MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 593 
at any time highly refracting granules of any kind that could 
possibly be mistaken for yolk. The germinal vesicle moreover 
shows no sign of budding until the yolk is fully formed. 
The yolk-spherules of Allopora are solely produced by the 
activity of the egg protoplasm. When they can first be clearly 
distinguished as yolk-spherules they are distributed equally 
through the protoplasm of the egg, the largest being from ‘005 
to Ol in diameter. At the tinie that the ovum is approaching 
maturity the largest yolk-spheres are on the proximal side of 
the egg. In the neighbourhood of the germinal vesicle they 
are much more numerous than elsewhere, but smaller. They 
are all perfect spheres. ‘The average size of the largest spheres 
is ‘02 mm., but occasionally one may be found with a diameter 
of 025 mm. 
As soon as the oosperm nucleus retreats from the periphery, 
some curious changes occur in the character of the yolk-spheres. 
The smallest spherules are not confined to the neighbourhood 
of the nucleus, but scattered over the periphery of the distal 
hemisphere, and the largest ones are not without exception 
perfect spheres as they were before, but some of them are oval 
or irregular in shape. Some careful measurements I made 
of the irregular yolk bodies gave me the following results: 
703 mm. x ‘02 mm., ‘(0275 mm. x ‘02 mm., ‘025 mm. x :02 
mm.; the numbers representing the longest and shortest 
diameters respectively (Pl. XX XVIII, fig. 19). The largest 
perfectly spherical yolk bodies in the same section were ‘01 
to ‘0125 in diameter. There can be little doubt from the 
appearance of the yolk bodies at this stage that a considerable 
disturbance is going on in the substance of the ovum; some 
of the yolk spheres are breaking up, others are fusing together. 
The rapid degeneration that takes place in the structure of the 
trophodisc, together with the curious changes that take place 
in the character of the yolk-spheres, suggest that at the time 
of maturation of the ovum it ceases to be nourished from with- 
out, and that the food material required for the further develop- 
ment is entirely procured by the breaking up and distribution 
of the stores (yolk bodies) already within the substance of the 
