30 CHARLES R. ESSICK 



The facial nucleus has a remarkably constant form, the outline of 

 which is similar both in sagittal and coronal sections. It might 

 be compared to a pear, the smaller cephalic extremity tapering 

 off bluntly. The nucleus preserves this constricted end in the 

 adult as has been brought out in a model made by Mr. Weed in 

 this laboratory. Cephalad through half of its extent it lies dorso- 

 lateral and parallel to the superior olive and extending far in front 

 of the outline of the nucleus n. abducentis. It will be observed 

 that the relation to the superior olive is that of the adult yet one 

 would miss the familiar appearance of the facial nucleus seen in 

 transverse sections through the cephalic pole of the inferior olive. 

 In other words, the caudal tip of the facial nucleus is distant a 

 considerable interval from the cephalic tip of the inferior olive. 

 The olivary complex, still very incompletely developed, is made 

 up of an elongated mass of cells situated near the raphe. It shows 

 a marked bend conforming to the flexure of the medulla in the 

 neck region. Its cephalic pole, as projected on the lateral sur- 

 face, falls behind the transverse level of the seventh nucleus. Of 

 the greatest importance is the histological appearance of the 

 rhombencephalon, the ventral surface of which is made up of the 

 marginal veil ('Randschleier' of His) and in its nuclear free net- 

 work run the fibers comprising the long association tracts. This 

 layer, striking in sections on account of the dearth of nuclear 

 material, forms a brilliant background which permits one to readily 

 outline the nuclei wandering over its surface later. 



From the embryological series of this laboratory definite evi- 

 dences of the migration leading up to the formation of pontine 

 nuclei appear in an embryo of 23 mm. (Mall Collection No. 382). 

 Fig. 2 was drawn from a wax-plate reconstruction of the rhomb- 

 encephalon of this embryo. Here the degree of medullary 

 development corresponds pretty accurately to that of the 22 mm. 

 embryo just described. The behavior of the cells lining the ven- 

 tricular cavity deserves particular attention, inasmuch as they fur- 

 nish the neuroblasts for the future pontine nuclei. The ependyma 

 covering the floor of the fourth ventricle over the basal and alar 

 plates has lost all signs of the great activity which it showed 

 during the formation of the tegmental structures. The cells 



