ON FECUNDATION, MATURATION, AND FERTILISATION. 3821 
withdraws somewhat from the periphery of the egg, though 
still maintaining an excentric position. It should here be 
remarked that throughout the whole process of maturation 
there is no sign of a centrosome or archoplasmic sphere. 
As is shown, however, in fig. 9 c, at either end of the spindle 
is a deeply-stained body, which may be called a pseudocentro- 
some. ‘This is nothing more than the point where the slightly 
stained spindle-threads meet. Although there is no astro- 
sphere, the protoplasm at the ends of the spindle is distiactly 
modified, the reticular structure giving way to a more granu- 
lated condition. 
Fertilisation.—The spermatozoa consist of the usual 
three pieces—head, middle piece, and tail. When ripe they are 
very active, piercing through the two layers of test and follicle 
cells in a very short space of time. The egg puts out a “ cone 
d’attraction,’ which embraces the head of the spermatozoa. 
The tail drops off, and the head rotates very rapidly. In fig. 13 
the stage is drawn where the head has rotated 90°. At the 
end of the middle piece is a deeply-staining body, which may 
be the centrosome. Already the rays of the astrosphere are 
apparent. The “cone d’attraction” subsides as soon as the 
sperm-head has made its way into the ovum. The head itself 
broadens and grows rapidly, until it reaches about twice its 
original size. It then suddenly splits into two. At first regular, 
these pieces gradually take on a beaded irregular shape, and 
subsequently break up into small irregularly-shaped chromo- 
somes, usually about eight or nine in number (figs. 14,15, and 
16). These chromosomes are only transitory structures, at 
least as far as outward appearances go, for very shortly after- 
wards the male pronucleus passes completely into the resting 
condition. Meanwhile the astrosphere has grown considerably, 
and passed through the same phases of formation as described 
in the case of Spherechinus. The rays are remarkable for 
their length and thickness, the whole structure being much 
coarser than the astrosphere of Spherechinus. The centro- 
some is also very large and distinct, and soon after the centre 
of the astrosphere has become finely granular, divides into 
