934 THE AMERICAN NATURALIST. [VoL. XXXIV. 
eighth, ninth, and tenth. As the section is remote from the 
median plane, little of the brain appears, there being only a 
shaving from the lateral wall of the fore-brain (#.) and a sec- 
tion of the widest part of the hind-brain, which shows the 
cavity or lateral recess (A.L.) of the fourth ventricle. The 
auditory vesicle or otocyst (O£. is cut; its narrow upward 
prolongation is the anlage of the ductus endolymphaticus. 
The otocyst lies in a line with the great cephalic ganglion, 
occupying its invariable and permanent position behind the 
acustico-facial ganglion (4c.F.) and in front of the glosso- 
pharyngeal. Only the lateral portion of the pharynx (PA.) 
appears ; this portion forms a wide diverticulum, almost slit- 
like (compare Fig. 4), from which extend the first and second 
entodermal gill-pouches. In the figure the pharynx has a 
small depression extending downward at the cesophageal end of. 
the part shown in the section; this depression marks the be- 
ginning of the second cleft. Nothing is seen of the third and 
fourth clefts in the section, as these both lie nearer the median 
plane. The pocket or diverticulum of the cervical sinus (com- 
pare Fig. 1) appears (Cerv.S.) near the ganglion nodosum 
(G.nod.). It might be mistaken for a gill-cleft,. but it is 
lined by ectoderm and its cavity can be easily traced through 
the series to the exterior. Cephalad from the sinus but 
close to it lies a small dark mass, the anlage of the thy- 
mus gland. The mass is produced by proliferation of the 
entodermal cells on the anterior side of the third cleft (com- 
pare Fig. 5, Zim.), and is penetrated by blood vessels which 
seem to be sinusoids, although their history has not been 
worked out. The jugular vein (/zg.), owing to its irregular 
course, is cut several times ; its main stem ( /ug."") rises nearly 
vertically through the cervical region, and is, relatively to the 
size of the embryo, of huge diameter, and continues (Jug.") 
upward along the dorsal side of the vagus nerve to a level 
about halfway between the ganglion nodosum and ganglion 
jugulare, and as the vessel there curves inwards and forwards 
it is not encountered again until it bends outwards (/ug.’) on 
its way past the trigeminal ganglion. A branch of the jugular 
vein (/ug.") is cut just above the ganglion. 
