THE ECTODERM AND ITS DERIVATIVES 167 



brain (inesocoel) connects the rhonibocoel (fourth ventricle) with 

 the third ventricle which becomes narrow and is known as the aque- 

 duct of Sylvius (also the iter e tertio ad quartum ventriculum). 



The Rhombencephalon (Hindbrain). 



This portion of the brain is clearly marked off from the mesen- 

 cephalon by a transverse constriction in the roof of the brain, at the 

 posterior limit of the dorsal thickening. It is not clearly divided farther. 

 There appears a slight transverse thickening in the roof of the rhom- 

 bencephalon which corresponds to the metencephalon of higher forms 

 and develops into the small cerebellum. Posterior to this the roof be- 

 comes broad, thin, and vascular, and folds into the rhombocoel 

 (fourth ventricle) as the posterior choroid plexus. The ventral and 

 ventro-lateral walls of the rhombencephalon are known as the medulla 

 oblongata from which arise the cranial nerves V to X inclusive. The 

 walls become thickened by fibers which form numerous pathways 

 from the brain and cord. 



The rhombocoel or cavity of the hindbrain is known as the fourth 

 ventricle which communicates posteriorly with the central canal of 

 the spinal cord and anteriorly with the aqueduct of Sylvius of the 

 mesencephalon. 



The Spinal Cord 



The neural or central canal (neurocoel) from the beginning of its 

 development is laterally compressed by the thick lateral walls of the 

 spinal cord derived from the original neural folds. The lining cells, 

 coming from the dorsal epithelial ectoderm, retain both their pigment 

 and their cilia, and are non-nervous in function. These cells, which 

 continue to line the central canal of the adult, are known as the non- 

 nervous ependymal cells. The thick lateral walls of the spinal cord are 

 made up of the rapidly multiplying germinal neuroblasts (primitive 

 or embryonic precursors of the neurons) and the supporting small 

 and stellate cells known as the glia (neuroglia) cells. These cells have 

 the function normally ascribed to connective tissue, namely support 

 for the neuroblasts, but they are of ectodermal origin. The compact 

 glia and neuroblasts close to the inner layer of ependyma comprise 

 the gray matter of the cord. It is within this layer that the bulk of the 

 cell bodies of neurons and the commissural fibers from one side of the 

 cord to the other will be seen. As the cord develops further, the 



