ii NERVOUS SYSTEM 85 



fowl embryo) the use of appropriate methods reveals the presence of 

 neurofibrils in their protoplasm. The tail-like prolongation of the 

 neurone which forms the neurite or axon is still believed by the 

 great majority of workers to arise as an actual outgrowth of the cell 

 body as was taught by His. Others regard the appearances upon 

 which this belief is based as being probably deceptive, as will be 

 explained later. 



The longitudinal axons of the spinal cord are concentrated in its 

 outer layers forming the " white substance " of the early anatomists. 

 This makes its appearance as a rule in the more primitive Vertebrates 

 as a continuous layer and in the higher forms as a sharply separated 

 dorsal and ventral portion upon each side. 



The enclosure of the axons in the insulating medullary sheaths 

 commences only within a few days of the end of incubation, in the 

 case of the bird, and at similarly advanced stages of development in 

 other Vertebrates. The sheath is generally believed to be secreted 

 by the protoplasm of the axon. Its formation tends to take place 

 approximately synchronously in all the axons belonging to a particular 

 group. This fact, in conjunction with the use of specific stains for 

 the insulating substance, facilitates the mapping out of the various 

 groups of axons. 



The spinal cord, like the rest of the central nervous system, 

 becomes invaded during ontogeny by immigrant mesenchymatous 

 tissue. This provides the capillary network which traverses the 

 nervous tissue, in addition doubtless to many other elements of a 

 less conspicuous kind. 



A curious detail which is noticed in studying sections of develop- 

 ing spinal cord (or brain) is that the active cell-multiplication is 

 confined to the layer next the central cavity, in other words to what 

 was originally the superficial region of the ectoderm. This is in 

 striking contrast with the general ectoderm of the surface of the body 

 where cell-multiplication is confined to the deep (Malpighian) layer. 



BRAIN. The anterior portion of the neural tube becomes en- 

 larged and dilated to form the brain and this gradually becomes so 

 modelled as to present the various regions seen in the brain of the 

 adult. The general course of this process will first be sketched as it 

 occurs in Lepidosiren one of the lower gnathostomatous Vertebrates 

 in which the egg is holoblastic. 



DIFFERENTIATION OF THE MAIN REGIONS OF THE BRAIN IN 

 LEPIDOSIREN. The brain rudiment becomes apparent as a slight 

 enlargement of the neural tube. The first sign of differentiation is 

 the appearance of a constriction marking off the primitive fore-brain 

 or cerebrum (archencephalon of Kupffer) from the primitive hind- 

 brain or rhombencephalon. As development goes on this boundary 

 becomes specially marked ventrally where the floor of the brain bulges 

 upwards into the cavity as a transverse fold l (see Figs. 52 and 53,/). 



1 This fold may in other vertebrates make its appearance before the medullary 

 is covered in. This is shown clearly in Polypterus Fig. 80, B, p. 146. 



