220 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in function, and to contribute to the mesenchyma of the head." Th 
ear capsule now lies with only its anterior portion opposite encephalo- 
mere VI. Behind the ear capsule and opposite the posterior portion 
of encephalomere VII lie the cells of the glossopharyngeus, as yet with- 
out fibrillar connections with the neural tube. Behind the glossopha- 
ryngeus and now separated from it lie the cells of the vagus, extending 
ventrally as a broad sheet between the mesoderm and ectoderm into 
the region of the pharynx, where the Anlage becomes segmented by the 
formation of the visceral clefts. The trochlear and thalamic portions 
of the trigeminus soon disappear without assuming fibrillar relation 
with the neural tube. 
At a stage with 52 somites, when the embryo is about 8 mm. in length, 
the thalamic portion remains as a group of cells lying in the constriction 
between the forebrain and midbrain vesicles (Plate 4, Fig. 18), but wi*h- 
out connection with the ophthalmicus profundus. It very soon disappears 
entirely, and I think probably contributes to the loose mesenchyma of 
this region. In precisely the same way the disassociation of cells of 
the trochlear portion takes place, scattered clumps of cells indicating 
its previous extent. The Gasserian ganglion and the ganglion of the 
ramus ophthalmicus profundus (mesocephalic ganglion) are both clearly 
differentiated. Three branches of the fifth nerve may now be distin- 
guished, viz. the two sensor branches, r. ophth. profundus and r. maxil- 
laris (inframaxillaris? Dohrn), and the mixed mandibular branch. Nerve 
relations to the neural tube remain the same as in the previous stage. 
d. DEVELOPMENT OF THE 
1. OCULOMOTORIUS, 
By the time the embryo has reached the length of 8 mm. (52 so- 
mites), the oculomotorius has however appeared as a fibrillar process 
from the base of the midbrain (encephalomere II, Figures F to H), arising 
as processes from neuroblast cells in the ventral horn of this encephalo- 
mere. Since this nerve throws light on the morphology of the pre- 
mandibular somite, whose musculature it innervates, its development is 
of great interest and has been studied by many investigators ; viz. Mar- 
shall (’81), Rabl (89), Dohrn (91), Platt (91), Mitrophanow (93), 
and Sedgwick (94). Neither Marshall ('81) nor Rabl (89) saw the 
1 Kastschenko (’88), Goronowitsch (’92), Miss Platt (93). 
2 This is, I believe, the nerve which in Ceratodus van Wijhe (’82) named ramus 
maxillaris superior, which in Amphibia Strong ('95) called accessory branch of 
the fifth, and Miss Platt (’96) r. buccalis profundus V. 
