TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 263 



marks in the study of -the topography of the embryonic head. The nerve-cells of 

 the ganglion are grouped, for the most part, on the side toward the ectoderm, 

 where they are closely crowded together, making a deeply staining mass. Nearer 

 the brain-wall the tissue of the ganglion is much less condensed, is somewhat 

 penetrated by small blood-vessels, and contains a considerable number of nerve- 

 fibers, which are gathered into small bundles. Toward the brain-wall the bundles 

 become distinct, and on the right side of the embryo the passage of the nerve- 

 fibers into the brain can be readily seen. The nerve-fibers -at this stage are 

 merely neuraxons; that is to say, thread-like prolongations of the bodies of the 

 nerve-cells (neurones). The fibers are entirely without sheaths. They stain very 

 lightly, and hence, in the preparation, may be detected by their light appearance. 

 The nerve-fibers may be conveniently rendered conspicuous by counterstaining the 

 sections with Lyons blue. The nerve-fibers of the trigeminus, which enter the wall 

 of the hind-brain, form in part a bundle of fibers, which extends along past the 

 acustico-facial ganglia within the medullary wall. These fibers represent the com- 

 mencement of the ascending trigeminal tract of anatomists. The other ganglia 

 associated with the hind-brain are not well shown in this section. The otocyst 

 (compare Fig. 42, p. 79) has a very sharply defined epithelial wall and is im- 

 bedded in loose mesenchymal tissue. On the. right side of the embryo we have 

 the ductus .endolymphaticus, D.E, the opening of which into the .main cavity of 

 the otocyst is shown on the left side. The epithelial wall of the ductus is thicker 

 than that of the greater part of the otocyst proper. The wall, Md,., of the hind 

 brain exhibits already characteristic differentiations^ for it shows clearly the 

 primitive laprs; the outer neuroglia layer (ectoglia^is thin, -and appears light in 

 the sectii- * :; use it takes the stain slightly. It is in this outer neuroglia layer 

 (ectoglia} that the entire sensory nerve-fibers are p ;.rnriiy distributed, and. there- 

 fore, it is in a po, r md l'-> ract 

 situated. Next to the ectoglia comes the middle layer, in \vbi 

 the medullary wall are situated, and which is, therefore, termed the neurone < > 

 gray layer (cinerea), easily recognizable under the microscope by its brighter color, 

 which is due principally to the fact that the nuclei in this layer, though numerous, 

 are much less crowded than in the innermost of the three layers, or primitive 

 ependymal layer, which at this stage is quite thick. Owing to the presence of 

 nuclei, the gray layer is, of course, stained much more than the ectoglia. The nu- 

 clei of the brain- wall shoty as yet very little differentiation. There are numerous 

 mitotic figures which are situated exclusively clpse to the inner surface of the brain- 

 wall in the fore-brain. The structure ,*' the for^-brain is similar, but the develo* 

 ment is less advanced; the differentiation, T)i~4ki neurone layer is only v* 

 ning, and it has acquired little thickness. In tho'^'Vl^bjain we ><> 

 along the region between the otocysts, a se* 

 a scalloped outline to the wall. A ^ 

 nex ^ one of the spac<- 



