FOURTH PERIOD OF DEVELOPMENT. 149 



misymmetrical manner. They both abstrict com- 

 pletely from the alimentary canal, which then is 

 completely withdrawn from the anterior end of the 

 body. The diverticulum on the right side extends 

 considerably, and its cells become flattened like an 

 endothelium. This thin-walled diverticulum encloses 

 then a large triangular open space occupying the 

 anterior body end of the notochord ventralwards. 

 The diverticulum on the left side remains round and 

 thick- walled ; it is composed of columnar cells. While 

 that on the right side moves more towards the 

 anterior, the other remains at the posterior end of the 

 head-prolongation somewhat further back than the 

 brain swelling of the neural canal. At the beginning 

 of the larval stage this diverticulum, which is 

 covered on the inner surface with cilia, breaks 

 through on the left side of the body with a small 

 opening outwards the preoral pit. 



The diverticulum on the left side was described by 

 Ko walevsky as u a peculiar sense organ " of the larva. 

 He did not recognise its development. 



AVe will now observe these processes more closely. 



The alterations of both hypoblast diverticula can be 

 studied in the living object by observation oi a con- 



