MORPHOLOGY OF EYE MUSCLE NERVES 



163 



vnj., myotome 



my .^•'^•^'^' myotomes of VanWijhe's so- 

 mites 1 to 8 

 my.'^^., dorso-lateral division of the 



second myotome 

 my.-^^., ventro-lateral division of the 



second myotome. 

 my.-™^., median division of the second 



myotome 

 my.'^^^., dorso-lateral division of the 



first post-otic myotome 

 my.'^^^., ventro-lateral division of the 



first post-otic myotome 

 my.'*™., median division of the first 



post-otic myotome 

 /(., olfactory pit 

 nidi., nidulus, or 'motor nucleus' of 



nerve 

 nidl.abd., nidulus of abducens nerve 

 nidl.oc, nidulus of oculomotor nerve 

 nidl.tr'ch., nidulus of trochlearis nerve 

 n'v.'', first post-otic nerve in Petromy- 



zon 



n'r.-, second post-otic nerve in Pctro- 

 myzon 



oc'mot., oculomotorius nerve anlage 



oL, otic vesicle 



pi., plasma 



rx.d., fiber bundle of dorsal nerve 



rx.v., fiber bundle of ventral nerve 



scL, sclerotome 



so., somite 



sp., spiracle 



subch., sub-chordal rod 



ib.n., neural tube 



thyr., thyreoid 



tr'ch., trochlearis 



tr'ch.dors., dorsal branch of the troch- 

 learis nerve 



tr'ch vent., ventral branch of the troch- 

 learis nerve 



tr.art., truncus arteriosus 



vac, vacuole 



vag., vagus nerve 



vn.crd., cardinal vein 



vs. opt., optic vesicle 



PLATE 1 



EXPLANATION OF FIGURES 



All the figures of this plate were drawn with Abbe camera, one-twelfth homo- 

 geneous oil immersion objective and No. 6 compensation ocular of Zeiss. In 

 reproduction the magnification has been reduced by one-third. The series of 

 drawings illustrates the stages just preceding and following the appearance of 

 the anlagen of somatic motor nerves in the trunk region of Squalus embryos. 



1 A portion of a cross-section (DK 2-3-10) of a Squalus embryo with eight 

 somites (Stage D of Balfour) in the middle trunk region showing the relations of 

 neural tube, somite, and chorda. The absence of plasmodesmata or protoplasmic 

 strands connecting neural tube and somite is to be noted. A vacuolated plasma 

 fills the intercellular space between the neural tube and the myotome. 



2 A portion of a cross-section (IK 3-2-37) of a Squalus embryo of 6 mm. 

 (Balfour's Stage I) in the middle of the trunk region, showing the conditions 

 just previous to the appearance of a ventral nerve anlage. The section is taken 

 from an embryo of about twice the length of the one from which figure 1 was 

 drawn. The section shows no indication of protoplasmic or nervous connection 

 between the neural tube and the somite. The outflow of processes from the 

 sclerotome cells {scl.) is the beginning of the movement of mesenchymatous cells 



