PLATE G 



EXPLANATION OF FIGURES 



A series of camera drawings illustrating the histogenesis of the abducens nerve 

 With the exception of figures 37. 38 and 40, all were drawn with one-twelfth 

 homogeneous oil-immersion objective and compensation ocular 6 or 8 of Zeiss. 



31 A portion of a parasagittal section (KP 2-1-6) of a 10 mm. embryo, show- 

 ing an early stage in the development of the abducens anlage, which makes its 

 appearance as a product of the union of the processes of neuroblasts in the ventro- 

 lateral wall of the medulla immediately posterior to the otic capsule. The pres- 

 ence of loose mesenchyma in the region of the nerve anlage complicates the 

 picture, but the processes of deeply stained neuroblasts may easily be traced 

 into the nerve anlage. The mesenchyma cells are granular, much vacuolated, 

 and stain lightly, showing in some cases protoplasmic connections with the ab- 

 ducens anlage. In every essential respect the picture resembles an early stage 

 in the histogenesis of a spinal somatic motor nerve. The chief differences con- 

 sist in the relatively early appearance of a loose mesenchyma and in the remote- 

 ness of the myotome from the point of emergence of the neuraxon processes of 

 the nerve anlage. The presence of a lightly staining granular protoplasm sur- 

 rounding the deeply stained processes of the neuroblasts is noteworthy, but it 

 is difficult to determine whether this ensheathing protoplasm is of mesenchyma- 

 tous or medullary in origin. Analogy with spinal nerves would favor the con- 

 clusion that it is an outflow from medullary cells. 



32 A portion of a parasagittal section (KR 1-3-4, 5, 6) — strictly, a combina- 

 tion of three sections — of an 11 mm. embryo, showing the anlage of the abducens 

 nerve, when it possesses two roots and when its growing tip extends for some 

 distance anteriorly in the mesenchyma lying at the base of the medulla. Deeply 

 stained processes of medullary neuroblasts may be traced into the nerve anlage 

 with its sheath of granular protoplasm. The highest powers of the microscope 

 fail to resolve the neuraxon processes into constituent neurofibrils. As no evi- 

 dence of the migration of medullary elements into the nerve anlage appears in 

 this or in preceding stages, the cells in the vicinity of the nerve anlage are pre- 

 sumably mesenchymatous. 



33 A portion of a parasagittal section (KS 2-2-7) of an 13 mm. embryo, show- 

 ing a portion of the abducens anlage in a later stage of development than in 

 figure 32. The nerve anlage consists at this stage of a compact bundle of deeply 

 stained neuraxons surrounded more or less completely by a sheath of mesenchy- 

 matous cells. The section is somewhat exceptional in showing two isolated neu- 

 raxons which join the nerve anlage and each of which consists of a deeply stained 

 fibril surrounded by a very thin, lightly stained granular protoplasmic sheath. 

 Each of the neuraxons is also associated with a nucleus presumably of mesenchy- 

 matous origin. 



34 A portion of a parasagittal section (LAA 2-1-3) of a 19 mm. embryo, 

 showing a peripheral portion of the abducens anlage, consisting of a bundle of 

 highly refractive neuraxons surrounded by a sheatli of mesenchymatous cells 

 and granular protoplasm. 



■ 35 to 44 (See pp. 178 and 179). 



177 



JOURNAI- OF MORPHOLOGY, VOL. 25, NO. 1 



