PLATE 7: EXPLANATION OF FIGURES (CONTINUED) 



52 A cross-section of the two roots of the trochlearis anlage near the point 

 of emergence from the neural tube as seen in a parasagittal section (MD 5-1-3) 

 of a 22 mm. embryo. At this stage the nerve anlage consists exclusively of 

 deeply stained neuraxon fibers devoid of nuclei. A sheath of granular proto- 

 plasm, however, may be distinguished and mesenchymatous cells show loose 

 relationships with the distal portion of the nerve. 



53 One of the roots of the trochlearis as seen in a parasagittal section OA 

 3-1-8) of a 45 mm. embryo, showing evidence of the migration of nuclei from the 

 neural tube into the roots of the nerve. Proximally the nerve root is cut longi- 

 tudinally, while distally it is cut transversely. Nuclei appear within the nerve 

 anlage among the neuraxon fibers and also partly within and partly without 

 the neural tube. No mesenchymatous cells are closely associated with the nerve 

 anlage near its point of emergence from the neural tube, and there is no evidence 

 that the nuclei lying within the roots of the nerve have any other than a medul- 

 lary origin. 



54 A portion of a cross-section (MAG 1-6-7) of a 25 mm. embryo in the region 

 of the optic vesicle, showing a peripheral portion of the trochlearis anlage at 

 a stage when it has reached its termination in the superior oblique muscle. 

 The section shows the relation of the trochlearis to the median surface of the 

 muscle and to the ramus ophthalmicus superficialis trigemini which extends 

 along the dorsal surface of the muscle, appearing in cross-section in the figure. 

 Between the two nerve anlagen lies a mass of cells (gn.sym.) presumably de- 

 rived from the r. ophth. sup. trigemini (Miss Piatt '91). Both of the distal 

 branches of the trochlearis anlage show a similar aggregation of cells where they 

 cross the ramus ophthalmicus superficialis trigemini. Their derivation and rela- 

 tions show that such groups of cells are to be regarded as sympathetic ganglia 

 and not as primitive sensory ganglia of the trochlearis. They afford as little 

 evidence of the sensory character of the trochlearis as do the sympathetic gan- 

 glia associated with somatic motor spinal nerves. 



55 A graphic reconstruction of the trochlearis anlage and the structures 

 associated with it made from a series of parasagittal sections (MAA 3, etc.) of 

 a 24 to 25 mm. embryo as seen from the right lateral aspect under low power 

 microscope. At its anterior extremity the ramus ophthalmicus superficialis tri- 

 gemini terminates in a group of cells derived from the neural crest. The group 

 of cells (gn.sym.) associated with the same nerve at the point of crossing of the 

 trochlearis have a similar origin and their relations to the trochlearis anlage 

 are secondary. 



56 A graphic reconstruction of the head of a 25 mm. embryo as viewed from 

 the left lateral aspect, based upon parasagittal sections (MAH 3, etc.), showing 

 the relations of the cranial nerves at this stage. In order not to complicate the 

 figure, only two of the eye muscles are shown. 



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