40 CHARLES R. ESSICK 



the transverse pontine fibers going into the brachium pontis. 

 Moreover the fasciculi of the pyramidal tract keep on growing so 

 that one must infer that some of the axones coming down from the 

 cortex add themselves to the bundles more deeply placed in the 

 pons. 



During the period of growth between 35 mm. and 46 mm. 

 enormous numbers of neuroblasts have come down from the lateral 

 boundaries of the fourth ventricle. These new cells together with 

 their processes sent out transversely and the cortical axones 

 threading their way among them have increased the thickness of 

 the basilar portion to 0.642 mm. There is a tendency for the cells 

 from both sides to crowd toward the midline, thus giving rise to 

 the typical crescentic shape of the pontine nuclei which one ob- 

 tains in transverse sections through the pons. The increase in 

 thickness is also accompanied by an increase in caudo-cephalic 

 extent. The latter, however, does not proceed with the same 

 proportional rate as the former, so that the sagittal sections of 

 pontine nuclei are becoming more and more oval. In spreading 

 caudad the interval between the nuclei pontis and the abducens 

 rootlets has been gradually reduced until in this fetus the more 

 cephalic axones are surrounded by pontine nerve cells. 



In fetus of 50 mm. (Nos. 84, 96, 184) the number of cerebro- 

 spinal neurones have increased to such an extent that it is now 

 possible to follow them with sufficient accuracy to be certain that 

 we are dealing with the axones of the cortical projection system. 

 The fibers splitting up among the pontine nuclei already form 

 comparatively large bundles (fig. 6) which are collected together 

 into a solid fasciculus at the cephalic end of the pons. Here they 

 come into close relationship with the lemniscus medialis, but it 

 is not impossible to trace the large fiber mass into the internal cap- 

 sule. Traced cerebrally the crura gradually diverge from the 

 midline and turning around the cephalic flexure they lie ventral 

 and lateral to the nucleus hypothalmicus, while the medial lem- 

 niscus has a more dorso-lateral position with regard to this 

 nucleus. The fibers making up the pyramidal tract can be traced 

 definitely into the internal capsule. Spinal-ward I have been 

 unable to differentiate the projection system from the medial lem- 



