THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHH. 313 



PLATE XXIV. 



All figures except Figures 29, 30, and 32 represent longitudinal sections of spinal ventral nerves in different 



stages of development. Figures 29 and 32 were drawn from cross-sections of spinal ventral nerves. All, except 



Figure 30, are camera drawings with compensation ocular 6 and iV oil-immersion objective. 



Fig. 24. The proximal portion of a nerve showing the bundle of neuraxones surrounded by a cellular sheath, whose 

 medullary origin has been traced. The section gives no evidence that the cells of the sheath participate 

 in the formation of the neuraxones. 



Fig. 25. A more distal portion of the same nerve, showing the same marked difference between fibrillar and cellular 

 components as in Figure 24. The sheath-cells are peripheral in position, as in the proximal portion of 

 the nerve. No mitoses were found at this stage nor in any of the previous stages figured. 



Fig. 26. A portion of a nerve from a 55-mm. embryo. Cells appear among the neuraxones as well as in the 

 peripheral sheaths. Since there is no other more probable source for the interfibrillar cells than the 

 peripheral sheath, they must be regarded as having migrated inward from this. They are, therefore, 

 ectodermal (medullary) in origin. They are destined to form the neurilemma. The bending of the 

 neuraxones makes exact longitudinal sections impossible. 



Fig. 27. A portion of a ventral nerve from an embryo of 125 mm. The nuclei of the neurilemma are more abun- 

 dant than in the last section, and the medullary sheath appears on the neuraxones. 



Fig. 28. A portion of a nerve of a 200-mm. embryo, showing conditions which are practically the same as adult. 

 Between the fibres, which now have medullary sheaths and neurilemma, a vacuolated interfibrillar 

 substance has made its appearance. 



Fig. 29. A cross-section of the same nerve as that shown in Figure 28, giving the typical elements of a nerve in 

 their well-known relations. 



Fig. 30. A cross-section of the spinal cord of a 200-mm. embryo, showing the nidulus of a ventral^nerve at a stage 

 like that shown in Figures 28 and 29. 



Figs. 31 and 32. Longitudinal and cross-sections of the ventral nerve of an adult Squalus. 



