MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 587 
complete series of sections with the camera lucida, and com- 
paring them with one another. 
When the ovum is mature the cavities of the pouches of the 
trophodise become obliterated, and soon afterwards their out- 
lines become obscure, and the whole structure degenerates into 
a multinucleate mass of endoderm (Pl. XX XVIII, fig.14). In 
its earlier stages the ovum is enclosed in a complete chorion 
composed of ectoderm, mesogloea, and endoderm, each of these 
tissues being directly continuous with the same tissues of the 
canal in which the ovum first began to grow. As the egg in- 
creases in size these membranes become much thinner, but they 
may always be distinguished on the distal side of the egg, even 
when it is fully grown (P]. XXXVIIL, fig. 18). On the proximal 
side, however, the ectoderm disappears from the area covered 
by the trophodisc as soon as the latter comes in contact with 
it, and at the same time the ectoderm of the outer walls of the 
cup of the trophodisc becomes continuous with the ectoderm 
covering the distal side of the ovum. In the later stages then 
the ovum and trophodise are completely enclosed in a con- 
tinuous ectodermal sheath. 
When the embryo escapes to the exterior the ampulla in 
which it developed is lined internally by a sheath of ectoderm, 
mesogloea, and endoderm, and on the proximal side of it, i. e. 
the side nearest to the central axis of the branch, there is an 
endodermal syncytium—the degenerated trophodisc—in which 
new young ova are frequently to be found. 
III. The Changes of the Germinal Vesicle. 
For a long time I was unable to discover any of the ova of 
Allopora younger than those already sunk in their diverticula. 
My attention was at last directed, however, to certain large 
nuclei occasionally to be seen in the endoderm of the canals, 
more especially in canals in the neighbourhood of already esta- 
blished trophodises and ova. These nuclei can be distin- 
guished from ordinary endodermal nuclei, not only by their 
large size, but also by their transparency, their large nucleolus, 
