312 THE DEVELOPMENT OP THE VENTRAL NERVES IN SELACHII. 



the inference that they are concerned in the formation of the neuraxones. The connection between the neuraxones 



and neuroblast cells in the neural tube, however, is seen at all stages of development. 



Fig. 12. A portion of a cross-section of a 12-13-mm. Squalus embryo, showing a ventral nerve at a stage of develop- 

 ment slightly in advance of that shown in Figure 9 (PL XXII). The neuraxon processes of the medul- 

 lary cells can be traced into the nerve, and the cells of the nerve are obviously of medullary origin. The 

 fact that the long axes of the cells of the nerve are at right angles to the neuraxones is against the 

 assumption that these cells form the neuraxones. The fact also that they are granular and vacuolated 

 points in the same direction. The continuity of the external limiting membrane of the neural tube 

 with that of the nerve points to the medullary origin of the cells in the nerve. 



Fig. 13. The nerve of the left side of the same metamere as that in Figure 14, showing clearly the double nature 

 of the nerve and the typical relations at this stage of the two components. The conditions do not differ 

 essentially from those shown in Figure 12. 



Fig. 14. A "giant ganglion cell" (Beard) from the same metamere as that drawn in Figure 13, showing a stage in 

 the development of its neuraxon process when the latter extends to a region between the somite and 

 the ectoderm. This single large neuraxon illustrates well the method of development of the neuraxones 

 of ventral nerves as protoplasmic processes of medullary cells. Such neuraxones as the one figured 

 have been traced from the time when they barely protrude beyond the medullary wall until they have 

 twice the extent of the one represented in this figure. All the evidence points to the conclusion that 

 this neuraxon is ah initio connected with its terminal organ, but not in the intimate way maintained 

 by some histologists. 



Fig. 15. The ventral nerve of the second metamere anterior to that shown in Figure 13, giving a more advanced 

 stage in the development of a ventral nerve. The number of cells in the nerve has increased entirely 

 through the migration of medullary cells. Neuraxon processes of medullary cells can be traced into 

 the nerve. There is no evidence that the migrated medullary elements (ms'ec'drm.) are concerned in 

 the formation of the fibrillar (neuraxon) portion of the nerve. 



Fig. 16. The ventral nerve (left side) of the metamere anterior to that shown in Figure 15. A slightly more 

 advanced stage. This section shows the migration of neurilemma (ms'ec'drm.) cells to the dorsal side 

 of the nerve, to form a cellular sheath around the neuraxones, that is, around the fibrillar portion of 

 the nerve. This dorsal migration of the cells is correlated with the growth of the neuraxones dorsad 

 along the myotome. 



Fig. 17. The ventral nerve (right side) of the metamere anterior to that shown in Figure 16. The division of the 

 fibrillar portion of the nerve into a ramus ventralis and a ramus dorsalis is clearly seen. The vacuolated 

 neurilemma cells give no evidence of participation in the genesis of the neuraxones. On the other hand J 

 the deeply stained and (in some cases clearly) fibrillar processes of the medullary cells can be traced 

 into the nerve. 



Fig. 18. The ventral nerve (left side) of the second metamere anterior to that shown in Figure 17. The section 

 shows two important conditions: first, the sharp demarkation between the fibrillar and the cellular por- 

 tions of the nerve, such as would not be expected did the cells "spin" the neuraxones; and secondly; 

 the continuity of the external limiting membrane of the neural tube with that of the nerve, evidence 

 pointing to the medullary origin of the cells in the nerve. 



Fig. 19. The ventral nerve (right side) of the eighth metamere anterior to that given in Figure 18, showing a con- 

 siderably more advanced stage in development. Except in the matter of length, however, the conditions 

 in the two stages are practically the same. In this stage and in subsequent stages there is no evidence 

 of a further migration of cells from the neural tube into the nerve. The processes of medullary cells 

 can be traced into the nerve. 



Figs. 20-23. A series of camera drawings of four stages in the development of ventral nerves from a 12-mm. embryo} 

 magnified about 50 diameters, of which the section shown in Figure 20 is the most posterior. The series 

 illustrates the change in the ventral contour of the neural tube accompanying the migration of medul- 

 lary cells, and the cessation of this migration. 



