318 M. D. HILL. 
up a central position. The two products of division both 
resemble the original astrosphere at the stage drawn in fig. 3. 
There is a finely reticular central mass in each, but in no single 
instance was I able to see a centrosome. Although I find a 
similar absence at this stage in Phallusia, still I believe that 
further examination is all that is necessary to prove that the 
centrosome exists at this stage also. The two astrospheres 
gradually travel to opposite poles of the ovum, as Wilson has 
already described. Shortly after taking up their positions 
there, they exhibit each a clear inside space! (thegranular central 
mass having disappeared), in the middle of which are two clearly 
distinct centrosomes (fig. 6). It is probably this stage that 
Fol interpreted as being the one in which the two halves of the 
egg- and sperm-centrosomes respectively are about to fuse, one 
sperm half with one egg half, to form two single composite 
centrosomes (cf. Fol, fig. 9). From his drawings, however, 
even when it is taken into consideration that they are but 
rough woodcuts, one is tempted to doubt whether Fol ever 
really saw a centrosome at all. It is quite certain, as Wilson 
points out, that the clear area round the nucleus in which 
the “ quadrille ” is drawn as taking place is an artefact. Ihave 
obtained like results after using certain reagents, especially 
with the eggs of sea-urchins which had been kept some time 
in the tanks previously. With regard to the decrease in the 
length of the rays (mentioned by Wilson) during the Amphiaster 
(two-starred) stage, I cannot confirm his results. On the con- 
trary, in some instances they are larger than at any other 
time, reaching nearly to the periphery of the ovum, and the 
“stratum corneum” is extremely well marked. 
Further than the Amphiaster (two-starred) stage of the first 
segmentation nucleus I have not investigated. The conclusions 
which I think may be drawn from these facts I shall reserve 
till later, in the general summary at the end of the paper, 
1 By the word “space” I do not mean a hollow cavity in which the 
centrosome is somehow suspended, but a portion of the protoplasm which is 
so homogeneous as to give the impression, even when looked at under high 
powers of the microscope, of being an empty space. 
