DEVELOPMENT OF NERVOUS SYSTEM 



759 



their final forms and positions more remote from the central nervous system, the axones ter- 

 minating in them must necessarily grow and be drawn out with the structures. At need, 

 later axones are sent out by neurones developing later to supply the growth demands. Such 

 axones follow the general paths made by those already extending to the tissues requiring them. 

 Being processes of the cytoplasm of the ceU-body, the growth and life of all axones (and dendrites) 

 is under the control of the nucleus in the cell-body. They grow by absorbing nourishment, 

 or having added to them substances, from the tissue stroma through which they pass, which 

 stroma may be either ectodermal or mesodermal in origin. 



The great majority of axones in the central nervous sytem and all in the peripheral system 

 have sheaths about them. The sheath is an acquired structure and is not added till a rela- 

 tively late period of development. These sheaths are of two general varieties, sheaths con- 



FiG. 602. — Diagram illustrating the Gross Divisions op the Central Nervous System. 



- .Prosencephalon 

 (fore -brain) 



Mesencephalon 

 (mid-brain) 



' Cerebrum 



OUvary body"--|{5ry,t " 



"Cerebellum 1 



•*% Pons (Varoli) 

 Myelencephalon 

 (medulla oblongata) 



Metencephalon 

 (hind-brain) Rhomben- 

 I cephalon 



Encephalon 



(brain) 



-Pars cervicalis 



■-Pars thoracalis 



■ Pars lumbalis 



Pars sacralis or 

 "" conus meduUaris 



Spinal cord 

 (medulla spinalis) 



sisting merely of a fibrous coat with the nuclei belonging to it, and sheaths in which there has 

 been added a coating of fat or myelin, medullary sheaths. A nerve fibre consists of an axone and 

 its sheath whether medullated or non-meduUated. 



In the embryo, axones are given off fiom the developing neurones at a time when the 

 entire ectodermic neural tube and embryonic ganglia and the mesodermic tissue surrounding 

 them are each void of definite cell boundaries, each being a continuous mass of nucleated 

 protoplasm, a syncytium. From these syncytia are developed the fibrous connective tissues 

 of the later framework supporting the nervous system. Of this, the fibrous tissue, neuroglia, 

 is derived from the ectodermal syncytium, while the white and elastic fibrous tissues are derived 

 from the mesodermal or mesenchymal syncytium. Before any connective tissue fibrils are 

 developed in either syncytium, before and at the time of the ingrowth of blood-vessels into the 

 developing gangha and the neural tube from the mesenchyme about them, there occurs an 

 invasion of the me.senchymal syncytium into the ectodermal syncytium. This invasion 

 occurs both as independent ingrowths and fusions at the periphery of the neural tube and by 



