548 PISCES—FISHES. 
dinal sinus, moored by a fold of peritoneum. In young 
skates they are like the young testes, but in the adults they 
are covered with large Graafian follicles, each containing an 
ovum. The ripe ova burst into the body cavity, and enter 
the single aperture of the oviducts, which are united an- 
teriorly just behind the heart. About the middle of each 
oviduct there is a large oviducal gland, which secretes the 
horny “purse”; the elastic lower portions open into the 
cloaca. 
_ Development.— The ripe 
ovum which bursts from the 
ovary is a large sphere, 
mostly of yolk, with the for- 
mative protoplasm  concen- 
trated at one pole. 
The formation of polar 
bodies (maturation) takes 
place at an early stage. 
Fertilisation occurs in the 
upper part of the oviduct. 
Some observers have de- 
scribed the occurrence of 
polyspermy. 
As the ovum descends farther, it 
Fic.290.—Elasmobranch develop- 
ment.—After Balfour. 
Uppermost figure shows blastoderm at 
an early stage. £f., Epiblast; sg.c., 
segmentation cavity; ., yolk-nuclei. 
Middle figure shows the invagination 
which forms the gut. .x., Blastopore ; 
&-, archenteron. Mesoderm dark. 
Lowest figure, a longitudinal section at 
a later stage. Zf., epiblast; ~.c., 
neural canal; #e.c., neurenteric canal ; 
is surrounded first by albuminous 
material, and then by the four- 
cornered ‘‘mermaid’s purse” se- 
creted by the walls of the oviducal 
gland. This purse is composed of 
&-y gut; #., notochord. Mesoderm 
keratin—a common skeletal sub- 
ark. 
stance which occurs for instance 
in hair and nails. Its corners 
are produced into long elastic tendrils, which may twine round’ 
seaweed, and thus moor the egg. Rocked by the waves, the 
embryo develops, and the young skate leaves the purse at one end. 
Development is very slow, and takes perhaps the greater part of a year. 
The egg-case of some sharks, e.g. the Port Jackson shark (Cestracton 
philippz), has elastic spiral fringes, and is found securely wedged among 
the rocks ; that of a neighbour species (C. ga/eatzs) has reduced spirals. 
ending in a couple of tendrils, which may be go in. in length, and 
serve very effectively to entangle the egg among seaweed. 
The segmentation is meroblastic, being confined to the 
disc of formative protoplasm. From the edge of the 
