156 THE LATER STAGES OF DEVELOPMENT 



nervous system. The size of the spinal ganglia depends to a 

 great extent upon the size of the peripheral area with which 

 they are connected. Increase of this area, e.g. by the implanta- 

 tion of a supernumerary limb in the flank, causes an increase 

 in the number of sensory neurones in these ganglia, and there- 

 by an increase in their size (Detwiler, 1920). If, on the other 

 hand, no skin is present on one side of the body, as in the case 

 of two amphibian embryos fused side by side, the ganglia on 

 this side remain very small (Detwiler, 1926). The number of 

 cells in the ventral half of the spinal cord, that will differentiate 

 into motor neurones, also varies with the size of the innervated 

 peripheral area (Hamburger and Keefe, 1944). The nerve fibres 

 growing out from the cells seem to play a role here, but the 

 effect is not dependent on the normal conduction of nervous 

 impulses. If amphibians are kept under permanent anesthesia, 

 their development proceeds normally, including that of the 

 nervous system. 



The growth processes in the brain are partly governed by 

 stimulation by the ingrowing nerves from the large sense organs 

 of the head. If these sense organs are extirpated at an early 

 stage, the brain centres with which they are normally connect- 

 ed do not attain their normal size. Conversely, the implantation 

 of a supernumerary eye or olfactory pit will cause an over- 

 development of certain parts of the brain, if the nerve coming 

 from the graft penetrates into the brain of the host. 



The growth of peripheral nerves towards their end organs 

 seems to be governed by attractions exerted on the nerves by 

 strongly growing organs. A limb transplanted into the flank 

 will establish connections with any nerve that happens to be in 

 the neighbourhood, even though normally this nerve has no- 

 thing to do with limb innervation. The nerve fibres enter into 

 the limb and become connected with its muscles, so that in 

 many cases the latter will function more or less normally 

 (Detwiler, Weiss). An eye or an olfactory pit, transplanted 

 into the flank, also attracts the neighbouring spinal nerves 

 (Detwiler, 1927), though in this case no functional connections 

 between nerve and organ are formed. 



Once the connections of the nervous system with the various 



