810 



THE SERVE 



Development of Nerve Tissue. 1 . In the Wall of the Neural Tube. The single layer of 

 nucleated epithelial cells of ectodermal origin which makes up the wall of the neural tube early 

 becomes modified into a layer of tall columnar cells called spongioblasts (Fig. 579). Their 

 protoplasmic ends undergo differentiation in that the central ends become elongated and atten- 

 uated or collapsed to form a series of striated pillars 

 with intervening spaces. The central ends retain 

 their breadth, however, and form an internal limiting 

 membrane. The ectal ends undergo differentiation 

 to form a spongv felt-work or reticulum (pcripht ral 

 rril of His) ; eventually these spongioblasts become 

 (a) ciliated ependymal cells and (6) neuroglia. 



In the intercellular spaces of the central zone there 

 appear spherical cells of different structure and 

 density. These are the germinal cells, seen in very 

 early stages and proliferating rapidly by karyokine- 

 sis. They soon lose their spherical form, becoming 

 pear-shaped as a protoplasmic process extends 

 ectad. These pear-shaped cells are now termed OLL 

 neuroblasts (Fig. 579), the protons of the neurones, 

 and as development advances they leave the central 

 zone and migrate into the marginal reticulum to 

 the positions in which they are found in the gray 

 substance of the brain and spinal cord. The proto- MYELI 

 plasmic process is at first slightly bulbous and elon- 



cell. 



Xenroblast. 



Nuclei of 

 spongioblasts. 



j \-Myelospongium 

 network. 



Fio. 579. Transverse section of the spinal cord of a 

 human embryo at the beginning of the fourth week. 

 Top of figure corresponds to lining of central canal 

 (After His.) 



FIG. 580. Scheme of central motor neurone. 

 (I. type of Gol^i.) The motor cell body, together 

 with all its protoplasmic processes, its axis-cylinder 

 process, collaterals, and end ramifications, repre- 

 sent parts of a single cell or neurone, a.h. Axone- 

 hillock devoid of Nissl bodies, and showing fibril- 

 ation. c. Cytoplasm showing Nissl bodies ami 

 lighter ground substance, n'. Nucleolus. (Barker. 



gates to form the axone extending toward other nerve-cells or to the peripheral tissue elements 

 with which they become associated by the contiguity of the terminal arborizations into which the 

 bulbous extremity develops. The precision with which the axones travel toward their allotted 

 goal is one of the most remarkable manifestations of organic development. An American experi- 

 menter, Ross G. Harrison, has devised a method for directly observing the living, growing 

 nerve. In isolated pieces of frog embryos the differentiation of the living nerve elements could 

 be observed from day to day during several weeks. The bulbous end of the outflowing pro- 

 toplasmic fibre, showing a faint fibrillation, was seen to reveal a continuous change of form 

 particularly in a number of fine simple and branched filaments which were in constant ameboid 



