298 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



applicable to yolk-ladened embryos (amphibians, teleosts, and Petromyzon in early 

 stages of development). It is a neurological rather than a histological method. 



III. THE HISTOGENESIS OF VENTRAL NERVES. 



The chief constituents of a differentiated ventral nerve are: (1) the nidulus or 

 "motor nucleus," which is in the ventral horn of the neural tube and consists of mul- 

 tipolar ganglion-cells and dendritic processes; (2) the nerve-fibres, each of which is 

 composed of a neuraxon (axis-cylinder process), a neurilemma (Schwann's sheath), 

 a medullary sheath, and a motor termination; and (3) the connective tissue sheath 

 (epi- and peri-neurium) . 



The histogenesis of intracellular structures such as centrosomes, neurofibril^, 

 etc., does not come within the scope of the present paper, the purpose of which is to 

 present evidence bearing on the question of the morphology of the eye-muscle nerves. 

 This evidence will be presented under three chief heads: the development of the 

 nidulus, the histogenesis of the neuraxon and the neurilemma, and the development of 

 the epi- and peri-neurium. 



i. The Development of the Nidulus — A. Historical. Most of our knowledge of 

 the histogenesis of the nidulus of ventral nerves we owe to His ('79, '89). The chief 

 facts, as stated by His, are as follows: At the time of closure of the neural tube 

 its lateral walls are composed of undifferentiated columnar epithelial cells. Out of 

 these are soon differentiated two kinds of cells : (1) epithelial cells, which undergo no 

 further division and which form the neuroglia cells of the adult ("spongioblasts" of 

 embryonic stages, His); and (2) germinative cells, rounded cells near the lumen of 

 the neural tube, which retain the power of multiplication. The germinative cells 

 migrate laterad in the neural tube (Herms, '84), become pear-shaped, and develop 

 neuraxon processes which grow toward the adjacent myotomes. They thus become 

 "neuroblast cells," a group of which gives rise through their neuraxon processes 

 to a ventral nerve and constitute the nidulus or "motor nucleus" of that nerve. The 

 dendritic processes of these cells are formed considerably later than the neuraxon 

 processes? The researches of von Kolliker ('85, '92, :00), Vignal ('88), Schaper ('94), 

 and Giglio-Tos (:02) have made it probable that the "Keimzellen" or germinative 

 cells of His are simply ordinary epithelial cells in mitosis and that they do not give 

 rise exclusively to neuroblasts. The only other disputed point is the extent of the 

 migration of the neuroblasts. Dohrn ('88) maintained that some of these migrate 

 from the neural tube into the roots of the nerve. His in a later study ('89) denied 



