PLATE 8: EXPLANATION OF FIGURES (cONTINUED) 



70 The nidulus and roots of the oculomotorius anlage in a parasagittal sec- 

 tion (AIE 3-1-10) of a 21 mm. embryo, showing the increase in the number of 

 roots and the relation of the fibers of the nerve to deeply staining neuroblasts 

 in the ventral wall of the midbrain. With the growth of marginal zone of fibers, 

 the neuroblasts have receded from the limiting membrane of the wall of the 

 midbrain, but they still retain their deeply staining properties and their neu- 

 raxon processes may be easily traced through the marginal zone of fibers into 

 the roots of the nerve. 



71 A single neuroblast of the nidulus of the oculomotorius in a 46 mm. em- 

 bryo, showing the beginning of multipolarity and the relation of the neuraxon 

 process. 



PLATE 9 



EXPLANATION OF FIGURES 



. A series of diagrams designed to show the metameric relations of the e3'e mus- 

 cle nerves and their comparability with spinal somatic motor nerves. 



72 A diagram showing the topographic relations of dorsal and sympathetic 

 ganglia to a somatic motor nerve in the trunk region of a Squalus embryo of 

 15 mm. 



73 A diagram showing the relations of the oculomotorius nerve to a sympa- 

 thetic (ciliary) ganglion and to the ganglion of the ramus ophthalmicus pro- 

 fundus trigemini in a Squalus embryo of 11 mm. A comparison of these rela- 

 tions with those of a spinal somatic motor nerve as shown in figure 72 shows that 

 the}^ are essentially the same. The differences are such as would be brought 

 about, following a reduction of the myotome. The evidence favors the view of 

 the similar mode of formation of the s>anpathetic ganglion. The reduction of 

 the myotome of VanWijhe's first somite allows the precocious connection of the 

 profundus nerve with the skin and brings about the slightly different topographic 

 relations of nerve, ganglia and myotome shown in the diagram. 



74 A diagram showing the relations of the abducens nerve in a Squalus em- 

 bryo of 13 mm. Like the primitive somatic motor nerves of Amphioxus, the 

 abducens does not become connected with a dorsal nerve and (therefore ?) is 

 not associated with any sjonpathetic ganglion. Its nidulus, that is, its central 

 relations, and its distribution to somatic musculature, however, sufficiently attest 

 its serial homology with spinal somatic motor nerves. The nidulus of the ab- 

 ducens is elongated, extending through two brain neuromeres (VH and VIII), 

 from both of which the cells which form the vagus ganglion are proliferated. 



75 A diagram showing the relations of the trochlearis in a Squalus embryo 

 of 25 mm. Except for the dorsal chiasma, the relations are similar to those of 

 the oculomotorius (fig. 73). The nidulus of the trochlearis lies in the same zone 

 as that of the oculomotorius and immediately behind it in the floor of the cere- 

 bellar anlage (third brain neuromere). In precisely the same way that the ocu- 

 lomotorius is associated with the profundus nerve and the ciliary ganglion, the 

 trochlearis is associated with the ramus ophthalmicus superficialis trigemini and 

 with a transient (?) sympathetic ganglion. 



76 to 82 (See p. 186). 



185 



