74 H. V. NEAL 



tained that the neurileniina was of mesenchymatoiis origin, 

 finally became convinced COo) of its ectodermal origin. 



By tracing the histogenesis of the nerve through closely con- 

 nected stages until adult conditions are established in Squalus 

 it is possible to demonstrate that a large number, if not all, 

 of the cells present in the anlage of the oculomotor become differ- 

 entiated as neurilemma cells. Some of these stages are shown in 

 the figures of plate 8 of this paper. 



The cells, which are at first peripheral to the bundle of fibers 

 (fig. 65), soon begin to penetrate among them (fig. 68) as stated 

 by Vignal ('83), Gurwitsch ('00), Bardeen ('03), Carpenter 

 ('06), and Gast ('09). Gast is probably correct in asserting 

 the centrifugal migration of the group of cells clustered about 

 the roots of the nerve anlage (figs. 64, 70) and their penetration 

 into the bundle of fibers. In fact the penetration of the fiber 

 bundle appears to occur largely in this manner, as stated by Gast. 



Altogether the evidence goes to show that the neurilemma 

 of the oculomotor has a similar derivation and differentiation 

 to that of somatic motor spinal nerves. 



h. What is the histogenesis of the neurilemma? Gast ('09, p. 

 428) summarizes the histogenesis of the neurilemma of the 

 oculomotor as follows: In its proximal portion the oculomotor 

 passes through four different developmental stages: (1) the naked 

 processes of the central neuroblasts extend to the mesocephalic 

 ganglion; (2) from the mesocephalic ganglion neurocj^tes (Kupffer 

 '90) wander centrad along the fibrillar bundle; these lie upon 

 and between the fibers and form about them, especiallj^ among 

 the roots, a loose plasmatic network; (3) the neurocytes arrange 

 themselves wholly on the surface of the nerve anlage, which at 

 this stage consists of a central fibrillar bundle with a cellular 

 sheath; (4) the neurocj'tes migrate back into the central fiber 

 bundle and are transformed into sheath cells. 



The results of a former ('98) and of the present study are 

 in essential agreement with this description of Gast. The evi- 

 dence derived from a study of Squalus embryos, however, seems 

 hardly to justify the inference of a forward and backward mi- 

 gration of neurocytes along the nerve anlage. Moreover, it 



