FIFTH PERIOD OF DEVELOPMENT. 161 



follows iii such a way that the place of the original 

 opening corresponds to the anterior edge of the later 

 and subsequent one. The opening of the mouth 

 remains continually surrounded by a thickened epi- 

 blast edge (Fig. 62). 



The origin of the first gill-slit is due to the fact 

 that a funnel-shaped depression is now formed on the 

 interior surface of the disc-shaped gill foundation, 

 which is turned towards the lumen of the alimentary 

 canal (Fig. G!A). This depression presses forward as 

 far as the outer surface, which is here in immediate 

 contact with the alimentary canal, and there is now 

 formed in this place a fine opening, which gradually 

 enlarges, similarly to the opening of the mouth. The 

 epiblast remains thin on the edge of the gill-slit ; the 

 hypoblast, however, which previously manifested here a 

 disc shape now forms a broad ring-shaped wall of thin 

 columnar ciliated cells, representing the interior edge 

 of the gill-slit. The latter is already on its first per- 

 foration moved slightly to the right, and during its 

 enlargement makes its way further and further up on 

 the right side of the body. 



The perforation of the outer opening of the ciliated 

 organ follows soon after that of the mouth and first 



M 



