NORMAL HISTOLOGY. 



the peculiarity of being stained yellowish brown by solutions containing 

 chromic acid or chromium salts. In recognition of this affinity, these ele- 

 ments are known as chromaffine cells and regarded as related to the sympa- 

 thetic system. Definite collections of such cells, associated with a complex 

 of blood-vessels along the course of large arteries, occur in the carotid and 

 aortic bodies, as well as within the medulla of the suprarenal body. 



DEVELOPMENT OF THE NERVOUS TISSUES. 



Although the reader must be referred to the larger or special books for a syste- 

 matic and detailed description of the development of the nervous system, an under- 

 standing of the chief features of its histogenesis is so important for an appreciation of 



Amniotic sac 



Closing neural canal 



Ectode 



Mesoderm 



Body-ca 



Body-cavity 



Visceral mesoderm Entoderm Chorda Open gut-tube 



Splanchnopleura 



FIG. 103. Transverse section of rabbit embryo of about nine and one-quarter days. X 80. Neural 



canal is just closing. 



the relations of its structural components, that a sketch of these processes finds here 

 an appropriate place. 



Among the very earliest phases of the embryo is the formation of a longitudinal 

 furrow, the neural groove, bounded by thickened ectoderm and corresponding with 



the long axis of the embryo. By the approxi- 

 mation and fusion of its dorsal lips, this groove 

 is gradually converted into a closed tube, the 

 neural canal. The walls of this canal, from 

 which all the essential nervous elements are 

 derived, consist at first of only a few layers 

 of the invaginated ectodermic epithelial cells. 

 The latter actively proliferate and become con- 

 verted into a multinucleated tissue in which 

 the cell-boundaries disappear and the nuclei 

 lie embedded within a general protoplasmic 

 tract or syncytium. The larger dividing ele- 

 ments, the germinal cells, conspicuous on 

 account of the mitotic figures, lie close to- 

 the lumen of the tube. Soon this continuity is 

 interrupted by the appearance of spaces within 

 the syncytium, the cell-substance being re- 

 solved into a delicate reticulum, the myelo- 

 spongium. The meshes of the reticulum 

 enlarge, the intervening nucleated tracts elon- 

 gate, and the increasing nuclei become radially disposed. Following these changes, 

 the elements next the lumen assume a columnar form and radial arrangement and 

 become the primary ependymal cells, while the remaining elements, the indifferent 



Urn 



FIG. 104. Segment from lateral wall of 

 neural tube of pig embryo of 5 mm.; syncyt- 

 ium replacing distinctly outlined cells, a, 

 inner zone ; g, germinal cells ; ilm, internal 

 limiting membrane; m, peripheral zone; r, 

 radial strands of cytoplasm. X 690. (Har- 

 desty.) 



