DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 37 



seen in the adult lower animals where the abducens nerve emerges 

 from the brain some distance behind the pontine formation. 



At 35 mm. (No. 199) the increased cellular activity around the 

 wall of the fourth ventricle is shown by the great numbers of 

 dividing cells and the twofold increase in depth of the migrating 

 column passing between the facial and acoustic nerves. Already 

 the neuroblasts which are crowding from both sides toward the 

 midline have piled up over the ventral surface of the brain, so 

 that, near the raphe, they are now four times (0.22 mm.) as deep 

 as the stage preceding. The free interval between the emergent 

 abducens and the caudal edge of the pons is decreased to half 

 what it was in the 33 mm. embryos. The important contribu- 

 tion which this embryo adds to the development of the basilar 

 part of the pons, consists in a few strands of longitudinal fibers 

 lying near the midline within the thin sheet of superficial neuro- 

 blasts newly descended from the lateral walls of the ventricle. 

 Extending in a direction parallel to the axis of the central nervous 

 system, these inconspicuous fiber bundles separate from the well 

 developed bundle of axones comprising the lemniscus medialis at 

 the level of the cephalic edge of the pontine sheet and plunge into 

 the latter where they take up a middle position as far as its caudal 

 edge. Here again they leave the nuclei pontis and join the fiber 

 mass constituting the medial lemniscus. It is impossible to 

 trace these isolated fibers except where they lie among the pon- 

 tine nuclei, but, as we shall see when, by continual addition to 

 their number, more of their course can be determined, these few 

 strands represent the first beginnings of the longitudinal fibers 

 which are recognized in the basilar part of the adult pons as the 

 cortical projection system. They are represented in fig. 5 by two 

 dotted lines traversing the pontine nuclei. Concerning the first 

 appearance of the pyramidal tract there has been a general unan- 

 imity of opinion, the most important work being that of Flech- 

 sig's work on myelinization time. Tiedemann ('16) thought he 

 saw pyramids in fetus of the third month but he was evidently 

 looking at the eminences formed by the inferior olives which at 

 this time lie adjacent to the ventral median fissure and cause an 

 elevation in the position occupied by the future pyramidal tract. 



