22 Professor W. Baldivin Spencer [Jan. 28, 



WEEKLY EVENING MEETING, 



Friday, January 28, 1887. 



Sir John Lubbock, Bart. M.P. D.C.L. LL.D. F.R.S. 



Vice-President, in the Chair. 



Professor W. Baldwin Spencer. 



TJie Pineal Eye in Lizards. 



There is present in the human brain, buried deeply beneath the 

 surface and hence concealed from view externally, a small blunt 

 process : it differs in nature from the surrounding nervous matter, 

 being hard to the touch. To this small process human anatomists 

 gave the name of pineal gland, and its meaning has always remained 

 a problem. Investigation of the brains of other animals showed that 

 in these the structure was present : in fact it is typical of the brains 

 of all true vertebrata, and not only this, the lower we descend in the 

 scale of vertebrate life, the more highly developed does it become in 

 comparison to the remaining parts of the brain. In such a mammal 

 as a rabbit it is, for example, larger than in man ; in a bird it is still 

 more highly developed ; whilst when we come down to fishes, the 

 pineal gland or epiphysis, as it is better called, assumes the form of 

 a forwardly directed hollow process whose distal extremity is swollen 

 out into a vesicle, the proximal part forming a hollow stalk running 

 back to the roof of the brain. If the brain of a lizard, such as 

 Hatteria, be examined, the epiphysis is seen to have undergone a very 

 definite change : it is divided more clearly than in other animals into 

 the two divisions above mentioned ; sections, however, show that the 

 stalk is solid and that the vesicle has become developed into an 

 organ of vision — into a pineal eye. This is the highest stage of 

 development reached by the epii)hysis, and is now preserved, so far as 

 is at present known, in lizards only amongst living animals. 



Turning to the development of the brain : it is seen in early 

 stages in all vertebrates to have the form of a simple tube running 

 the whole length of the back of the animal ; a little later the walls of 

 the anterior division have bulged out and given rise to three vesicles 

 which are known as the fore, mid, and hind brains; the part of the 

 tube posterior to these forms the spinal cord. As development 

 proceeds, from the fore brain on either side is given off a hollow 

 process, each one of which forms an optic vesicle, and at the same 

 time there grow forward two outgrowths of the fore brain which give 

 rise to the cerebral hemispheres ; in addition to these structures, the 

 roof of the fore brain gives off a single median outgrowth which grows 

 forward within the skull cavity, swelling out distally into a small 



