FOURTH PERIOD OF DEVELOPMENT. 153 



the next stages this fold is deepened, and in its 

 appearance it soon reminds us of the completed gland, 

 although still, in its whole extent, it is open towards 

 the alimentary canal (Figs. 55, 57). It is strongest 

 on the right side of the body, where along the whole 

 surface of the wall of the alimentary canal it descends 

 somewhat obliquely in an anterior direction, and 

 continues, on the left side, considerably smaller, and 

 only as far as the middle of the left wall of the ali- 

 mentary canal (Fig. 60). 



Towards the end of the embryonal period follows 

 the closing of the canal, and the abstraction of this 

 formation from the alimentary canal (Fig. 61). This 

 formation now exhibits a gland situated on the right 

 side. This continues into a thin duct, which ven- 

 tralwards curves round the alimentary canal, and 

 on the left side rises till near the middle of the 

 latter (Fig. 63). There, later on, the thin duct 

 opens outwards. Just as, later on, the mouth per- 

 forates in this part of the left wall of the body, 

 so the gland then opens at the exterior edge of the 

 mouth. 



This canal, which runs obliquely over the alimen- 

 tary canal, and which divides into a glandular portion 



