PLATE 6: EXPLANATION OF FIGURES (CONTINUED) 



41 A portion of the same section (LAD 1-7-17) enlarged to show the struc- 

 ture of the neuroblasts. These are pear-shaped, deeply stained cells with pro- 

 toplasm extending into the fibrillar neuraxon process. 



42, 43, 44 A series of cross-sections of the anlage of the abducens nerve as 

 seen in a 19 mm. embryo. Figure 42 is the most posterior of the sections. The 

 nerve anlage at this stage consists chiefly of a bundle of deeply stained neuraxons 

 irregularly grouped together among vacuolar spaces and partly enclosed by a 

 layer of mesenchymatous cells. In figure 44, the relations of the neuraxons of 

 the nerve as it subdivides into a branch are shown. The highly refractive gran- 

 ules seen in cross-section correspond with the deeply stained fibers appearing in 

 longitudinal sections. 



PLATE 7 



EXPLANATION OF FIGURES 



Most of the figures are camera drawings with one-twelfth homogeneous oil- 

 immersion objective and No. 6 compensation ocular of Zeiss showing stages in 

 the histogenesis of the trochlearis nerve. Figures 55 and 56 are graphic recon- 

 structions from parasagittal sections as seen under low power objective. 



45 and 46 Portions of cross-sections (LAG 1-6-1, 2) of a 19 mm. embryo 

 through the region of constriction between midbrain and hindbrain vesicles, 

 showing portions of the trochlearis anlage as it passes through the mesenchyma 

 lateral to the wall of the brain. The growing tip of the nerve is not shown. At 

 this stage the anlage is wholly fibrillar and its deeply staining fibrils contrast 

 strikingly with the granular vacuolated protoplasm of the surrounding mesen- 

 chyma. A thin layer of granular protoplasm may be seen covering portions of 

 the nerve anlage. The fibrils of the nerve may be traced dorsally into the chi- 

 asma at the anterior boundary of the cerebellar neuromere, but the nerve has 

 not yet become connected with the myotome of VanWijhe's second somite. 



47 and 48 Proximal and distal portions of the trochlearis anlage in cross- 

 sections (AIBB 1-4-17 and MBB 1-5-2) of a 21 mm. embryo in the region of the 

 cerebellar neuromere. The nerve anlage differs from the previous stage only 

 in the larger number of fibers composing the nerve bundle. No cells closely 

 associated with the nerve anlage are to be found. The growing tip of the nerve 

 is not shown. 



49 A group of neuraxon fibers near the peripheral termination of the troch- 

 learis nerve anlage as seen in a parasagittal section (ME 5-1-4) of a 21 mm. 

 embryo, showing the nerve anlage as a loose brush of neuraxons without any 

 closely associated cells. The highly refractive fibrils show a thin sheath of 

 granular protoplasm. 



50 and 51 Portions of the trochlearis nerve anlage as seen in parasagittal 

 sections (MD 2-2-5 and MD 2-2-8) of a 22 mm. embryo, showing the fibrillar 

 character of the nerve. Figure 51 shows a distal and figure 50 a proximal por- 

 tion of the nerve anlage. In its proximal portion, the trochlearis consists of a 

 compact bundle of neuraxon fibers, naked or with a thin coating of granular 

 protoplasm and devoid of closely associated cells. Distally, however, where 

 the nerve anlage breaks up into a brush of loose fibers, mesenchymatous cells 

 are more closely associated with the individual cells, but there is no evidence 

 of a genetic relation between these cells and the neuraxon fibers. 



52 to 56 (See p. 180). 



179 



