SPINAL CORD AXD MEDULLA OF CYCLOSTOMES 2'.^ 



that the roof plate has been weakened more in the medulla region 

 through a greater migration of neural crest cells. This explana- 

 tion, however, is not entirely satisfactory; since it was shown for 

 Petromy^on (fig. 32) that the cranial ganglia were well-difYer- 

 entiated while the medulla was still a solid cord of cells. If then 

 this were the determining factor it must have cast its shadow a 

 long way ahead. Also it is apparent when the difference in size 

 is considered between the medulla and the spinal cord that there 

 are relatively no more neural crest cells extruded from the 

 medulla. 3j The roof expansion appeared at the same time in 

 the region of the X ganglion as in the region of the VIII and \ , 

 and developed at a uniform rate throughout the medulla. 4) 

 Two factqrs are evident in the formation of the fourth ventricle: 

 first, an upward and outward migration of the roof plate nuclei 

 followed by a disintegration of the inner protoplasm: second, 

 internal ])ressure exerted by the rapidly increasing cerebro-spiiial 

 fluid, infiltrated and secreted by the roof plate itself. 



D. Selachians and Amphibia. As types of these clas.ses I had 

 access to very complete sets of serial sections of Squalus and 

 Amblystoma embryos, and to one transverse series of a 15 nun. 

 Necturus. A careful examination of this material contributed 

 nothing new to the ontogeny of tiio fourth ventricle. It was 

 possible, however, to confirm in these forms many of the con- 

 clusions arrived at in Petromyzon. In both Squalus and Ambly- 

 stoma the nmch earlier appearance of the central canal brings 

 about a much earlier and more extensive expansion of the fourth 

 ventricle. The sections shown in figures 72 and 73, while pos- 

 sessing enormous expansions of the roof plate, exhibit less progress 

 in the differentiation of the structure of the medulla than was 

 revealed in a 12 day Petromyzon (fig. 41), in which very little 

 or no expansion had taken place in the roof plate. 



In Squalus and Amblystoma the typical embryonic central 

 canal, in the form of a vertical cleft, is a comparatively late 

 production. The canal appears soon after the neural folds co- 

 alesce as a horizontal cleft, its presence often being indicated 

 only by a layer of pigment. This cleft becomes elliptical, then 

 more or less circular, and finally changes to a vertical or dorso- 



