282 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ment of the secondary roots is the same as that described for the pri- 
mary one, viz. as processes from neuroblast cells in the ventral horn. 
By following the fibres of the roots in the wall of the brain, it is 
easily ascertained that the motor “nucleus” of the abducens is a 
very elongated one, as is known to be the case in higher Vertebrates 
(see Edinger, *96). 
The study of the development of the abducens is simpler than that of 
the oculomotorius, since the nerve never comes into relation with a 
ganglion, and thus resembles the primitive ventral nerves of Amphioxus 
more than do the ventral spinal nerves. The gradual extension of its 
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fibres through the mesenchymatous tissue at the base of the medulla 
may therefore be easily followed. It is a matter of some morphological 
importance, in my opinion, that not all the nerve fibrils extend anteriorly 
toward the third somite (van Wijhe’s), but that in later stages of develop- 
ment, e.g. in embryos with 78-80 somites (Plate 4, Fig. 20), a nerve 
fibril is seen to pass from the posterior root of the nerve in a posterior 
direction toward the myotome of the sixth somite, which has at this stage, 
however, only a few rudimentary muscle fibres. Miss Platt (°91) like- 
wise has mentioned the fact that this nerve also distributes fibres to 
mesoderm posterior to the third somite (muse. rectus posterior). In the 
abducens, therefore, we have to do with a post-otic ventral nerve, 
Fic. J. Parasagittal section of a Squalus embryo with 60 somites, showing the 
abducens as a fibril formed by the processes of at least four neuroblast cells. 
X 447. abd., abducens; cl. n’bl., neuroblast cell. 
