THE TRACTS 267 



The animal, being now elongated along one axis, develops 

 at one end (the front end) epiblastic cells speciaUsed to 

 receive stimuh from a distance — hght, smell and, later, 

 sound-waves. In this way it is enabled to explore new 

 territory before moving into it. The information gained 

 from these sensory cells largely determines the reaction of 

 the organism, the rest of the body becoming subservient 

 to the advancing end. With the greater responsibihty 

 thrown upon this region, the neurones belonging to it 

 undergo considerable increase in number and complexity — 

 in this way the cerebrum is formed. 



But in addition to knowledge of the external world, the 

 animal requires information regarding its own position. In 

 different parts of the body special cells are developed to 

 be excited by position and by change of position. These 

 impulses converge upon masses of nerve-cells lying behind 

 the cerebrum and forming the cerebellum. 



THE TRACTS OF THE CENTRAL NERVOUS SYSTEM 



The following methods have been employed for tracing 

 the course of fibres within the central nervous system :■ — • 



1. Fleschig's Myelination Method. — This depends upon 

 the fact that in different tracts the axis cylinders acquire 

 their myelin sheaths at different stages of embryonic 

 development. 



2. Wallerian Degeneration. — The histological changes 

 which follow the separation of a nerve-fibre from its cell- 

 body have already been noted. In about three weeks 

 after section, the myehn is converted into a simple fat 

 which can be stained with osmic acid (Marchi's method). 



3. Successive Degeneration. — This is a modification of the 

 above method. Fig. 42 represents a longitudinal section 

 of the cord, A, B, C and D being the segments. It is 

 desired to find out what descending neurones arise in the 

 segment B. The cord is transected between A and B and 

 several months allowed to elapse, so that all fibres arising 



