70 AMPHIOXUS. 



a. The anterior gut diverticula. At a stage with seven 

 pairs of somites, a pair of lateral diverticula arise from the dilated 

 anterior end of the mesenteron. These are situated (Figs. 30, 

 31, DL, DK), near the dorsal surface of the mesenteron, just in 

 front of the first pair of somites, and ventral to the anterior 

 prolongations of these somites. 



The two diverticula soon separate from the mesenteron, which 

 then shrinks back from the anterior end of the body. They are , 

 at first of equal size, but from a stage with about ten pairs of 

 somites, onwards, they develop very unequally. 



The right anterior gut diverticulum (Fig. 33, DR) forms a thin- 

 walled sac, which extends forwards so as to occupy a large space 

 at the anterior end of the body, below the notochord ; its walls 

 become flattened epithelial cells, and the space which they inclose 

 may be spoken of as the head-cavity. 



The left anterior gut diverticulum (Fig. 33, DL) remains ot 

 small size, and forms a spherical thick-walled sac, lying on the 

 left side of the head, just in front of the mesenteron and a little 

 way behind the level of the neuropore ; its wall consists of a 

 single layer of columnar ciliated epithelial cells. Towards the 

 close of the embryonic period it opens to the surface by a small 

 pore on the left side of the head (Fig. 34, DL), and from this 

 time is spoken of as the praeoral pit. 



The homologies of these anterior gut diverticula with organs 

 of higher Vertebrates are very uncertain. They are probably to 

 be regarded as parts of the body cavity or ccelom, though it 

 must be admitted that their development differs in important 

 respects from the rest of the ccelom. In the mode of their 

 origin, in their asymmetry, and in the fact that the left diver- 

 ticulum early acquires an opening to the exterior, they resemble 

 the anterior ccelomic diverticula of Balanoglossus, and the 

 enteroccelic outgrowths of Echinoderms, with which they have 

 by some observers been held to correspond. 



b. The club-shaped gland. In embryos with nine or ten 

 pairs of somites a shallow transverse groove appears across the 

 floor of the mesenteron, and extending up its sides, opposite the 

 septum between the first and second pairs of somites. The first 

 commencement of this groove is seen in Fig. 30, opposite the 

 ventral end of the first somite, but is not indicated by a refer- 

 ence letter. Towards the end of embryonic life the lips of the 



