subsequent stages of segmentation. The blastoderm is, there- 
fore, circular in outline during the unsegmented stage, the four-cell- 
stage and the sixteen-cell-stage, bilateral during the two-cell-stage 
and the eight-cell-stage. Even in the four-cell-stage and the sixteen- 
cell-stage however the bilateral period lasts longer than the circular 
period. 
So conspicuous are the bilateral stages of the segmenting egg 
that irresistly they lead to the supposition that there exists some 
causal relation between this bilaterality and that of the adult body. 
Miss CLapp has shown in the egg of the toad-fish (Batracus) that no 
such relation exists and I have tested her conclusion on the small 
pelagic eggs of Ctenolabrus and Serranus in which the bilateral cleav- 
age is more strongly marked than in the toad-fish egg. My results 
show that there is no relation whatsoever between the cleavage planes 
of the egg and the median plane of the adult body. I have definite 
records of twenty-two eggs carefully marked. The results, expressed 
as nearly as possible, show for Ctenolabrus the plane of first cleav- 
age agrees approximately with the medium plane of the body in five 
cases, the plane of second cleavage with the medium plane in 
ten cases. The medium plane of the adult body lay between the first 
and second cleavage planes in two cases. For Serranus the first cleav- 
age agreed in one case, the second agreed in two cases, and neither 
in two cases. A word as to the method. All attempts by breaking 
through the membrane to mark the blastoderm lead to failure. Two 
methods remained possible: 1st to follow through step by step an in- 
dividual egg from the two-cell stage to the appearance of the germ-< 
ring; 2nd to mark the membrane above the blastoderm and to use 
such marks as points of orientation. The first method is extremely 
tedious taking a whole day of close observation to a single egg. This 
was done in one case and the median plane of the embryo corresponded 
to a plane midway between the first and second cleavage planes. 
The success of the second method depended upon the close ad- 
herence between the egg and its membrane. If the egg revolved (as 
some fish eggs do) the method was useless; if the egg remained fixed 
the method would give definite results. After a very careful test it 
was found that even after rough treatment the egg retained a fixed 
position relatively to the surface markings. And again by watching 
individual eggs for some hours it was seen that the egg did not rotate 
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