DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 43 



the recess wall from those of the ventricular roof is purely arbi- 

 trary, since the two origins are really continuous with each other. 

 This association in the adult was first pointed out by Orzechow- 

 ski ('08, p. 41). For embryological reasons the nuclear thicken- 

 ings of the lateral recess wall with their accompanying fibers 

 occurring in the adult should be included in the structure which I 

 have termed the corpus ponto-bulbare. At this stage the cavity 

 of the fourth ventricle shows a peculiar tendency to form small 

 outpouchings along the attachment of the roof at the place where 

 the pontine nuclei are being formed. From two to four such 

 recesses can be made out extending laterally for a considerable 

 distance from the main ventricular cavity and causing the exter- 

 nal surface to be thrown up into ridges. In section they may be 

 round or slit-like and are lined with deeply staining cells, great 

 numbers of which are found in process of karyokinetic division. 

 The production of neuroblasts at this stage is enormous and these 

 lateral extensions from the ventricle furnish a greater expansion 

 of ependymal surface and thus increase the germ layer where cell 

 division can take place. 



In the older fetus the system of ventricular outpouchings be- 

 comes more complicated and secondary processes are formed 

 which are distinctly tubular. The size of the lumen varies, being 

 sometimes less than the width of a single nucleus. It is always 

 lined with a simple layer of cells which are definitely ependymal 

 and as long as pontine nerve cells are being formed these tubules 

 can be made out with a little difficulty among the closely packed 

 neuroblasts but always the center of mitotic activity. With the 

 emigration of the last of the new elements the ventricular pro- 

 longations stand out with much greater clearness. This is partic- 

 ularly well shown in the five and eight months fetus — in the latter, 

 one is struck by the greater number of such tubules both in the 

 roof attachment and the caudal wall of the lateral recess. 



At the beginning of the fourth month, as shown by No. 172 

 (80 mm.), the evidences of marked cellular activity, i.e., extensive 

 mitosis and deeper staining are still present around the roof 

 attachment of the fourth ventricle and the caudal wall of the 

 lateral recess. This fetus does not illustrate any new principle 



