MATURATION OF OVUM AND DEVELOPMENT OF ALLOPORA. 583 
the main features in the histology of the general canal system. 
The general arrangement of the canals is very similar to that 
described by Moseley in A. profunda. “The ccenosarcal 
canals form a fine superficial reticulation at the surface of the 
coral beneath the surface layer and spring from a deeper 
meshwork of larger canals which, as in the Stylaster already 
described, have a mainly longitudinal course within the thick- 
ness of the walls of the pore system . . . “i 
The canals vary cousiderably in size, the ares ones being 
about ‘05 mm. in diameter, the smaller ones ‘(02 mm. in 
diameter. In young branches the soft or fleshy parts fit very 
accurately into the corresponding channels in the calcareous 
cenosteum. The soft parts are composed externally of a thin 
ectoderm and internally of a thick endoderm. The two layers 
are separated from one another by a thin homogeneous 
mesoglea. The ectoderm (Pl. XX XVIII, fig. 1) isa thin sheet 
of tissue divided into cell areas by fairly well-marked limits, 
each area provided with a spherical, oval, or flat nucleus. 
Both in the cell substance and nuclei a protoplasmic sponge- 
work may be clearly seen in well-stained preparations. The 
endoderm is practically a solid or tubular rod of cell substance 
bearing at intervals large spherical nuclei. I have not yet 
been able to distinguish in any of my preparations any traces 
of the division of this cell substance into cellular areas. The 
endoderm bears a very well-marked protoplasmic spongework, 
and the nuclei a chromatin spongework with nodal thickenings. 
The mesoglea varies in thickness in different places ; it always 
stains deeply and never exhibits anything but a clear homo- 
geneous structure. The young ova, whose early history I 
shall describe more fully under section 3, cause a swelling or 
protrusion of the endoderm. Now, as I have previously pointed 
out that the fleshy canals of young branches of Allopora fit 
accurately into the canals of the skeleton, the swelling of the 
canals must be accompanied by a process of absorption of the 
skeleton in the immediate neighbourhood of the ovum. This 
process of absorption continues until the large cavity is formed 
called the ampulla, in which the embryo lies before it is dis- 
