DEVELOPMENT OF LAMPREY Ig! 
future dorsal region of the embryo; on this side the 
margin of the blastopore is known as the dorsal lip, DL, 
while to the right the ventral lip is seen greatly enlarged 
by the yolk-bearing cells, Y. A somewhat later stage 
(Fig. 208) shows the blastopore as a narrowly constricted 
opening, BP, whose dorsal lip is slightly raised at its left- 
hand margin. The head of the embryo is to arise near 
the opposite pole (as in Fig. 210), and is thence to elon- 
gate into neck and trunk (Fig. 212). A sagittal section of 
a stage, slightly older than Fig. 208, shows admirably the 
structures of the embryo that have thus far been differ- 
entiated (Fig. 209). Contrasting with Fig. 207, it will 
thus be seen that the ccelenteron, arising at BP, has 
become greatly elongated ; at its blind end its lining mem- 
brane, entoderm, ZV, is in contact with an indented por- 
tion of the ectoderm, at S, where later the opening of the 
mouth will be established ; and that ventrally the ccelen- 
teron has given off a pouch which passes into the yolk, and 
will later be differentiated as the liver. That the entire 
dorsal wall of the ccelenteron has become thickened, con- 
stitutes the main difference between the sections of Figs. 
207 and 209; there have, in other words, arisen between 
the entoderm and ectoderm of Fig. 207 the central ner- 
vous system, or medullary cord, WY, and the notochord, CH. 
The origin of these structures may best be traced in the 
cross-section of a slightly earlier stage (Fig. 213); the 
ceelenteron, or gut, is at G, the ectoderm at ZC, the yolk 
cells intervening at Y; and the notochord and medullary 
cord, CH, and M, in the sagittal region immediately be- 
tween the gut and the ectoderm. In the medullary region 
the ectoderm cells are seen pressed together, growing down- 
ward and sidewise, forming altogether a compact cell cord * 
* As in Teleosts, but unlike other vertebrates. 
