MUSEUM OF COMPARATIVE ZOOLOGY. 125 
A furrow appears at one pole. This furrow penetrates into the yoll, 
forming in profile a slit-like structure, which in this way divides the 
yolk into the 2-cell stage. In Strongylocentrotus, figured by A. Agassiz, 
we notice that a flattening of each cell of the 4-cell stage occurs prepara- 
tory to the passage into the 8-cell stage. This flattening occurs on one 
side at first (p. 710, fig. 27). Several eggs of Echinarachnius, PI. III. 
fiz. 6, were taken in a similar condition. In many others, however, 
each of the four cells of the 4-cell stage is divided from the very first by 
a constriction reaching wholly around the cell, Pl. II. figs. 4, 8. 
Tn several ova the following modification of development was observed 
after the 4-cell stage. An egg was found in the 4-cell stage apparently 
normally formed, Pl. II. fig. 9. Immediately after two of the spheres 
begin to fuse, and the wall of the cleavage plane separating them is 
broken down. In this way we pass by retrogression from an egg with 
four, Pl. III. fig. 9, into one with three segment spheres, Pl. III. fig. 12. 
Whether the many eggs in a 3-cell stage which were observed were all 
.formed in this manner or not, cannot be stated. It was not observed 
how the 4-cell stage in this abnormal mode of development is formed 
from the 2-cell stage. Segmented ova with three segmentation spheres 
are quite common in some trials for artificial fecundation. 
An egg fertilized at noon was found in the 2-cell stage at 1.30 P. M., 
and passed into the 4-cell stage at 2Pp.mM. At3 P.M. it was in the 
8-cell stage. We can, therefore, roughly say that the formation of a 
fresh cleavage plane occupies approximately an hour’s time. By a 
comparison with the rate of growth of the starfish it will be seen that 
the rate of development of Echinarachnius is more rapid. The water 
in which my eggs were kept. was evidently warmer than that in which 
Strongylocentrotus was reared. 
The mode of formation of the 8-cell stage from the 4-cell does not 
differ from that of the 4-cell from the 2-cell. The segments of the 
4-cell stage are, however, not always bisected, and here appears the 
first indication of an unequal segmentation. The spheres of the egg 
even in the 8-cell stage have a peripheral tendency. In the 8-cell stage 
it will be noticed, Pl. Il. fig. 11, that the eight spheres cannot be so 
brought together as to touch each other on adjacent sides. A recess, 
cav, is thus early left, which later forms in the interior of the ovum a 
“seomentation cavity.” This cavity increases in size as the size of the 
segmentation spheres diminishes in the progress of segmentation. An 
egg in the 32-cell stage was found four hours after impregnation, Pl. II. 
fig. 12. 
