388 



THE FURTHER DIFFERENTIATION OF THE 



The discovery of the factors controlling neuron-proliferation is of 

 theoretical interest from two further points of view. In the first 

 place, it is clear why the brain differs in form from the spinal cord. 

 The anterior end of the body is occupied by the organs of special 

 sense, from which an enormous number of fibres enters the neural 

 tube. These fibres induce the multiplication of neurons where they 



The effect of eye-extirpation on the development of the mid-brain. Transverse 

 section through the mid-brain of a larva of Rana fusca from which at an early 

 stage an eye was removed. The roof of the mid-brain on the operated side is 

 markedly under-developed (left half of figure), as may be seen by comparing it 

 with that of a normal control (right side of figure) ; the outer molecular layer (2) 

 and the stratum medullare superficiale (6) are absent on the operated side, (From 

 Diirken, Biol. Gen. vi, 1930,) 



end, and the large numbers of neurons so formed find expression 

 in the bulges and prominences familiar as the optic lobes, restiform 

 bodies, and olfactory lobes, which differentiate the brain morpho- 

 logically from the spinal cord. It has been shown that the grafting 

 of an extra eye or an extra nasal pit on the head results in an in- 

 growth of fibres from the graft to the brain, and an increase in the 

 number of neurons in the brain at that point. ^ Conversely, the 



^ May and Detwiler, 1925; May, 1927. 



