190 Observations on the Development [*^SSi,^STiS^ 
of granules. On each side of the median furrow the granules or 
cells collect, forming a little mound co-extensive with the furrow in 
length. These mounds eventually arch over the furrow and unite, 
thus enclosing a space, in wliich is formed the spinal cord and 
brain. Kunning across the two longitudinal lines (the axis of the 
germ) are lines which represent the vertebral rudiments. (Fig. 6 
shows the median furrow, the two straight lines parallel with it, and 
one extremity of the embryo.) On each side of the median furrow, 
and above the notochord, is formed the half of the spinal cord and 
brain, so that the cord contains a cavity within, when the two halves 
unite above by arching over. At this period we have, next to the 
yelk, a cylindrical body, the notochord: above it, the spinal-cord 
rudiment, separated into halves by the median furrow, and termi- 
nating in a cephalic, and a caudal, rounded extremity : the whole 
encircling one-half or three-fourths of the yelk. 40 hours. — The 
cephalic extremity is pointed: on either side is an inbending of 
• the lateral line, marking off the position of the mesencephalon or 
middle lobe of the brain (Fig. 7). In profile the embryo is seen to 
form an elevated mass on the surface of the yelk ; the rudiments of 
four vertebrae, or bones composing the spinal column, are observable, 
commencing below the cephalic inflection. (Fig. 8 : a, cephalic ex- 
tremity ; h, rudiments of vertebrae, being segmentation or division 
of the notochord ; c, caudal extremity.) 52 hours. — The cephalic 
extremity resembles a leaf of clover, being divided into three lobes : 
the anterior one, the anterior lobe of the brain ; the two lateral 
lobes, the median lobe of the brain, and the eyes. Behind the 
middle lobe of the brain is a third and smaller one. (Fig. 9, the 
three lobes seen laterally, at a ; a wave of indentation is also seen.) 
(Fig. 10 shows, a, the anterior lobe; h, median lobe, with a vesicle 
on either side, the rudiment of the eye ; c, the posterior lobe ; also 
several rudimentary vertebrae, and the ventricle or cavity between 
the halves of the spinal cord.) Twelve vertebral segments are 
discernible : the cavity occupied by the spinal cord is seen running 
along their summit. 58 hours. — The spinal cord is seen extend- 
ing from the eyes to the tail, widening just below the eyes, 
and then preserving the same width to the tail. It is divided 
into halves by a median sulcus or furrow. Ylth Ajoril, 10 a.m., 
72 hours. — The eye has a central lens, an iris, and a round band 
encircling both. The ear appears as a somewhat quadrangular 
capsule, enclosing smaller ones, on the side of the brain mass. The 
heart also is seen, as a tube bent upon itself, somewhat anterior to 
the position of the ear. From the heart passes a vessel upwards 
and towards the tail, but at present it extends no farther than 
first vertebra. A fold of membrane is reflected from the under- 
part of the head, which includes the rudimentary heart. (Fig. 11 
shows, a, the eye ; h, the including membrane ; c, the heart, with 
