ON FECUNDATION, MATURATION, AND FERTILISATION. 317 
surroundings. After being hardened in alcohol, the eggs were 
embedded in paraffin, cut into sections, and stained with 
Heidenhain’s iron-hematoxylin. 
Fig. 1 is a drawing of a section of an unfertilised ovum, 
killed immediately after leaving the parent’s body. The polar 
bodies are given off before the egg leaves the mother, but the 
nucleus has not yet taken up a central position. At this 
stage there is no sign of an astrosphere or of a centrosome. 
The next stage is taken (fig. 2) shortly after the entrance of 
the spermatozoon, which may be at any point on the surface, 
and is not affected by the position of the female pronucleus. 
The tail of the spermatozoon has dropped off, and the head 
and middle piece have turned completely round, so that the 
latter is nearest the centre of the egg. That this rotation 
takes place, as Wilson is the first to point out for Echinoderm 
eggs, I had convinced myself long before seeing his paper. 
The process is exactly parallel to that described by Fick (8) as 
occurring in the egg of the axolotl. For avery short time 
the middle piece persists as a small faintly-stained body attached 
to the sperm head. It soon becomes separated, and is con- 
verted into the astrosphere. At first the rays are very short, 
and all start from a central point, but gradually they lengthen 
out, and a finely granulated central mass makes its appear- 
ance, in the midst of which lies a single centrosome. As the 
astrosphere grows the granular central mass at first becomes 
reticulate, and in this condition the division of the astrosphere 
takes place, Fig. 5, but finally the network disappears, leaving 
a clear homogeneous central mass. The centrosome is of 
extreme minuteness. Figs. 2, 38, and 4 show the above stages. 
The sperm head consists of a mass of chromatin enclosed in 
a loose membrane. At first cone-shaped, it gradually becomes 
more irregular in contour, till it appears as a roundish or oval 
lump, and the membrane so closely approximated as to become 
indistinguishable. As regards the so-called ‘fusion’ I have 
nothing to add to Wilson’s account. The astrosphere divides 
into two about the same time as the sperm head comes iuto 
contact with the egg-nucleus, which has by this time taken 
