DEVELOPMENT OF SHARKS AND RATS. 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 vertebrae 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 vertebrae 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 Chimcera. 



Order 1. Plagiostomi, In the sharks and skates the teeth 

 are very numerous ; the gill-slits are uncovered. 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 



