RATA. 329 
Development.—The skate lays its eggs in the autumn and the 
young are hatched in early spring. 
The eggs are large yellow spheres which break away from the ovary 
into the abdominal cavity. Thence they pass into the Fallopian tubes 
by their internal openings. The male skate is said'to thrust the claspers 
into the cloaca and the base of the oviducts of the female, and to, dis- 
charge sperms down the grooves of the claspers into the oviduct. The 
sperms then appear to pass up the oviduct and to fertilise the egg 
in the Fallopian tube. After fertilisation the egg passes down to the 
oviducal gland in which is secreted an enveloping egg-case or ‘‘ purse.” 
The eggs contained in these purses are deposited two at a time in 
moderately deep water, usually amongst dark seaweed. The ‘‘ purse” 
is of a tough consistency and a dark greenish-black colour. It is 
flattened and has long processes at the four corners. The ‘‘ purse” has 
the edges of its two walls at one end lying loosely against each other, 
allowing free egress but making ingress impossible. In this purse the 
egg develops slowly, and the young skate on emergence is practically 
a diminutive adult. During all this period it is sustained by the maternal 
“‘ yolk,” hence the skate has a purely embryonic development and only 
a lecithal type of nutrition (see page 427). 
Segmentation.—The segmentation is meroblastic, z.¢., the proto- 
plasm is largely aggregated to one pole of the large egg, and there segments 
or divides into a multicellular disc or cap called the blastoderm. Therest 
of the protoplasm with few nuclei is scattered throughout the yolk. 
These nuclei divide and are gradually added to the blastoderm during 
development. At completion of segmentation the blastoderm has an 
outer layer or epithelium of cells which represents the efzd/ast and an 
inner mass which, with the rest of the egg, forms the hyfob/ast. 
Gastrulation.—One part of the rim of the blastoderm can soon 
be distinguished by its greater thickness and is called the embryonic 
vim. This represents the future hind end of the embryo, and immedi- 
ately below it the blastoderm-cells commence to be invaginated, forming 
an archenteron. Hence this rim is comparable to the dorsal edge of 
the blastopore in Amphioxus. 
Two separate processes now take place contemporaneously. Firstly, 
the whole blastoderm commences to envelop the lower yolk-cells by 
increase of cells at the rim, partly by cells added from the yolk-mass, 
and partly by division of the blastoderm-cells. This enveloping process 
does not take place equally all round the edge of the blastoderm or the 
last point of meeting would be the lower pole, but the embryonic rim 
does not progress over the yolk ; hence the rest of the rim grows over, 
and the whole rim gradually closes in immediately behind the blastopore. 
If it be recollected that the edge of the blastoderm is the line of 
junction of the epiblast and the hypoblast, it is clear that the growth 
of the former over the yolk-mass is a modified and retarded form of 
archiblastic invagination, which is called epzbolic. 
‘The process is so slow that at the same time the embryo becomes 
differentiated in the middle: line forwards’ from the embryonte rim. 
The nervous system arises along this region as a median dorsal 
medullary groove which, by the upgrowth and meeting above of its 
edges.or medullary folds, becomes converted into a complete tube. 
