SPINAL CORD AND MEDULLA OF CYCLOSTOMES 15 



which is not ependjinal cilia or a tangled Reissner's fiber. Also 

 throughout its entire length, as was noted by Sterzi, the fourth 

 ventricle is enveloped by a rather thick layer of spongy and very 

 vascular ependyma, which would be distinctly favorable for 

 infiltration and possibly for secretion into the ventricle. This 

 rich blood supply is from the blood vessels and sinuses travers- 

 ing the meningeal membranes, and especially from two large 

 arteries (A. rhombencephalica of Sterzi), one of which appears 

 in figure 64 (M.A.). Also it would be quite possible in the 

 anterior part of the fourth (A.:iV.) for the various canals, which 

 run close to the dorsal, to receive infiltration direct from the 

 outer meningeal blood and lymph sinuses. 



Coagulated cerebro-spinal fluid is also to be seen in reduced 

 amounts in the mesencephalic ventricles designated as the 

 posterior mesocoele and the sinus mesocoelicus. They are also 

 surrounded by a layer of vascular ependyma, which, while much 

 thinner than the corresponding layer of the fourth ventricle, 

 doubtless functions as a cerebro-spinal fluid forming organ. 



A careful examination of this peculiar modification of the 

 chorioid plexus of the fourth \entricle in Polistotrema leads 

 one to believe that this is not as efficient an organ for the pro- 

 duction of cerebro-spinal fluid as the more expanded tela chori- 

 oidea of Petromyzon and higher vertebrates. 



C. Petromyzon. Petromyzon is apparently the best type that 

 could be selected for obtaining definite information concerning 

 the early history of the formation of the fourth ventricle and its 

 expanded roof plate. 1) It is the lowest living vertebrate that 

 possesses a well-de\eloped fourth ventricle and expanded tela 

 chorioidea in the adult. 2) At no times does the medulla have 

 a pontine flexure. 3) Its central nervous system remains a 

 solid cord of ectoderm until after the cranial and spinal ganglia 

 are well-differentiated. For these and other reasons the early 

 history of the fourth ventricle in Petromyzon has been studied 

 in the effort to determine the essential factors involved in its 

 appearance and growth. 



In order to arrive at the fundamental factors involved in the 

 anlage and development of the fourth ventricle in Petromyzon 



