3S WILLIAM F. ALLEN 



terior i)()rti()ns has developed into specialized organs for the pro- 

 duction of cerebro-spinal fluid. Notwithstanding this speciali- 

 zation, the fourth ventricle is thought to be decidedly inferior 

 to the tela chorioidea of Petromyzon as an organ for the produc- 

 tion of cerebro-spinal fhiid. 



6) In the spinal cord of one individual of Polistotrema there 

 occurred at least three expansions of the roof plate which re- 

 semble the roof of the fourth ventricle in other vertebrates. 

 From the fact that these expansions were very vascular and 

 their cells granular it is inferred that they functioned as cho- 

 roid plexuses for the formation of cerebro-spinal fluid. The 

 writer presents the hypothesis that the fourth ventricle in ances- 

 tral vertebrates may have originated as a mutation, similar to 

 this sport plexus in the spinal cord of Polistotrema; that such 

 sport expansions may have occurred at various places such as 

 the diencephalic segment, the roof of the mesencephalon where 

 a choroid plexus still exists in Petromyzon, and in the hind 

 brain and spinal cord. Such mutations, proving to be useful 

 have been preserved in the vertebrate race. 



Concerning the flattening of the spinal cord in Cyclostomes 



7) A great variation in the shape of the so-called typical 

 embryonic spinal cord is to be recorded. In Petromyzon it was 

 found to be nearly cylindrical, to be moderately compressed in 

 Squalus, Amblystoma, and in the turtle, and decidedly com- 

 pressed in the chick and the pig. 



8) To obtain this typical stage the original compressed spinal 

 cord of Petromyzon must have undergone a marked depression, 

 and the early depressed neural tubes of Squalus, Amblystoma, 

 turtle, chick, and pig must have undergone a decided com- 

 pression. The main factor causing this depression in the former 

 was thought to be ventral pressure from a growing notochord, 

 and the compression of the latter was attributed to lateral pres- 

 sure from the growing myotomes. 



9) Transverse sections immediately before and during the time 

 that the greatest depression of the spinal cord is taking place 



