No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 927 
protuberant first or hyoid branchial arch (77y.), and this arch is 
separated from the mandibular region by a groove (4z.) or ecto- 
dermal depression, which is later transformed into the meatus 
auditorius externus. In mammals there are four pairs of gill- 
pouches, or so-called clefts, all of which are lateral diverticula 
of the pharynx. Owing to the curvature of the pharynx these 
diverticula are brought to different levels, and into different 
planes in the pig embryo at the stage we are considering; 
hence in a series of sections the various gill-pouches are 
encountered in different portions of the series, and thus it 
occurs that only one pouch, the third, is shown in the sec- 
tion. The student should clearly understand that the median 
region of the two-thirds gill-pouches in Fig. 5 is the pharynx 
proper, and that it gives rise to the anlage of the median thy- 
roid, of which both the stalk (JZ¢/.) and the glandular portion 
(Thyr.) appear. Just above the third gill-cleft may be seen the 
large, darkly stained lower ganglion (ganglion nodosum) of the 
vagus nerve (G.zod.), and just above the ganglion is situated 
the section of the jugular vein (/ug.). Close to the ganglion 
on its ectal side appear two fibrous nerve trunks, of which the 
one nearest the pharynx is the accessorius or eleventh nerve 
(N.x.t.) while the other nearest the jugular vein is the hypo- 
glossus, which reappears (JV.xzz.) below the aortic arch. A 
little above the jugular vein is the section of the first cer- 
vical nerve (Cerv.7) laterad from which is the external 
branch (A.ex.xz. of the spinal accessory nerve. This branch 
in the adult innervates the sternocleido-mastoid and trapezius 
muscles. 
The lower part of Fig. 5 represents part of the head and 
shows the two nasal fossz (/Va.) closed towards the mouth side 
by the olfactory plate (O/.p/.), an epithelial membrane some- 
what similar to the closing plate of a gill-cleft. On the dorsal 
side of the olfactory fosse — below in Fig. 5 — the cerebral 
hemispheres are cut, their darkly stained wall bounding on each 
side the large lateral ventricle (Z. V.). 
The next section figured is No. 470, and is therefore much 
lower in the series. It was selected in order to show the 
anterior limb-buds, the ducts of Cuvier, and the heart. The 
