i;\ri,A.\.\TI()N OK I'KidKKS 



',i2 to M A .st'iics of triUisvtT.sc scclioiis througli tlic region of llic V, VIII, 

 ;ui(l X Hiinglia in embryos of Pctromyzon of ages varying from 10 to 2G days. It 

 will hv seen from these sections that Pctromyzon develops an extensive roof 

 expansion willioiit the aid of a pontine flexure, and the cranial and spinal ganglia 

 aic wcll-fornicd while the (•ciitral nervous system is a solid cord. All of the 

 figures were ilrawn with the aid of an Edinger-Leitz drawing apparatus. With 

 figure 54 a magnification of 76.6 diameters was used, while 250 diameters was used 

 for the others. In reproduction they were all reduced one-half. 



32 Transverse section through the medulla in the region of the auditory 

 vesicle from a 10 day Pctromyzon. This is my oldest embryo in which the cen- 

 tral nervous system has remained a solid cord. Ordinarily it becomes tubular 

 during the seventh day. This section shows the medulla to consist of a syncy- 

 tium of protoplasm, consisting of a mass of round nuclei, much yolk, and a few 

 fibers in the marginal layer. The nuclei have migrated a short distance to either 

 side of the median dorso-ventral line. A seam (CCS.) has appeared here, which 

 marks the position and beginning of the embryonic central canal. The proto- 

 plasm bordering the central canal seam is finely granular and may be assum- 

 ing a secretory function. The acustic ganglia and fibers are shown to be well- 

 dififerentiated on both sides. X 125. 



33 to 35 Three transverse sections passing through the medulla region of 

 another 10 day Pctromyzon embryo, in which the central canal has been somewhat 

 retarded in development. These sections pass through the V, VIII, and X gan- 

 glia respectively, and with the exception that the central canal furrow or seam 

 (CCS.) has expanded into small dorsal and ventral cavities (no dorsal cavity 

 has appeared in fig. 35) the general structure of the medulla is about the same 

 as in figure 32. Later these cavities will become the dorsal and ventral ex- 

 pansions of the embryonic central canal of the medulla. The protoplasm in the 

 region of this seam is granular and may be secreting an embryonic cerebro- 

 spinal fluid. From these figures it will be seen that the beginning of the central 

 canal occurs at the same time throughout the entire rhombic brain. The anterior 

 portion of the spinal cord, while not figured, contains a central canal furrow in 

 the same stage. X 125. 



{Continued on page 60) 



ABBREVIATIONS 



And. v., auditory vesicle or otocyst Mes., mesencephalon 



Br. A., branchial arch M.L., mantle layer 



B.V., blood vessel My:)., myotomes 



CC, central canal Nc. notochord 



CCC, central canal closure, caused P.B., pineal body 



by fusion of lateral plates R.P., roof plate of the central ner- 

 CCS., central canal seam or furrow, vous system 



in Petromyzon Sp.M., spinal cord 



Ep.N., layer of ependymal nuclei Sy.P., syncytium of protoplasm 



F.L.P., fused lateral plates of the Tel., telencephalon 



spinal cord V.G., Gasserian or semilunar ganglion 



F.P., floor plate of the central nervous VIII.VIL, acustico-fascialis ganglion 



system; W.M., white matter 



G.C., germinal cell X.G., vagus ganglion (nodosum). 

 Mar.L., marginal layer ' Y ., yolk granules 



58 



