PLATE 8: EXPLANATION OF FIGURES (CONTINUED) 



62 A single neuraxon of the oculomotorius anlage in a parasagittal section 

 (KL 1-5-12) of a 10 mm. embryo, showing its relations with mesenchymatous 

 cells at the base of the midbrain vesicle. The fibril shows the characteristic 

 granular sheath and the evidence which has been interpreted by some investi- 

 gators in favor of the theory that the neuraxons are differentiated from a pro- 

 toplasmic reticulum. Such a conclusion, however, ignores the evidence of the 

 peripheral growth of the neuraxons from medullary neuroblasts. The picture 

 is strikingly similar to Cast's ('09) figure 3, plate 13. 



63 A portion of the oculomotorius anlage in a parasagittal section (LA 4-2-8) 

 of a 16 mm. embryo, showing the axial fibrillar bundle and the cellular sheath. 

 The conditions are the same as those seen in spinal somatic motor nerves and in 

 the other eye-muscle nerves at similar stages of development. There is no doubt 

 as to the neuroblastic origin of the fibrillar portion of the nerve anlage, but it 

 is not possible to determine positively whether the cellular sheath is ectodermal 

 or mesodermal in origin. 



64 A portion of a parasagittal section (LC 2-1-10) of a 16 mm. embryo in 

 the region of the roots of the oculomotorius anlage, showing the relation of fibers 

 of the nerve anlage to neuroblasts lying in the base of the midbrain vesicle. 

 No evidence of cellular migration from the brain into the nerve anlage can be 

 seen, and the source of the cells grouped at the roots of the nerve is doubtful. 



65 A cross-section (LG 2-2-9) of the oculomotorius nerve anlage in its pe- 

 ripheral portion in a 15 mm. embryo, showing the bundle of neuraxon fibers with 

 the sheath of mesenchymatous cells. The structure is essentially the same as 

 that in similar sections of the anlagen of spinal ventral nerves. The nerve figured 

 shows approximately two hundred highly refractive granules in the axial bundle, 

 evincing approximately that number of neuraxons in the nerve anlage. 



66. A longitudinal section of the oculomotorius anlage in the distal portion 

 of the nerve of a 17 mm. embryo, showing the protoplasmic sheath — no nuclei 

 appear in the portion figured — and the axial bundle of fibers. 



67 The oculomotorius anlage as frontal sections (LAG 1-4-24 and 25) of a 

 16 mm. embryo, showing the relations of the nerve to the ganglion of the ramus 

 ophthalmicus profundus trigemini and to the first somite of VanWijhe. The 

 group of cells lying between the ganglion of the profundus and the oculomotorius 

 may be traced in later stages into the ciliary ganglion of the adult and thus form 

 the anlage of that sympathetic ganglion. The evidence in Squalus seems to 

 favor the view that these cells are derived from the profundus nerve; the possi- 

 bility, however, that they are in part derived by migration from the brain is 

 not excluded. Of this, however, I am able to find no evidence. 



68 A cross-section (LAB 8-13-20) of the oculomotorius anlage in a 17 mm. 

 embryo, showing an early stage in the penetration of the bundle of neuraxon 

 fibers by neurilemma cells. As development continues the number of such cells 

 within the fibrillar bundle steadily increases, beginning at the proximal and 

 distal ends of the nerve anlage. 



69 A part of the pro.ximal portion of the oculomotorius anlage in a parasagittal 

 section (MC 4-1-4) of a 21 mm. embryo, showing the penetration of the neuraxon 

 bundle by neurilemma cells. 



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