94 ' H. V. NEAL 



anlage (plate 8). The earliest stage represented (fig. 31) shows 

 that the anlage, which already has two roots of origin, contains 

 deeply staining fibers surrounded by an envelope of granular 

 protoplasm. Mesenchymatous cells crowd thickly around the 

 roots of the anlage so that it is impossible to distinguish their 

 boundaries from the protoplasmic envelope. In this and later 

 stages (figs. 32, 41, 37) the continuity of the processes of the 

 medullary cells with the fibers of the anlage is unmistakable. 



In later stages the number of roots increases to the number 

 of five or six and there is a correlated increase in the number 

 of medullary cells, the processes of which may be traced towards 

 or into the roots of the anlage. In the meanwhile cells make 

 their appearance in the nerve anlage (figs. 33, 36), but the com- 

 parison of closely connected stages indicates that these cells 

 secondarily attach themselves to the anlage or, in advanced 

 stages, migrate from the neural tube (fig. 39). None of them 

 ever has the form or staining properties of neuroblastic cells, 

 such as appear within the ganglionic nerves or within the wall 

 of the medulla. Also, during the stages during which the fibers 

 most rapidly increase in number these cells are distinctly periph- 

 eral in relation to the bundle of fibers. Furthermore, the fact 

 that the fibers in successive stages grow centrifugally toward the 

 myotome of somite 3, and that, in the earlier stages of growth, 

 the number of fibers is greater in the proximal portion of the 

 anlage than in the distal portion (figs. 42-44), accords with the 

 supposition that the fibers have a medullary origin in central 

 neuroblasts. 



c. Have these protoplasmic connections a genetic relatio7i to the 

 neurofibrils? Students of the histogenesis of the abducens have 

 paid little or no attention to the differentiation of the neuro- 

 fibrils. There seems, however, little reason to doubt the genetic 

 connection between the deeply stained fibers of the nerve anlage 

 and the finer fibrils of the fully differentiated nerve. Their 

 most important histogenetic change appears to consist in the 

 splitting of the fibers into finer fibrils as the diameter of the 

 fiber increases during growth. The granular envelope, so con- 

 spicuous in the anlage, gradually disappears and is replaced by 



