940 THE AMERICAN NATURALIST. [VoL. XXXIV. 
beginning of the first and second gill-pouches. Between the 
pharynx and the raphe the basilar artery (a77.) has been cut 
transversely. The otocyst is a large epithelial vesicle with 
three well-marked divisions: first, the common chamber (.S.c.), 
out of which the three semicircular canals are to be differenti- 
ated; second, the slender canal (D.e.), the anlage of the ductus 
endolymphaticus, which lies between the semicircular canals 
and the medulla; third, the long, curving, but not spiral, 
cochlea (Coch.). The common chamber formed by the union 
of these three divisions is later subdivided to form the upper 
utriculus and lower sacculus. Outside the cochlea lies the 
cross-section of the jugular vein (/zg.), just below which is 
the section of the motor portion (Fac.m.) of the facial nerve. 
The sensory portion of the facial is much smaller, runs only a 
short distance downwards from the geniculate ganglion, and is 
entirely separate from the motor portion. It seems to me, 
therefore, that the anatomical description of this nerve, in view 
of the embryonic conditions, needs revision. | 
The absence of advanced stages of histological differentiation 
at this period of development is a general and characteristic state 
of the tissues. Especially noteworthy is the simple character 
of the mesenchyma, for it offers no marked differentiation 
except that of the merely endothelial blood vessels and their 
contained blood. The blood corpuscles are all nucleated red 
cells, which are for the most part in embryos of twelve milli- 
meters, in a transitional stage. In younger pigs the corpuscles 
are in the ichthyopsidan stage and have a full-sized nucleus 
with quite distinct chromatin granules. In older pigs the cor- 
puscles have much-contracted nuclei, which stain darkly, as in 
the blood cells of Sauropsida. In our embryo the nuclei are 
partly contracted and modified. The mesenchyma proper is 
very voluminous and constitutes a large proportion of the 
total bulk of the body. Its cells vary much in the degree in 
which they are crowded in the different territories of the tissue, 
and they vary also in form, but they can hardly be said to offer 
any positive specialization, and there is probably at this stage 
no differentiation of the fibrils and elastic networks, which are 
so conspicuous in many of the adult tissues which are derived 
