DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 33 



between the seventh and eighth nerves and where it ends behind 

 the fifth nerve being but a single cell in depth. 



We have then a narrow well-defined band of neuroblasts derived 

 from germinal centers situated along the attachment of the roof 

 of fourth ventricle to the medulla, and moving over the surface of 

 the brain toward the pontine flexure. The histological character- 

 istics make it possible to trace them as far as the trigeminal 

 nerve as a sheet which gradually thins out toward its advancing 

 edge. One might well compare the process to ice growing over a 

 pond, yet unlike the latter the new material is formed at the shores 

 only and the whole sheet moves out over the surface its thin 

 advancing edge to meet its fellow from the opposite side. 



It should be noted that the degree of development of the 

 rhombencephalon does not always correspond absolutely with the 

 measurements of the human embryos given in this table. A 

 priori, it would not be expected that at any given stage each organ 

 would always be found to correspond to those of another embryo 

 of like measurement, but in addition to the personal elements in 

 measuring, the fluid in which they are measured often accounts for 

 the difference of a few millimeters more or less.. Embryo No. 

 405 measuring 24 mm. shows a younger stage from the standpoint 

 of pontine development than No. 382 just described. In the 

 former embryo we can see the same active participation of the 

 ventricular epithelium in the production of new elements and the 

 same distribution of karyokinetic figures, yet the front ranks 

 of advancing neuroblasts have only reached the level of the facial 

 and acoustic nerves. This would give us a stage slightly younger 

 than No. 382 where the advancing edge has gained the transverse 

 level of the trigeminal nerve. Furthermore another possible error 

 is introduced bj r the measurements which I have made to show 

 the growth of the pons and they must be interpreted freely since 

 the plane of section is rarely perfect. Obliqueness of section 

 therefore precludes accurate comparison yet the differences are 

 great enough to draw general conclusions. 



Nos. 227, 75, and 86 (measuring 30 mm.) furnish valuable 

 steps leading up to the fusion of the columns of the advancing 

 neuroblasts derived from the two halves of the brain. The fre- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 1 



