SPINAL CORD. 



d.m.s. 



d.r. 



from the spinal ganglia enter its dorsal portion and grow up and down the 

 medullary tube through its meshes, thus forming the oval bundles. Mean- 

 while the nucleated layer becomes divisible into two portions, a thick 

 ependymal layer composed of undifferentiated cells around the central 

 canal; and a gray layer [mantle layer] composed of cells which have 

 moved outward and become partly differentiated. The gray layer is at 

 first triangular, being thick ventrally and narrow dorsally. It consists of 

 two sorts of cells, the neuroglia cells (glia cells), which are the cells of the 

 protoplasmic syncytium; and the nerve cells (in their young stage, called 

 neuroblasts}, which are imbedded in the neuroglia network and send out 

 processes to ramify among its meshes. The neuraxons of the motor 

 cells grow out from neuroblasts in the ventro-lateral part of the gray layer; 

 after crossing the white layer, they 

 pass out of the medullary tube as 

 fibers of the ventral roots. This 

 stage of development is shown in 

 Fig. 109, E. Blood vessels are seen 

 growing into the tube under the 

 dorsal roots and near the ventro- 

 median line. They carry some 

 connective tissue cells with them, 

 to mingle with the neuroblasts and 

 neuroglia, both of which are ecto- 

 dermal. 



Fig. 139 represents a later stage 

 in which the form of the adult cord 

 is clearly suggested. The walls of 

 the dorsal portion of the central 

 canal have fused and disappeared 

 so that the canal is reduced in size. 



It is surrounded by an ependymal layer which is becoming thinner, since its 

 cells are being added to the gray layer faster than they are replaced by 

 mitosis of the inner cells. The gray layer in the preceding stage showed 

 two ventral protuberances, one on each side. These extend the length 

 of the cord and are known as the ventral columns [horns]. In the present 

 stage in addition to these, there are two dorsal columns [ horns] which have 

 been formed by the dorsal proliferation of the ependymal layer. As a 

 whole the gray is shaped like an H. That portion which extends from 

 side to side beneath the central canal is the ventral gray commissure. The 

 white layer has become wider. Its neuroglia network has a predominant 

 radial arrangement. Nuclei are found in its strands of neuroglia which 



FIG. 139. SPINAL CORD OP A RABBIT EMBRYO 

 OF 20 DAYS. 



c. c., Central canal; d. c., dorsal column; d. m. s., 

 dorsal median sulcus; d. r., dorsal root; ep., 

 ependymal layer ; v. c., ventral column ; v. g. c., 

 ventral gray commissure; v. m. f., ventral 

 median fissure; v. r., ventral root; v. w. c., 

 ventral white commissure; w. 1., white layer 

 (lateral funiculus). 



