DEVELOPMENT OF SHARKS AND RAYS. 419 
The next external change is the division of the tail-end 
into two caudal lobes. The notochord arises as a rod-like 
thickening of the third germ-layer, from which it afterwards 
entirely separates, so that the germ, if cut transversely, 
would appear somewhat as in the embryo bird. 
The primitive vertebre next arise, and about this time the 
throat becomes a closed tube. ‘The head is now formed by 
a singular flattening-out of the germ, like a spatula, while 
the medullary groove is at first entirely absent. The brain 
then forms, with its three divisions, into a fore, middle, and 
hind brain. Soon about twenty primitive vertebre arise, 
and by this time the embryo is very similar, in external 
form, to any other vertebrate embryo, and finally hatches in 
the form of the adult. 
The skate was found by Wyman to be at first long and 
narrow, the dorsal and anal fins extending to the end of the 
tail, as in the eel. Soon after it becomes shark-shaped, and 
finally assumes the skate form. Thus skates pass through 
a shark-stage, and this accords with the position in nature 
of skates, since they are, as a whole, a more specialized as 
well as more modern group than the sharks. Wyman found 
that there are in the skate at first seven branchial fissures, 
the most anterior of which is converted into the spiracle, 
which is the homologue of the Eustachian tube and the 
outer ear-canal ; the seventh is wholly closed up, no trace re- 
maining, while the five others remain permanently open. 
The Elasmobranchs are subdivided into two orders (re- 
garded by Gill as super-orders, the Plagiostomi, represented 
by the sharks and rays, and the Holocephali, the type of 
which is Chimera. 
Order 1. Plagiostom?.—In the sharks and skates the teeth 
are very numerous; the gill-slits are unovvered. The rays 
or skates differ from the sharks in their broad, flat bodies, 
with the gill-slits opening below; the great breadth of the 
body is due to the enlargement of the pectoral fins which 
are connected by cartilages to the skull ; there is likewise no 
median articular facet upon the occiput or base of the 
skull, for the first vertebra. 
The most common of our Selachians is the mackerel shark 
