DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 29 



covered around the ventricular cavity where great numbers were 

 present in the earlier stages. With the cessation of its activity, 

 the epithelium lining the cavity of the fourth ventricle becomes 

 very sharply marked off from the underlying nervous tissue and 

 it would be expected that the various nuclear masses in the me- 

 dulla have received their allotment of cells, further growth consist- 

 ing of increase in size of individual elements and the addition of 

 nerve fibers. 



Fig. 1 Profile reconstruction of 22 mm. embryo. X 4.5. Taken from His — 

 Entw. (1. mensch. Rautenhirns, fig. 5. I have drawn in the abducens nerve (N. 



vii and its nucleus (Nu. vi). 



If now one looks at fig. 1 (a profile drawing taken from His, 

 ('91), to which I have added the sixth nucleus and its nerve) many 

 differences from the adult are evident, the most striking perhaps 

 being the great flexure in the pontine region — the cerebellar thick- 

 ening almost touching the medulla. Just under the floor of the 

 fourth ventricle appears the nucleus n. abducentis usually an 

 elongated mass of cells lying immediately behind the ventricular 

 furrow formed by the bend in the neural tube. From this nucleus 

 the axones pass obliquely vent rally through the tegmentum to 

 emerge just behind the summit of the pontine flexure in a series 

 of rootlets which behave much as the hypoglossal nerve roots. 

 They are quickly gathered together to form a single nerve trunk. 



