LACERTA 113 



sperms are acted on by a secretion, as a result of which they 

 complete their development and acquire the power of individual 

 movement. The Wolffian duct is the vas deferens leading to 

 the cloaca. As already mentioned, the wall of the cloaca 

 bears two eversible copulatory organs, for since the egg is 

 surrounded by a shell when it leaves the oviduct, it is obvious 

 that fertilisation must take place in the oviduct itself. 



Nervous System. — The brain is built on the same plan as 

 that oL Triton, but it shows an advance in the increased size 

 of the cerebral hemispheres. In these there is a small amount 

 of superficial grey matter or cortex ; in lower forms the grey 

 matter is almost entirely within the white. This is a very 

 important advance from the point of view of the evolution of 

 the human brain. The sides of the telecephalon (corpus 

 striatum) and of the diencephalon (thalamus) are enlarged, and 

 the cavities of the 1st, 2nd, and 3rd ventricles are consequently 

 reduced. There is a well-developed pineal eye, arising from the 

 diencephalon, and connected with the right habenular ganglion. 

 The cranial nerves are similar to those of Triton except for 

 the fact that the hypoglossus (12th nerve) is included among 

 them, and that there is a spinal accessory nerve (nth nerve). 

 The spinal accessory supplies the dorsal muscles of the shoulder 

 girdle, and represents a specialised portion of the vagus of 

 lower forms. 



Sense-organs. — The lateral line sense-organs are no longer 

 present, and the sole representative of the system to which 

 they belong is the ear. In addition to being an organ of balance, 

 the ear is also stimulated by vibrations in air, or sound. (This 

 is also the case in Amphibia such as the frog, but not so typically 

 in Triton in which the ear is degenerate (see p. 100).) The 

 vibrations impinge on the ear-drum (tympanic membrane) 

 and are communicated to the columella auris (hyomandibula) ; 

 the latter conveys the vibrations across the cavity of the 

 middle-ear or tympanic cavity (spiracular visceral cleft) to 

 the fenestra ovalis in the wall of the auditory capsule. The 

 auditory capsule contains the auditory vesicle ; between the 

 latter and the wall of the capsule is a fluid called perilymph, 

 while the auditory vesicle itself contains endolymph. The 



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