FOURTH PERIOD OF DEVELOPMENT. 157 



pally owes its origin to a thickening of the epiblasfc, 

 the formation of the first gill-slit begins with remark- 

 able alterations in the hypoblast. 



We were already able, in the stage of Fig. 5i, to 

 observe a small indentation of the hypoblast, on the 

 ventral side, in the region of the second segment. The 

 cells multiply here very rapidly. While the wall of 

 the anterior end of the alimentary canal, with the 

 exception of a small-celled streak in front of the 

 club-shaped gland, is composed of large granulated 

 cells, the foundation of the gill comes at once into 

 view, owing to the fact that it consists of somewhat 

 high thin columnar cells of clear appearance (Fig. 

 60). These form a disc-like thickening of the wall 

 of the alimentary canal. 



This thickening lies at first, more or less, in the 

 ventral middle line, but before the perforation of the 

 gill-opening moves a little to the right ; and when we 

 think that the boundaries of the segments are elon- 

 gated, we see that this opening is situated in the 

 second segment. This disc is distinguished also by 

 its stronger ciliation. 



We have already described above the relation of 

 this foundation to the blood vessel. 



