PLATE 5 



EXPLAXATIOX OF FIGURES 



All the figures are based on camera drawings. Figures 23 to 28 inclusive are 

 a series of semi-diagrammatic drawings, illustrating the more important stages 

 in the development of spinal somatic motor nerves and summarizing the essen- 

 tial features of the histogenesis of spinal ventral nerves. Figures 29 and 30 are 

 camera drawings of spinal nerves with one-twelfth homogeneous oil-immersion 

 objective, No. 6 compensation ocular of Zeiss, and camera lucida of Abbe. Re- 

 duction one-third in reproduction. 



23 A semi-diagrammatic drawing of a portion of a cross-section (IH 1-6-9) 

 of a 6 mm. embryo in the region of the cloaca. The anlage of the somatic motor 

 nerve is seen as a structure consisting of the protoplasmic processes of neuro- 

 blasts in the ventrolateral wall of the neural tube. The migration of the mesen- 

 chyma of the sclerotome has already begun. 



24 A portion of a cross-section (IE 2-1-last), of a 9 mm. embryo in the caudal 

 region, showing a somewhat more advanced stage in the development of a ventral 

 nerve. The neuraxon processes of the medullary neuroblasts have extended 

 farther along the median surface of the myotome and the mesenchyma cells 

 have advanced to a point nearer the neural tube. 



25 A semi-diagrammatic drawing of a cross-section (IE 2-7-27) of a 7 mm. 

 embryo in the middle trunk region, showing the beginning of the migration of 

 medullary cells into the anlagen of somatic motor nerve and neural crest. The 

 relation of the fibrillar portion of the somatic motor nerve to neuroblasts is shown 

 in the figure. 



26 A semi-diagrammatic drawing of a portion of a cross-section (IH 2-1-20) 

 of a 9 mm. embryo in the trunk region showing a somewhat more advanced stage 

 in the development of a somatic motor nerve. Medullary cells are still in the 

 process of migration from the neural tube along the nerve anlage. The ganglion 

 of the dorsal nerve has been formed by the migration and aggregation of neural 

 crest cells between myotome and neural tube. A Rohon-Beard cell in an early 

 stage of formation of a neuraxon process appears in the dorsal wall of the neural 

 tube. 



27 A semi-diagrammatic drawing of a cross-section (KJ 3-4-middle) of an 

 11 mm. embryo in the middle trunk region, showing a more advanced stage in 

 the differentiation of a somatic motor nerve. The emigrated medullary cells 

 at this stage form a sheath enclosing the fibrillar portion of the nerve. Neuro- 

 blasts have become differentiated within the dorsal ganglion. A Rohon-Beard 

 cell with neuraxon process extending between myotome and ectoderm is shown. 



28 A semi-diagrammatic drawing of a cross-section (LAB 5-3-17) in the 

 middle trunk region of a 17 mm. embryo, showing a stage in the development 

 of a somatic motor nerve, when the anlage of a sympathetic ganglion has made 

 its appearance at a point opposite the dorsal aorta and median to the trunk of 

 the spinal nerve of the metamere. At this stage fibrous connection of the dorsal 

 ganglion with the neural tube has been established and both dorsal and ventral 

 rami have reached a considerable extent in their development. The migration 

 of medullary cells into the somatic motor root has ceased. The cells that have 



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