MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 597 
Fie. 2.—A small portion of the canal system of Allopora, showing a young 
ovum lying in the endoderm. The ovum is not surrounded by any trace of a 
vitelline membrane, and the structure of its periphery is very similar in 
character to that of the endoderm. It is consequently very difficult to 
distinguish precisely the exact outline of the ovum at this stage. In the 
neighbourhood of the germinal vesicle the protoplasm of the ovum is rather 
more cloudy in appearance, and the meshwork it contains is finer and closer in 
texture than it is in the endoderm. The germinal vesicle is large, and con- 
tains a sparse chromatin meshwork and a large germinal spot. It is sur- 
rounded by a fairly well-defined membrane. At J. a large nucleus may be 
seen in the endoderm. It is possible that this is the nucleus of a still younger 
ovi-cell, the outline of which cannot be distinguished. 
Fig. 3.—A young ovum lying in the degenerated syncytium of the tropho- 
dise of an embryo. 
Fie. 4.—A sinall portion of a branch of Allopora, showing a young ovum 
and trophodise growing in the ampulla (sp.) formerly occupied by an embryo. 
ap. Aperture by which the embryo escaped. In the walls of the trophodisc, 
and in a large canal lying immediately below it, may be seen young ova 
(ov?. ov*.). 
Fic. 5.—The young trophodisc, the ovum, and a part of the subjacent canals 
of the last figure more highly magnified. The young ovum in the trophodise 
may be seen in this figure to be surrounded by a chorion (ch.) of ectoderm 
mesogloea, and endoderm continuous at z, with the ectoderm mesoglea and 
endoderm of the trophodise. 
Fic. 6.—A stage rather more advanced than that shown in Fig. 5. The 
trophodisc is more involved, and the substance of the ovum filled with small 
spherical yolk-globules. 
Fie. 7.—A stage still more advanced than that shown in Fig. 6. The 
germinal vesicle is irregularly oval in shape, and is situated not centrally, as 
in the earlier stages, but close to the distal pole of the ovum. It is surrounded 
by numerous small yolk-spherules. The trophodise is a flattened disc-shaped 
structure, and the lumina of its folds are almost obliterated. 
Fic. 8.—The germinal vesicle of the ovum represented in Fig. 7, more 
highly magnified, to show the large germinal spot, the chromatin meshwork 
with its nodal thickenings, and the amceboid process on the distal side. 
Fic. 9.—Germinal spot of the same, still more highly magnified. 
Fie. 10.—The germinal vesicle and a small portion of the substance of the 
ovum surrounding it, at the time when the polar bodies are being thrown out. 
Fie. 11.—Stage in which the germinal vesicle, after the polar bodies have 
been thrown out, becomes flattened on the distal pole of the ovum. 
Fic. 12.—Later stage of the germinal vesicle (oosperm nucleus) when it 
retires from the distal pole of the ovum into the yolk again. 
Fic. 13.—Stage in the development of Allopora when the germinal vesicle 
VOL. XXX, PART 4,.—NEW SER. QQ 
