9 
MUSEUM OF COMPARATIVE ZOOLOGY. 113 
the cells are likewise separated by a transparent wall of the same general 
appearance. ach of the cells of the 4-cell stage has a nucleus. 
The 8-cell stage, Figs. 8, 9, follows close upon the 4-cell, the addi- 
tional cells forming by a subdivision of those already existing. This 
division is generally regular, all the cells being of like size. In some ova 
smaller cells were observed with larger in the 8-cell stage. The cells 
have a centrifugal tendency, and a central unoccupied cavity, cav, can be 
seen enclosed by them. This cavity, which is the cavity of the blasto- 
sphere, grows in size as the larva matures. There is no solid morula stage, 
but a segmentation-cavity can be recognized in eggs as young as the 
4-cell stage. The transparent layer ¢, which envelops the 8-cell stage, 
and which is thought to be the same as the thin superficial layer of ear- 
liest stages, is somewhat diminished in thickness. It is seen to be spread 
over the surface of the cells, and to separate the spheres of segmentation 
from each other.* A day after the eggs were laid they had developed 
into free swimming spheres, Pl. I. fig. 10, covered externally with cilia. 
These larvee were found in great numbers free in the jars. The egg has 
developed into a larval stage, which has burst the imprisonment of the 
capsule, and the blastomeres have arranged themselves on the periphery 
of a hollow sphere. The superficial layer of cells, still more transparent 
than the profound, bears long vibratile cilia ; the larva moves readily from 
place to place. 
At one pole of this larva the blastoderm, or that shell of cells which 
encloses the cavity, is slightly thickened and more deeply colored than 
the remainder. This pole is the pole where the invagination to form 
the archenteron takes place, and it may be said at once that this pole 
is the seat of the next important change in the growth of the larva. 
At this point, Pl. I. fig. 11, ach., the blastoderm begins to fold inward, 
forming an invagination, which later becomes the stomach. The posi- 
tion where this infolding begins is the pole at which the mouth of the 
gastrula, or, is later situated. At this time in the career of the young 
Ophiopholis it begins to depart widely in form from that of the genus 
Ophiothrix, as figured and described by Apostolides. The segmentation 
of the egg is very similar in the two genera, but the form of the blasto- 
pheres is somewhat different. The blastodermic cells are very much 
* This condition of the plasmic cortical layer in stages of cleavage as old as the 
8-cell condition, is thought to indicate that the layer may be something more than 
_ a vitelline membrane, although the vitelline membrane may be formed from some 
portion of it. It is not possible for me to arrive at any good interpretation of the 
homology of this structure. 
VOL. XII. — NO. 4, 8 
