38 CHARLES R. ESSICK 



The latter subsequently forces its way between the olives and 

 gradually displaces them laterally. Certainly at this time the 

 number of fibers making up the pyramidal tract is insufficient 

 to cause the surface markings on the medulla which we know as 

 pyramids. Reasoning back from the interval of time — four 

 months — between the appearance and myelinization of other 

 systems, Flechsig came to the conclusion that the pyramids must 

 first be laid down between the middle and the end of the fifth 

 month. In cross sections through the olive of an 80 mm. J (crown- 

 rump) fetus he is unable to recognize any tissue which may be 

 regarded as matrix for the pyramids but thinks they arise from 

 fibers growing down from the cerebral cortex with remarkable 

 rapidity in the second half of the fifth month. W. His ('04) has 

 given a valuable table (p. 155) showing the various fiber systems 

 which he was able to identify at each stage of embryonic growth. 

 In a fetus of 83 mm. he was unable to find the pyramidal tract, 

 but at a length of 120 mm. he saw evidences of its appearance 

 together with cross pontine fibers. He gives us no statement 

 as to the part of the brain in which he observed the pyramidal 

 tract, merely noting its presence or absence in the various embryos 

 in his collection. With this statement of the present knowledge 

 of the cortical projection system, I shall omit until later the 

 reasons for believing that the few axones isolated by the early 

 pontine nuclei represent the anlage of the cortico-spinal tract in 

 this 35 mm. fetus. 



No. 95 (46 mm.) is the youngest stage in which I could determine 

 cross fibers among the pontine nuclei. They are most conspic- 

 uous at the lateral borders where they gather together into com- 

 pact strands to form the brachia pontis. Here the fibers have 

 a superficial position, embracing laterally the corpora restiformia 

 as the latter turn sharply into the cerebellar hemispheres. It is 

 possible to trace the axones coming from the pontine nuclei for 

 some distance into the cerebellum until their course parallels the 



1 The crown heel measurement which Flechsig used was 11 centimeters. For 

 the sake of ready comparison with my study I have put his measurements into this 

 form from the table given by Mall in Handbuch der Entwicklungsgeschichte des 

 Menschen-Keibel und Mall, Leipzig, 1910, p. 205. 



