34 CHARLES R. ESSICK 



quency and wide distribution of the karyokinetic figures occurring 

 around the attachment of the ventricular roof as well as the caudal 

 wall of the lateral recess, speak for the active participation that 

 the rhombic lip is taking in the production of pontine nuclei and 

 as a result the roof and recess wall are thickened perceptibly. The 

 last fetus has been sectioned transversely through the medulla 

 and fig. 3. shows well the large production of cells along the roof 

 attachment. It is impossible to figure dividing cells at this mag- 

 nification, yet in this one section which I have illustrated, I was 

 able to count as many as 75 karyokinetic figures immediately 

 beneath the membrana limitans interna. A very few could be 



*^^/\ 



Fig. 3 Germinal centers for the basilar nuclei at the roof attachment of the 

 fourth ventricle in a 30 mm. fetus. X 6.0. (No. 86, slide 35, section 1). 



made out where the cells converge at the surface, while only an 

 occasional mitotic figure is met with among the cells turning 

 around the restiform body. No evidences of indirect division 

 could be found in the layer when it has arrived in front of the 

 pontine flexure. We see, then, that the mitosis is confined sharply 

 to the central canal. The increased production of new elements 

 is also brought to one's attention by the increase in depth of the 

 column passing between the facial and acoustic nerves to 10-15 

 superimposed neuroblasts; moreover those which had gained the 

 trigeminal nerve in the 23 mm. embryo have moved forward and 

 mesially in front of the pontine flexure. 



