PLATE 8 



EXPLANATION OF FIGURES 



A series of camera drawings, showing the histogenesis of the oculomotorius 

 nerve. Most of the drawings were made with one-twelfth homogeneous oil- 

 immersion objective, No. 6 compensation ocular, and Abbe camera. 



57 A parasagittal section (KQ 1-4-18) of a 9.5 mm. embryo viewed from the 

 right side, showing the relations of the oculomotorius anlage at the time of its 

 first appearance. The nerve anlage passes directly from the base of the mid- 

 brain vesicle towards the first somite of VanWijhe, and closely associated with 

 the pre-cardinal vein. 



58 A portion of the same section as in figure 57 (KQ 1-4-18) as seen under a 

 4 mm. apochromatic objective and No. 6 compensation ocular, showing the ocu- 

 lomotorius anlage as a product of neuroblasts in the base of the midbrain vesicle. 

 Shrinkage has resulted in breaking the roots of the nerve anlage, but there seems 

 no reason to doubt the genetic relation of the nerve anlage with deeply stained 

 neuroblasts lying in the ventral wall of the midbrain, the deeply stained proc- 

 esses of which may be traced to, and beyond the limiting membrane of the brain 

 wall. The nerve anlage shows a deeply stained fibril surrounded by a sheath 

 of granular protoplasm. At the point of union of the two roots of the nerve anlage 

 is a mesenchymatous cell. There is no evidence that this cell has any genetic 

 relation to the nerve anlage, nor is there any evidence of a migration of medul- 

 lary elements from the neural tube at this stage. 



59 A portion of a parasagittal section (KH 2-1-1) of a 9.5 mm. embryo, show- 

 ing the oculomotorius anlage at a somewhat more advanced stage of histogenesis 

 than that shown in the previous figure, as evinced by the greater number of 

 neuroblasts and neuraxons. There is no evidence of migration of cells from the 

 neural tube at this stage, but several mesenchymatous cells are somewhat closely 

 associated with the nerve anlage. The highly refractive fibrils of the latter are 

 in sharp contrast with the lightly staining vacuolated and granular protoplasm 

 of the mesenchymatous cells. 



60. A combination of four sections (KL 2-2-1, 2, 3, 4) of a 10 mm. Squalus 

 embryo in the head region, showing the relations of the oculomotorius anlage, 

 which extends from the midbrain to the first myotome, as seen under the low 

 power microscope. 



61 A combination of four sections (KL 2-2-1, 2, 3, 4) showing a part of the 

 oculomotorius anlage in a 10 mm. Squalus embryo. The association of the fibril- 

 lar portion of the nerve anlage with medullary neuroblasts in the ba^e of the 

 midbrain is clearly seen. Distally, near the myotome, the nerve breaks up into 

 a brush of separate fibrils, more or less closely associated with mesenchymatous 

 cells. As there is no evidence up to this stage of the migration of medullary 

 cells from the neural tube into the nerve anlage, and as the cells associated with 

 the nerve anlage are more abundant in its peripheral portion near the myotome 

 and the profundus ganglion, the evidence favors the conclusion that these cells 

 are either mesenchymatous or that they are derived from the profundus gan- 

 glion, or from both sources. 



62 to 71 (See pp. 184 and 185). 



182 



