The Development of the Olfactory Organ in the Teleostei. 623 
It can be seen how the groove deepens and in Fig. d, it is 
closed and situated under the ectoderm. In Fig. e, the solid base 
of the organ is found separated from the ectoderm and the olfactory 
nerve connecting the fore brain with the organ is fully formed. The 
exterior parts of the fosse retain the character of ordinary epithe- 
lium, but in the interior part the epithelium has developed into an 
olfactory one, with plenty of nerve cells. The nerve divides into a 
large number of fibres intersecting the base of the organ. In series 
from somewhat older embryos I have found the nerve divided into 
two or three branches, which enter the organ from different sides. 
From this time until hatching there is little to be found that is new 
except an increase in size, but then the cartilage of Tegmen Cranie 
and the Palato Pterygoid bar commence to develop and with that 
an alteration in the position of the organ takes place. It has before 
been on the ventral side, but through this development it is gradu- 
ally pushed forwards and upwards, so that in an embryo of 18 mm 
length (of which a sagittal section is shown in Fig. 5) the organ is 
laying in the front as far as possible. The ciliated epithelium has 
now appeared and the character of the organ is much like fully 
developed ones, with the exception of size and lack of folds. 
Hither to there has been plenty of room for both the organ and 
the cartilage to develop, but from this time the forming bone pre- 
vents any further growth inwards, as is shown in Fig. 6, repre- 
senting a transverse section of a somewhat older stage. The base 
of the organ has approached the Ethmoid closely and any further 
growth must take place sideways. Fig. 7, Plate XVII, shows a 
transverse section through the organ of a young fish of 60 mm length, 
probably a year old or more, where the lateral extension and the 
folds have developed to a great extent. The olfactory membrane 
has different thicknesses, being thicker in the grooves than on the 
protuberances; it is ciliated and has the characteristic appearance 
of olfactory epithelium. It is thus seen that the development of 
the olfactory organ of the Salmo salar more closely resembles that 
of the Elasmobranch fishes than that of Lepidosteus, where, as 
known, the pit is caused by a hollow being formed between the 
exterior cell layers and the interior ones of the thickened patch of 
ectoderm. The interior part develops into olfactory epithelium, the 
exterior is broken through and thus the pit is formed. 
Since the conclusion of this work, Kuprrmr’s research on the 
single olfactory organ in Acipencer embryos and Ammocoetes has 
