DEVELOPMENT OF LAMPREY l g l 



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 coelenteron, arising at BP, has 

 become greatly elongated ; at its blind end its lining mem- 

 brane, entoderm, EN, is in contact with an indented por- 

 tion of the ectoderm, at 5, where later the opening of the 

 mouth will be established ; and that ventrally the coelen- 

 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 coelenteron 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, M, 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 

 coelenteron, or gut, is at G, the ectoderm at EC, 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. 



