THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 311 



Fig. 1. A cross-section of an 8-mm. embryo showing the most posterior and youngest ventral nerve of the em- 

 bryo. The nerve consists of a single deeply stained process of a medullary cell. The process extends 

 a short distance along the median surface of the myotome. Except for the possession of a process the 

 cell is not different from the adjacent epithelial cells. The migration of mespnchyme cells from the 

 sclerotome of the somite has begun. Oc. 1, obj. yV homogeneous oil immersion. 



Fig. 2. The ventral nerve of the left side of the second metamere, anterior to that shown in Figure 1. The processes 

 of several medullary cells now form the nerve, and these have extended farther along the face of the 

 myotome. The peripheral termination of the nerve shows no intimate connection with the muscle, i.e., a 

 cell membrane separates them. Magnification as Figure 1. 



Fig. 3. The ventral nerve of the right side of the metamere next anterior to that shown in Figure 2. The processes 

 of four or five medullary cells may be traced into the forming nerve, of which the peripheral termina- 

 tion is seen to lie median to the myotome. The protoplasm of the neuroblast cells is vacuolated and 

 granular like that of adjacent epithelial cells. Comp. Oc. 6, obj. iV homogeneous oil immersion. 



Fig. 4. A cross-section of a 10-mm. embryo showing one of the more posterior of the forming ventral nerves. The 

 nerve is in a stage of development slightly in advance of that shown in Figure 3. The nerve is slightly 

 more elongated and some evidence of fibrillation appears in the cell processes that compose the nerve. 

 Oc. 1, Zeiss 4.0 apochromatic objective. 



Fig. 5. The entire cross-section from which Figure 4 was drawn. A circle indicates the portion shown in that 

 figure. The other figures on this plate and the next were taken from similar regions. 



Fig. 6. The ventral nerve of the second metamere anterior to that shown in Figure 4. Compared with the latter 

 the nerve is more elongated and fibrillar. No nuclei have yet appeared in the course of the nerve. 

 Magnification as in Figure 1. 



Fig. 7. The nerve of the third metamere anterior to that shown in Figure 6. The section shows only the posterior 

 portion of the nerve and gives evidence of the beginning of the migration of medullary cells from the 

 neural tube. The unbroken continuity of the outer limiting membrane of the neural tube and of the 

 nerve makes it possible to distinguish between cells of medullary origin and those of the adjacent mesen- 

 chyme. Magnification as in Figure 1. 



Fig. 8. The posterior portion of the nerve next anterior to that shown in Figure 7. The exit of medullary cells 

 and the continuity of the outer limiting membranes of the neural tube and the nerve can be clearly seen. 



Fig. 9. The ventral nerve from the third segment anterior to that of Figure 8, showing the two chief constituents 

 of the ventral nerve : neuraxon processes of medullary cells, and cells whose medullary origin is shown 

 by comparison with earlier stages and by the continuity of outer limiting membranes of the nerve and 

 neural tube. In the section shown, the process (n'ax.) of a neuroblast cell (cl. n'bl.) can be clearly traced 

 into the nerve as a very deeply stained fibre. The migrating medullary cells at tliis stage all lie ventral 

 and posterior to the neuraxonic processes. Magnification as in Figure 1. 



Fig. 10. A frontal section of a 9-10-mm. embryo, showing neuroblast cells with deeply stained neuraxones 

 extending into the nerve. The lateral migration of the neuroblasts has begun, and they have lost 

 their connection with the inner limiting membrane of the neural tube. In some cases traces of this 

 connection remain as deeply staining processes which extend toward the inner membrane. Comp. 

 Oc. 6, obj. TI homogeneous oil immersion. 



Fig. 11. A frontal section of a much later stage (14-mm. embryo) showing the increased thickness of the wall of 

 the neural tube as the result of multiplication of cells, and the increased separation of the neuroblast 

 cells from the inner limiting membrane. Evidence of the continued migration of medullary cells into 

 the nerve is also shown. Magnification as in Figure 1. 



PLATE XXIII. 



All the figures are camera drawings of portions of cross-sections of a Squalus acanthias embryo of 12-13 mm. 

 All except Figures 20-23 are magnified 358 diameters. Figures 12 to 19 form a series, of which Figure 12 is the 

 most posterior, and show the histogenesis of ventral nerves in stages immediately following that represented in 

 Figure 9 (PI. XXII). Figure 14, however, is a drawing of one of the so-called transient ganglion cells (Beard, '96). 

 In all figures are seen the two chief elements of ventral nerves, the fibrillar (neuraxones) and the cellular (neuri- 

 lemma), the relations of which are seen to change gradually in the series until the cells completely surround the 

 neuraxones as sheaths (Fig. 19). The relations of the cells in the forming nerve to the neuraxones do not justify 



