810 THE NERVOUS SYSTEM 



The cells of the neuroglia proper lie within the substance of the nerve 

 tube between the neuron-cell bodies of the gray matter and also between the 

 nerve fibers of the white matter (fig. 353H). Conspicuous among the neuroglia 

 cells are the protoplasmic astrocytes (fig. 353D) which reside mainly among 

 the neurons of the gray matter and the fibrous astrocytes (fig. 353B) found 

 in the white matter. The processes of the fibrous astrocytes are longer and 

 finer than those of the protoplasmic astrocytes, and they may attach to blood 

 vessels (fig. 353B). Two other cellular types of neuroglia, the oligodendroglia 

 and the microglia cells, also are present (fig. 353C and E). The microglia 

 cells presumably are of mesodermal origin (Ranson, '39, p. 57). 



B. Basic Developmental Features 



1. The Embryonic Origin of Nervous Tissues 



The ectoderm of the late gastrula is composed of two general organ-forming 

 areas, namely, neural plate and epidermal areas (fig. 192A). Both of these 

 primitive ectodermal areas are concerned with the development of the future 

 nervous system and associated sensory structures. From the neural plate region 

 arises the primitive neural tube (Chap. 10), the basic rudiment of the central 

 nervous system, whereas the line of union between the neural plate and the 

 epidermal areas gives origin to the ganglionic or neural crest cells which con- 

 tribute much to the formation of the peripheral nervous system. As observed 

 in Chapters 9 and 10, the determination of the neural plate material and the 

 formation of the neural tube are phenomena dependent upon the inductive 

 powers of the underlying notochord and somitic mesoderm in the Amphibia. 

 Presumably the same basic conditions obtain in other vertebrate embryos. 



Fig. 353. Structure of the developing neural tube. (A) Ciliated ependymal cells 

 from ependymal layer of the fourth ventricle of a cat. (Redrawn from Maximow and 

 Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Rubaschkin.) 

 (B-E) Various types of neuroglia cells. (Redrawn from Ranson, 1939, The Anatomy 

 of the Nervous System, Philadelphia, Saunders, after Rio Hortega.) (F) Transverse 

 section of neural tube of three-day chick embryo. The spongioblasts are stained black 

 after the method of Golgi. (Redrawn from Maximow and Bloom, 1942. See reference 

 under A, after Cajal.) (G) Transverse section of part of spinal cord of 15 mm. pig 

 embryo showing structural details. This section was constructed from several sections. 

 The part of the section to the left reveals the neuroglial support of the developing 

 neuroblasts. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (H) Transverse sec- 

 tion, constructed from sections, of part of the spinal cord of 55 mm. pig embryo 

 showing neuroglial support for developing neuron cells. (Redrawn from Hardesty, 1904. 

 Am. J. Anat.. 3.) (I) Transverse section of spinal cord of newborn mouse depicting 

 spongioblasts which are moving peripherally from the central canal. These spongioblasts 

 are in the process of transforming into stellate neuroglia cells or astrocytes. (J) 

 Transverse section of 9 mm. pig embryo portraying ependymal, mantle, and marginal 

 layers, external and internal limiting membranes, and blood vessels growing into the 

 nerve substance. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (K) Transverse 

 section of spinal cord of 20 mm. opossum embryo indicating general structure of the 

 spinal cord. Observe dorsal root of spinal nerve growing into nerve cord at the right 

 of the section. 



