PLATE 6: EXPLANATION OF FIGURES (CONTINUED) 



35 A portion of a parasagittal section (LB 2-2-6) of a 17 mm. embryo, sliow- 

 ing the peripheral termination and growing tip of the abducens nerve anlage 

 at a stage when it has nearly reached the posterior rectus muscle. The amoeboid 

 termination closely resembles those of somatic motor spinal nerves and of the 

 Rohon-Beard cells, and the section thus strongly favors the outgrowth theory 

 of nerve histogenesis and the opinion that the abducens in its histogenesis 

 resembles the spinal somatic motor nerves. 



36 A portion of a parasagittal section (LK 2-2-10) of a 19.25 mm. embryo 

 showing a portion of the abducens anlage in the vicinity of its point of emergence 

 from the medulla. The section is of interest chiefly because it shows a single 

 deeply stained fibril surrounded by a sheath of spindle-shaped cells. It was such 

 evidence as this that led Dohrn ('91) at one time to infer that the spindle-shaped 

 cells were genetically related to the neuraxon fiber. The evidence presented in 

 this paper, however, tends to show that there is no genetic relation between the 

 fiber and its surrounding cellular sheath. At this stage of development of the 

 nerve anlage the process of migration of medullary cells has already begun and 

 the ensheathing cells shown in the figure may be of medullary origin. 



37 A graphic reconstruction of the anlage of the abducens nerve as seen in 

 parasagittal sections of a 19 mm. embryo. The nerve arises from the base of the 

 seventh and eighth brain neuromere (Neal), four roots emerging from the seventh 

 and one only from the eighth. Its connection with these two distinct segments 

 of the brain suggests the inference that the abducens is the nerve of at least two 

 metameres. The posteriorly directed ramus recurrens of the abducens also sug- 

 gests that the nerve was once distributed to musculature posterior to that which 

 it now innervates. This branch soon atrophies. The roots of the nerve show 

 a tendency to form a network or plexus. 



38 A semi-diagrammatic drawing based on a parasagittal section through 

 the head of a 19 mm. embryo, intended to show the position of the niduli of the 

 eye muscle nerves. The niduli of the oculomotorius and of the trochlearis belong 

 to adjacent metameres, while the nidulus of the abducens is several neuromeres 

 removed. 



39 A portion of a cross-section (LF 3-5-19) in the region of the nidulus of the 

 abducens, showing evidence of the migration of medullary cells into the anlage 

 of the nerve. The phenomena are similar to those which appear in somewhat 

 advanced stages of histogenesis of spinal somatic motor nerves. As in the latter, 

 the nerve anlage contains both fibrillar and cellular components, the fibrillar 

 portion showing genetic relations with neuroblast cells in the ventro-lateral wall 

 of the neural tube. Evidence that the cellular portion of the nerve is medullary 

 and not mesenchymatous in origin is found in the fact that the cells first appear 

 in the proximal portion of the nerve anlage and that, in successive stages, tliere 

 appear to be more and more nuclei in this portion of the nerve, and a larger 

 number apparently in the process of migration. 



40 A portion of a cross-section (LAD 1-7-17) in the region of the nidulus of 

 the abducens of a 19 mm. embryo, showing the ventral half of the medulla as 

 seen under the low power microscope. The section shows the position of the 

 nidulus of the abducens and its relation to the bundle of deeply stained neuraxon 

 fibers which traverse the marginal zone and enter the nerve root. 



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