4o; 



tliemsel-ves in^ five rovrs that surround the embrvo. Frequently 

 the fifth, last, row of test cells, is not complete on the dorsal 

 side at this stage but this does not long remain so. Occasional- 

 Iv a si^cth more or less complete row is present. The blastopore 

 and the apical plate occupy nearly opposite ends of the embryo, 

 but for some time the blastopore may be twisted around somewhat 

 to one side. The position of the group of cells from which the 

 cerebral ganglia are formed, is indicated in tZae external view of 

 the embryo by a depression, x, where the colls of the first, or 

 anterior row of test cells, are pushed away from the second row. 

 As later stages show this to be the ventral side of the embryo, 

 it is easy to determine that the blastopore is always twisted 

 tovmrd the same, that is, the dorsal side. 



The test cells are distinctly rounded and the cilia on 

 each of the three intermediate rows are collected into a band. 

 The embryo now swims in more or less definite lines, rotating 

 the while upon its longitudinal axis. The rotation is q'aite 

 rapid but is not always in the same direction, there being fre- 

 quent changes. During activity the apical cilia are b-onched 

 together into a sort of whip, which lashes vigorously from side 

 to side, and always precedes the embryo in its movements. 



Inside the test the cells in the region of the blasto- 

 pore are still activel:'- undergoing division and are quite small. 



