K) WILLIAM F. ALLEN 



it is necessary to go back in the ontogeny of the central nervous 

 system to the time when it was a solid cord. Of my embryos 

 which represent stages killed at 5, 7, 9, 10, 11, 12, 14, 16, 18, 20, 

 and 26 days after fertilization, all of the 5 day series revealed 

 the central nervous system as a solid cord or keel of ectoderm 

 cells, formed according to Calberla by a process of delamination. 

 In places neural crest cells can apparently be seen budding off 

 from the dorso-lateral surface of the brain and spinal cord al- 

 most identically as described and figured by von Kupffer ('90). 

 It is obvious that the preponderance of yolk granules in all 

 tissues makes accurate observation in these stages very difficult. 

 The majority of the 7 day embryos disclosed a central canal 

 either formed or in the process of formation. Out of a great 

 number of series of 10 day embryos, three were found in which 

 the opening of the central canal had been considerably retarded. 

 In one series the central nervous system was still a solid cord of 

 cells, and in the other two the central canal was in the early 

 stages of formation. These series were selected in preference 

 to similar stages of 7 day embryos for the reason that less yolk 

 was present to obscure the various structures. 



To facilitate comparison of similar sections of successive 

 stages, figures 33 to 38 and figures 40 to 53 have been so arranged 

 as to bring several regions of series in a horizontal row, while 

 successive stages of the same region are placed in a vertical row. 

 The conditions at the level of the trigeminal, auditory and vagal 

 ganglia are thus readily compared. 



In figure 32 we have a section through the medulla in the region 

 of the VIII ganglion, which portrays very well an extremely 

 early stage in the formation of the central canal. Here the 

 medulla will be seen to be composed of a mass of nuclei imbedded 

 in a syncytium of protoplasm. Excepting in the roof and floor 

 plates, the nuclei have migrated some little distance to either 

 side of the median sagittal plane, presenting the appearance of 

 a rather broad light line surrounded by nuclei. The center of 

 this light streak of protoplasm discloses a very conspicuous 

 furrow or seam {C.C.S.), which appears in every section of the 

 central nervous system of this series, very much as it looks in 



