DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 47 



tricle and the mesial wall of the lateral recess. A great many of 

 the cells can be recognized as ganglion cells of the corpus ponto- 

 bulbare, but as a whole the appearance is one of immaturity. 

 Here and there can be found a cell whose protoplasm stains but as 

 a rule the large vesicular nuclei are surrounded by a colorless 

 zone. The migration of nuclear material in the medulla has 

 ceased entirely in this stage and one has to expect only the matur- 

 ity of those elements already present. 



Throughout the description I have disregarded a very impor- 

 tant factor in development which transforms the hind brain of 

 an embryo of the second month into the adult form : I refer to the 

 obliteration of the pontine flexure. The maximum flexure in the 

 neural tube occurs about the time of the appearance of the first 

 nerve cells on the ventral surface and then diminishes gradually 

 so that at birth there is still an indication of it on the ventricular 

 floor in front of the emmentia abducentis by a furrow running 

 transversely. The relations of the tegmental nuclei to one 

 another as well as to the olivary complex are distorted by the 

 extreme flexion of the brain in the pontine region. The reduction 

 of this may be regarded as taking place around the nucleus n. 

 abducentis as an axis, since it is situated immediately beneath 

 the ventricular floor just behind the bend in the brain. The other 

 nuclei, superior olive, facial nucleus and olivary complex are dis- 

 tributed around its circumference and are consequently separated 

 from one another. A glance at fig. 1 will show this arrangement. 

 Figs. 4, 5, 6, and 7, are camera lucida drawings of sections through 

 the nucleus n. abducentis with its emergent root bundles, which 

 were selected from such sagittal series as illustrated the change in 

 position of the nuclear masses during the obliteration of the pon- 

 tine flexure. The nucleus facialis is projected into the section as 

 indicated by the broken lines. As the neural tube unbends, 

 the olivary complex and pontine nuclei are gradually pushed 

 toward one another until the cephalic tip of the former comes to 

 be covered by the latter. The abducens nerve which in younger 

 stages (figs. 4, 5 and 6) pursues a straight course within the me- 

 dulla is bent by this process, so that it takes a caudal direction in 

 order to reach the surface of the brain (fig. 7). The facial nu- 



