268 COMPARATIVE ANATOMY 



of the peritoneal cavity. At first the genital ridges, as the 

 primordia of the generative organs are called, are nothing 

 more than modified tracts of the ccelomic epithelium which, 

 elsewhere flattened, here becomes a columnar epithelium. 

 Presently the columnar cells increase by division, and become 

 several cells thick ; a mesoblastic thickening is formed below 

 each ridge, and they project into the ccelomic cavity, but it 

 is not till the time of the metamorphosis of the tadpole into 

 the frog that any distinction of sex can be detected. The 

 further development of the ovaries and testes has been de- 

 scribed in vol. i. p. 1 1 6. 



In the tadpole, up to the time of the appearance of the 

 limbs, the axial skeleton is only represented by the notochord, 

 which consists, as in Amphioxus and the dogfish, of an elastic 

 rod composed of vacuolated cells. At about the time that 

 the rudiments of the hind limbs make their appearance the 

 notochord is invested by a continuous cellular sheath, and a 

 series of cartilaginous arches, the rudiments of the neural 

 arches of the vertebrae, are formed in the mesoblastic tissue 

 investing the spinal cord and rest upon the vertebral regions 

 of the notochordal sheath, which has meanwhile become 

 segmented into a series of nine rings followed by a posterior 

 unsegmented portion, the urostyle. Shortly after the meta- 

 morphosis the nine rings in question are surrounded by as 

 many thin bony rings, each of which is slightly constricted in 

 its centre like a napkin ring. These are the true vertebral 

 centra, and at the time of their first appearance they are 

 amphiccelous, like the centra of a fish. But this condition 

 does not persist long. An annular thickening of cartilage is 

 formed from the notochordal sheath, in each intervertebral 

 region, and this thickening extends inwards till it finally 

 obliterates the notochord intervertebrally. The intervertebral 

 thickenings are then divided into anterior and posterior 

 portions which unite with and form the articular surfaces of 

 two contiguous vertebral centra. The articular surfaces are 

 subsequently ossified, and the original ring-shaped bone form- 

 ing each centrum is greatly thickened, but a portion of the 

 notochord remains and persists throughout life in the middle 

 of each centrum. 



The cranium is formed in the tadpole at an earlier stage 

 than the vertebrae. It is at first entirely cartilaginous and 



