32 CHAKLES K. ESSICK 



composing it are sharply marked out into a definite lamina and 

 only after searching through many microscopic fields is one able 

 to detect evidences of cell division. In marked contrast to this 

 inactive region, the lip-plate which makes up the caudal wall of 

 the lateral recess and the roof of the fourth ventricle just behind 

 it, is found busily engaged in producing new elements. The fur- 

 row formed by the attachment of the roof plate to the medulla, 

 contains great numbers of karyokinetic figures in every high power 

 field of the microscope and in this neighborhood the ependymal 

 zone is not so sharply differentiated into such a thin layer as covers 

 the medullary floor nearer the midline. Its cells are more closely 

 packed, its nuclei take on a deeper stain, and the line of demarca- 

 tion from the subjacent tissue is partly destroyed by the proto- 

 plasmic processes of the new neuroblasts which are beginning to 

 push toward the surface of the brain. The exact manner of arriv- 

 ing at the surface is illustrated by a more fortunate section (fig. 

 3) through a slightly older embryo. Here the deeply staining 

 cells, poor in protoplasmic envelope, may be seen to leave their 

 position near the ventricular cavity, and to come together at the 

 surface where they form a tin sheet of closely arranged cellular 

 material. When once they have gained the surface of the brain 

 they migrate toward the pontine flexure, always preserving their 

 superficial position. 



By referring to fig. 2 a very good idea can be obtained of the 

 zone of proliferating cells and the area covered by the migrating 

 neuroblasts that have gained the surface of the rhombencephalon. 

 I shall omit the description of the arcuate formation for the pres- 

 ent and consider only that narrow elongated column of cells which 

 is seen to turn toward the pontine flexure. It is very easy to 

 identify the densely-staining closely-arranged nerve cells in sec- 

 tions and I have imitated the appearance one gets specimens 

 stained in toto by shading this column. The cells, that have left 

 the ventricle, converge into a well-defined band which, as it curves 

 around the restiform body, embraces the more a'nteriorof the root- 

 lets of the glossopharyngeal nerve and passes between the facial 

 and acoustic nerves as far forward as the trigeminal nerve. At 

 this stage the cellular sheet is very thin, being but 4-5 cells deep 



