198 MEAD. [VoL. XIV. 
gradually evanesce. The two arise separately, sometimes near 
together, sometimes far apart. 
When one of these living primary asters is examined under 
a high power, the yolk-granules can be seen to move away from 
the centre with a trembling, vibratory motion. Later the areas 
free from yolk extend to the region between the two asters, 
which represents the central spindle, and within about ten 
minutes after the egg is placed in sea-water the whole amphi- 
aster migrates to its definitive position at the periphery of the 
ege. 
The changes in the contour of the germinal vesicle and its 
final disappearance can also be followed in the living egg. All 
these phenomena afford a complete confirmation of the results 
already obtained from the study of preserved material. 
(e) Fertilization. 
The spermatozoon of Chzetopterus has a bullet-shaped head 
and a long vibratile tail. When first teased out into sea-water, 
it remains motionless for a few minutes, but soon becomes 
extremely active. Apparently it may penetrate the egg at any 
point on the surface, though it usually enters nearer the vege- 
tative pole. Polyspermy is rare. The entrance of the sper- 
matozoon initiates profound changes in all parts of the egg. 
The latent activity of the maturation-amphiaster is revived ; 
the polar globules and the female pronucleus are formed while 
the sperm is but a very minute and inconspicuous body in a 
distant portion of the egg. 
Other changes are begun in the vicinity of the spermatozo6én 
itself. After it has penetrated a little distance, a diminutive 
aster with two centrosomes lying close together and surrounded 
by a minute centrosphere may be seen near it (Fig. 19). These 
two centrosomes are the sperm-centrosomes, though in Che- 
topterus I am not sure that they are actually carried in by the 
spermatozoon. However this may be, the sperm-centrosomes 
separate as the head of the spermatozoon enlarges to form the 
male pronucleus, and, as they separate, the rays diverging from 
them become more and more extensive (Figs. 21, 22, 25, 30, 
31, 34, 36-38). 
