MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 581 
time. Fertilisation was not observed, but it seems probable 
that it takes place at this stage (Pl. XX XVIII, figs. 10, 11). 
6. The germinal vesicle, or perhaps it ought to be called the 
oosperm nucleus, next withdraws from the chorion, and is then 
seen to be hemispherical on its proximal aspect but irregular, 
and provided with several amoeboid, finger-like processes on its 
distal side (Pl. XX XVIII, fig. 12). 
7. The membrane surrounding the oosperm nucleus gradually 
disappears, and it becomes impossible to distinguish its proto- 
plasm from the general protoplasm of the ovum. At the time 
of the disappearance of the oosperm nucleus certain changes 
may be observed in the character and arrangement of the yolk- 
globules, the most important of them being the distribution of 
a number of small globules in the peripheral region of the 
distal hemisphere of the ovum (Pl. XX XVIII, fig. 13). 
8. In the next stage one or two irregular lumps of nuclear 
protoplasm may be found in the distal hemisphere of the 
ovum, and in their neighbourhood a few spherical, oval, or 
dumb-bell shaped fragments of the same material(Pl. XXX VIII, 
fig. 14). 
9. A little later the irregular lumps of nuclear protoplasm 
disappear and the smaller spherical ones increase in number. 
All stages in the division of these smaller nuclear frag- 
ments may be observed, and it is very probable that their 
multiplication in this manner is very rapid. In ova of this 
stage vacuolation of the protoplasm usually takes place and 
the larger yolk-globules become irregular in shape and show 
signs of splitting into smaller ones. The lumina of the tro- 
phodise are at this stage quite obliterated and the organ dimin- 
ishes in actual size. A little later it degenerates into a mere 
syncytium crowded with nuclei (Pl. XX XVIII, fig. 15). 
10. As the vacuolation of the ovum proceeds, a thin mem- 
brane of clear protoplasm containing only a few small yolk- 
spherules may be seen to separate from the distal periphery 
of the young embryo. Into this a number of the nuclear 
fragments wander and arrange themselves side by side. 
(Pl. XXXVIITT, fig. 18). This is the primitive ectoderm. 
VOL, XXX, PART 4,—NEW SER. PP 
