SPINAL CORD AXD MEDULLA OF CYCLOSTOMES 35 



a millimeter apart. Also the same relationship could be shown 

 in sections anterior to figure 62, and from similar sections of a 

 Petromyzon larva. A po.ssible objection to applying this argu- 

 ment in the tail region might arise from the fact that the extreme 

 posterior end of the spinal cord is non-nervous, consisting en- 

 tirely of ependyma and undifferentiated embryonic cells. In 

 reply to this we would invite comj)arison of figures 21 and 22, 

 where the structure is non-nervous in both cases, and where the 

 effect of the notochord is obvious. 



That the spinal cord of the higher vertel)rat('s has not been 

 depressed by pressure from the notochord is due ()i)viously to 

 the fact that the notochord is an embryonic structure, which 

 never attains sufficient size to have any influence on the spinal 

 cord. Tliis is clear from figures 85 and 86. In Cyclostomes, 

 liowcNcr, Ihc notochord is a very important structure, develops 

 early and grows foi- a long period of time, and serves as the 

 skeletal axis of tli(> :i(lult. in fishes. Aiiii)liil)ia, and reptiles th(> 

 growing notochord iiia> have some slight (effect in flattening of 

 the adult spinal conl. hi Ami)hioxus the ventral surface of the 

 spinal cord clearlx shows the indenting effect of the growing 

 notochord ' lig. 70 ). and in the trunk region where the diameter 

 of \hr noto('h()i-(l is gicatest. th(> s|)inal cord is most depressed. 



(;i;\i;k \i. coxsidi;!} a rioxs and si .\imai;v 



l'"roiii th(> foi'cgoing tacts the following conclusions .seem full\- 

 warranted : 



1) In the dc\ clopinciU of the loot' ])lale ('\|)aiisioii itela 

 chorioideai in the medulla oblongata of most vertebrates three 

 separate stages or (>pochs of expansion should be recognized: a) 

 A first enlargement of the dorsal portion of the embryonic cen- 

 tral canal took place from a migration outward and upward of 

 certain of the roof plate cells, or as was the case in Petromyzon 

 from the migration of the nuclei and jirobable disintegration of 

 the cytoplasm, b) The second stage in the expansion of the 

 roof plate was the direct result of an increase of pressure from 

 the cerebro-spinal fluid, produced from at least two possible 



