34 OUTLINES OF CHORDATE DEVELOPMENT 



opposite the first two somites (Fig. 9, C). Throughout the 

 embryonic period, as described above, the only opening to the 

 exterior is by way of the neurenteric canal, neurocosl, and neuro- 

 pore, the essential relations among which do not change during 

 this period. We may describe briefly the chief structures 

 arising in connection with the enteron during the embryonic 

 period. 



A. THE CLUB-SHAPED GLAND AND ENDOSTYLE 



In the floor of the enteron, just back of the anterior gut 

 diverticula, and therefore opposite the anterior margin of the 

 first somite, a transverse groove appears about the time nine 

 or ten somites are present (Figs. 9, B; 11). During the latter 

 part of the embryonic period this groove constricts off as a 

 narrow tube, separate from and below the gut, and acquires an 

 opening at its left end to the exterior in the region below the 

 preoral pit, while the right end dilates into a conspicuous sac, 

 closed at this time (Fig. 9, C). 



Lying just in front of the grooved rudiment of the club-shaped 

 gland, is a strip of high ciliated cells which seems to be the first 

 indication of the endostyle. The later development of the 

 gland and endostyle is described in connection with the 

 history of the larval period. 



B. THE MOUTH 



The mouth, first gill slit, and anus develop almost simul- 

 taneously, toward the close of the embryonic period. At the 

 age of fourteen somites a large patch of ectoderm on the 

 left side of the head, along the margin of the first somite, 

 thickens considerably and with this thickened area the adjacent 

 wall of the enteron fuses. This fused region then becomes per- 

 forated by a small opening (Fig. 9, C), which rapidly enlarges 

 and becomes the relatively enormous mouth opening bordered 

 with elongated cilia (Fig. 12). 



