DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 35 



The three specimens measuring 30 mm. give us the final steps 

 in the completion of the first anlagen of the nuclei pontis. In 

 No. 227 the cell lamina has not reached the ventral median fis- 

 sure, it being possible to trace it from the border of the fourth 

 ventricle forward to the fifth nerve where it curves mesially at 

 almost a right angle toward its counterpart from the other side. 

 The sagittal sections containing the sixth nerves mark the thin 

 advancing edges of the cellular sheets approaching each other from 

 the two sides of the brain. In No. 75 the most advanced cells 

 have almost succeeded in gaining the midline, while in No. 86 

 the two columns have fused across the raphe. During their 

 entire course from the rhombic lip on the dorsal surface to the 

 raphe on the ventral surface, the wandering neuroblasts have 

 kept a superficial position, only occasionally is there any tendency 

 for any of the cells to penetrate into the clear, almost nuclear-free 

 marginal veil. As they pass between the seventh and eighth 

 nerves the cells are constricted into a narrow band 0.2 mm. wide, 

 but on reaching the pontine flexure they spread out into a fan- 

 shaped layer 0.6 mm. in caudocephalic extent. The lemnisci 

 medialis and lateralis, which up to this stage had occupied a super- 

 ficial position, are now covered over by a thin bridge of tissue and 

 we can begin to speak of a tegmental and basilar part of the pons. 



Nos. 211 and 145 (33 mm.) are cut sagitally and give us an 

 opportunity to study the earliest pontine nuclei in their relation 

 to the emergent nervus abducens. Fig. 4 is a camera lucida out- 

 line of the section through this nerve. The axones after leaving 

 their nucleus take a ventro-cephalic course through the tegmen- 

 tum and emerge from the neural tube just behind the most 

 prominent part of the pontine flexure. The young pontine neuro- 

 blasts, on the other hand, lie wholly in front of this flexure, 

 spread out into a sheet whose caudo-cephalic extent is 1.25 mm. 

 and whose depth is 0.057 mm. at its thickest part, tapering down 

 to the thickness of a single cell both caudally and cephalically. 

 Between the most cephalic rootlets of the sixth nerve and the most 

 caudal cells of the pontine nuclei is an appreciable interval 

 (almost 0.5 mm.) so that one is at once reminded of the condition 



