THE DEVELOPMENT OF AMPHIOXUS 33 



and have been commonly regarded as endodermal derivatives 

 of peculiar character, but although their exact homologies 

 remain rather obscure it seems clear that they are essentially 

 a pair of mesodermal somites which develop late, on account 

 of the delayed forward extension of the whole anterior tip of 

 the body. The anterior gut diverticula first appear in the 

 embryo with seven pairs of somites, as a pair of narrow antero- 

 dorsal evaginations from an expanded anterior region of the 

 enteron (Figs. 9, B; 11, A, B}. These push forward much 

 like the somites, and quickly pinch off from the enteron as 

 a pair of closed sacs (Fig. 11, C). 



Although symmetrical in their origin they differ greatly in 

 their subsequent development. In the embryo with ten pairs 

 of somites the right diverticulum begins to grow forward and to 

 enlarge while its wall becomes very thin. Soon it extends 

 across the mid-line and finally, as a median structure, it occu- 

 pies the whole of the space below the chorda and from the en- 

 teron forward to the tip of the embryo. It remains wholly 

 in front of the first somite and forms what is known as the 

 preoral cavity or head cavity (Fig. 9, C). The left anterior gut 

 diverticulum differs widely from its antimere. It remains 

 quite small and unmodified throughout the embryonic period, 

 shortly after which it acquires an opening to the surface in the 

 left side of the head below the level of the chorda, and is then 

 known as the preoral pit (Fig. 9, C). Its history is quite com- 

 plicated and will be considered later. 



4. The Enteron and its Appendages 



After the wall of the archenteric cavity has lost successively 

 the mesoblast folds, notochord, and anterior gut diverticula, the 

 endodermal tube remains as the wall of the enteron or mesen- 

 teron. This gives rise to the epithelial lining of the alimentary 

 canal and related structures. After the time of hatching the 

 enteron elongates rapidly and, except at its anterior extremity, 

 narrows very markedly, differentiating in this way the posterior 

 stomach-intestine from the wide anterior pharyngeal region 



