DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 49 



ARCUATE NUCLEI 



Examination of different adult brains in microscopical sections 

 reveals a great variation in the amount of nuclear material which 

 goes to make up the basilar portions of the brain stem. This 

 is especially true of the arcuate nuclei where small, more or less 

 isolated patches of nuclear material may often be scattered along 

 the ventral and lateral surfaces of the medulla as far as the resti- 

 form body. The arcuate nucleus proper, the most constant of 

 these masses, lies near the ventral median fissure superficial to 

 the pyramidal tract, extending fr6m a point caudal to the olive 

 up to and fusing with the pontine nuclei. At its caudal extrem- 

 ity, under the olive, this mass is always of greater dimensions and 

 tapers off somewhat as it is followed toward the pons — in some 

 brains disappearing here and there for a. few sections^ in others 

 forming a continuous narrow strip under the whole length of the 

 medulla. The arcuate nuclei proper, as well as these superficial 

 isolated masses lying more laterally, will be shown to have a com- 

 mon origin and at one time to be actually continuous with one 

 another. The principles governing their development are iden- 

 tical with those which we have studied in connection with the 

 pontine formation. The same germ centers around the attach- 

 ment of the rhombic lip contribute cells which migrate superfi- 

 cially over the medulla in front of the cervical flexure. 



The formation of the arcuate nucleus, unfortunately, is not so 

 simple as that of the pontine nuclei, but is complicated by the 

 simultaneous development of the olivary complex. As His has 

 shown, the latter begins as a migration from the alar plate of the 

 rhombencephalon early in the second month. Toward the end 

 of this month the olive can be outlined readily although it has 

 only a small fraction of the cells which it contains in the adult. 

 At this time one can make out in embryos of about 20 mm. large 

 elongated nuclei, almost devoid of a protoplasmic body leaving 

 the ventricular margin along the attachment of the rhombic 

 lip. They are arranged in strands of a single cell in depth and two 

 or three in width, streaming over the surface of the medulla just 

 under the external limiting membrane. This migration is directed 



