8o 



THE BIOLOGY OF THE FROG 



system. This neural cavity, as we may well call it, extends 

 farther forward than the coelom, and is separated from the 

 latter by the bases or centra of the vertebrae. The anterior 



portion of the central nerv- 

 ous system, or brain, lies 

 in the skull, and is contin- 

 br ued posteriorly as the spi- 

 nal cord, which is inclosed 

 within the vertebral col- 

 umn. If we make a cross 

 section through the frog 

 somewhere near the middle, 

 we shall find that the body 

 contains two longitudinal 

 cavities separated by the 

 centra of the vertebrae, — 

 the ccelom below, and the 

 neural tube above. Around 

 both of these is a layer 

 of muscles which is much 

 thicker dorsally although it 

 completely surrounds the 

 ccelom below. And outside 

 of the muscles, from which 

 it is separated by large 



FIG. 13. — Organs in the neural cavity. 

 The anterior part of tins cavity con- 

 tains the brain, which is composed of 

 the olfactory lobes, olf. I; the cere- 

 bral hemispheres, crb. h ; the di- 

 encephalon, diet? ; the optic lobes, 

 opt. I; cerebellum, cblm ; and me- 

 dulla oblongata, med. obi. n. c, neu- 

 ral canal; sp. cd, spinal cord ending 

 in the filum terminale, f. t; e, eye. 

 (After Parker and Parker.) 



lymph spaces crossed by a few bands of connective tissue, 

 is the skin. 



With the exception of the loose attachment of the skin, 

 all the features of structure mentioned in the last paragraph 

 belong to the vertebrate animals in general. We have next 

 to see how these fundamental features of structure came to 

 be established. 



