262 . 
epithelium, the motor elements of the anterior segment yield to the 
sensory. 
Prior to the development of the oculomotorius, both the „thalamic‘“ 
nerve and the ganglion cells which have fused with the nasal epithe- 
lium lose their connection with that part of the primitive Anlage 
which was common to both, and which now remains as the distal 
portion of the ophthalmicus profundus trigemini, while the nerve tract 
connecting originally the ciliary ganglion with the trigeminal and trochlear 
Anlagen remains as the proximal portion of the ophthalmicus pro- 
fundus. 
Fragments of the atrophying „thalamic“ nerve are to be found, 
after the establishment of the oculomotorius, in the shape of small 
groups of nerve cells, resembling those left by the primitive trochlearis. 
as it also disappears. 
I have given in fig. 13 the relation to the fifth and seventh 
nerves, and to the hyomandibular and first gill clefts, of the head 
somites posterior to the praemandibular cavity. At this stage the 
first two clefts have not opened to the exterior, and are consequently 
blind pockets of the alimentary canal. It will be seen that three 
somites lie above the hyoid arch, where van WIHE (loc. cit.) found 
in Seyllium but two. Anterior to the first gill cleft (el. 7), Acan- 
thias has therefore seven pairs of somites, three for the hyoid arch, 
two for the mandibular, one praemandibular, and one „anterior“. 
- 
5. The mouth and the praemandibular cavity in 
Batrachus Tau. 
My study of the development of the mouth in Batrachus, but 
confirms the work published many years ago by DonrNn!) on the deve- 
lopment and significance of the Teleostean mouth. At an early stage 
a pair of pockets from the alimentary canal open to the exterior, 
anterior to the hyomandibular clefts (fig. 15 m). Much later in the 
development of the embryo, the mouth breaks through in the ventral 
region of these pockets as a bilateral involution of the ecto- 
derm, fusing with the endoderm and opening to each 
side of a central partition, sometime before the median 
line is crossed. The little fish is at this stage so large that the 
double nature of the mouth involution may be seen without the aid 
of a lens. In early stages the mouth cleft resembles in every parti- 
cular the hyomandibular cleft, and anterior to it there is found a 
1) A. Donen, „Der Mund der Knochenfische“. Mitteilungen a. d. 
Zool. Station z. Neapel, Vol. II, 1881. 
