DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 41 



niscus soon after it has left the caudal border of the pons. There 

 is then no question but that we have been dealing with the begin- 

 nings of the pyramidal tract as early as 35 mm.; its behavior 

 among the pontine nuclei making identification certain. Former 

 observers have confined most of their attention to the medulla 

 oblongata where it is hopeless to try to pick out the few strands of 

 fibers when they first grow down from the cortex. The increase 

 in number of these axones is so gradual that it is only in the older 

 fetus where enough fibers are collected to form the surface mark- 

 ing on the medulla which we can recognize as pyramids. Flechsig 

 is sure that there are no pyramids at 80 mm. and probably "the 

 pyramidal tract is completely lacking." To harmonize the sys- 

 tem with other observed facts he assumes that they must grow 

 down rather rapidly from the cortex when once they start, since 

 their myelinization occurs after birth and the usual interval be- 

 tween the formation of a nerve fiber and its acquirement of a mye- 

 lin sheath is about four months. This of necessity would have the 

 pyramidal tract appear about the middle to the end of the fifth 

 month or 14 to 16 cm. To this one must answer that a myelin 

 sheath does not appear on every axone of this system simultane- 

 ously ; it begins rather on isolated fibers and is first complete at the 

 age of two years. 



In these fetus of the eleventh week the basilar part of the pons 

 has reached a thickness of 0.7 mm. (fig. 6). The abducens nerve 

 rootlets are almost entirely surrounded by nuclear material after 

 they leave the tegmentum, only the caudal two or three fasciculi 

 being free. Great numbers of neuroblasts are encountered pass- 

 ing between the seventh and eighth nerves, forming a stream 

 0.16 mm. deep, while the germ centers at the ventricular margin 

 are busily producing new cells. A fortunate sagittal section 

 through No. 96 has been illustrated to show the participation 

 which the greatly thickened lateral recess wall takes in contribut- 

 ing cells to the pons. For purposes of orientation a wax-plate 

 reconstruction was made with the section drawn on its cut sur- 

 face (fig. 8) . As the cerebellum in its growth crowds against the 

 medulla, this caudal wall is flattened out and becomes part of the 

 mesial wall of the recess. A separation of the cells coming from 



