FORMATION OF GERM-LAYERS IN CRANGON VULGARIS. 345 
undivided. Kingsley, who has described the unsegmented 
egg and the first two nuclear divisions, states that no furrows 
appear until four nuclei are present. The rudimentary con- 
dition of the furrows in my specimens with four nuclei affords 
a strong confirmation of this statement. 
In the next stage observed eight cells were present, arranged 
in the manner shown in fig. 2.. Round the equator of the egg 
were four cells arranged in a riug, while at each pole were two 
cells. The upper and lower cells of fig. 2 are unfortunately 
so placed that each conceals a second cell. Sections through 
the eight-celled embryo show that the segmentation furrows 
are now much deeper than during the four-celled stage. The 
drawing given in Pl. XX, fig. 10, represents, as accurately as 
may be, the appearance of a median section. Two of the blas- 
tomeres appear to be completely separated, by well-marked 
fissures, from the rest ; but the three remaining cells, through 
which the section passes, are connected by their central ends 
with a small, irregular, unsegmented mass of yolk. In other 
sections of the series a similar communication with the central 
mass could be demonstrated in the case of every cell. 
I have taken some little trouble in order to determine 
whether this central mass of yolk, with which all the cells 
appear to communicate, is a mere result of manipulation, or 
whether it is really present in the living egg. In transparent 
preparations of the hardened egg, or in living specimens, it 
seems easy to trace the cell outlines almost to the centre ; and 
examination of specimens in this manner led E. van Beneden 
to the conclusion that the segmentation was total. But it is 
difficult to be quite sure of the distinction of the central 
extremities of cells which are seen through a maze of spherical 
outlines due to the superficial yolk; and, on the whole, from 
the great uniformity of the appearances seen in section, it 
seems probable that the segmentation is really centrolecithal 
and incomplete, though the central mass of unsegmented yolk 
is smaller than in many Decapods. 
The stage with sixteen cells appears to follow immediately 
upon that just described, by division of each of the eight blas- 
