FROM TWELVE TO THIRTY-SIX SOMITES 
169 
apex, so that by the stage of 29 somites the opening is reduced 
to a minute ellipse situated on the external side of the dorsalmost 
portion of the otocyst (see Fig. 93). 
now begins to form a small conical 
elevation, and the final closure takes 
place on the external side of this 
elevation, which is destined to 
form the endolymphatic duct. The 
latter remains united to the epi- 
dermis at this point for a consid- 
erable period of time by a strand 
of cells which preserve a 
lumen up to 104 hours (Fig. 98). 
The final point of closure of the oto- 
cyst is thus very definitely placed, 
and it coincides with the middle of 
the endolymphatic duct, that is, 
with the junction of the later formed 
saecus and ductus endolymphaticus. 
In the Selachia this duct remains 
in open communication with the 
exterior throughout life; the rela- 
tively long persistence of its con- 
nection with the epidermis in the 
chick may thus be interpreted as ¢ 
phylogenic reminiscence of the an- 
cestral condition. 
The Nose (Olfactory Pits). At 
about the 28s stage, the ectoderm 
on the sides of the head a short dis- 
tance in front of the eyes appears 
thickened. Two circular patches of 
ectoderm are thus marked off, the 
beginning of the olfactory epithe- 
lium; at first this grades almost im- 
the 
may 
perceptibly into neighboring 
ectoderm. 
This portion of the otocyst 
Fig. 98. — Section of the otocyst 
of an embryo of 104 hours. The 
original opening of the otocyst 
is drawn out into a narrow ca- 
nal which connects with the 
endolymphatic duct (recessus 
labyrinthi). 
a., Ball of cells in the otocyst 
(otolith?). b., Canal leading from 
the surface to the otocyst. D. 
end’]., Endolymphatie duct. D., 
Dorsal. Ect., Eetoderm of the 
surface of the head. Gn., Audi- 
tory ganglion. L., Lateral. M., 
Median. V., ventral. 
In the stages immediately following the olfactory 
plates appear to sink down towards the ventral surface of the 
head, due no doubt to more rapid growth of the dorsal portion 
of the head. 
Thus they appear at the ventro-lateral angles of 
