22 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Most of the outer cells now take on a decidedly different appear- 

 ance from the internal cells. There is a layer of protoplasm against 

 the outer wall of each cell that stains rather deeply, but the remainder 

 of the cell is occupied by vacuolated protoplasm. These vacuoles become 

 larger with age. 



Beside the vacuolated cells, there is a group of cells, Figs. 38 and 

 39 eg., and Fig. 40, ap., which are largely surface cells and later give 

 rise to the cerebral ganglia and the apical plate. The apical plate, 

 Fig. 40, ap., is already distinguishable. 



Whether there is a general migration of surface cells into the 

 interior is doubtful, but it is certain that their number is reduced. 

 The reduction is probably not more than is accounted for by the 

 pushing in of those around the blastopore, and the setting aside of 

 the group from which the cerebral ganglia and apical plate arise. 



The embryo now elongates, Fig. 31, the apical plate acquires a 

 tuft of apical cilia, ac., and the outer, vacuolated cells, which will here- 

 after be referred to as test-cells, arrange themselves in five rows 

 that surround the embryo. Frequently the fifth, last, row of test-cells, 

 counting from the anterior end, is not complete on the dorsal side at this 

 stage, but this does not long remain so. Occasionally a sixth 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 the 

 external view of the embryo by a depression, x, where the cells 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 toward the same, that 

 is, toward 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 longi- 

 tudinal axis. The rotation is quite rapid, but is not always in the same 

 direction, there being frequent changes. During activity the apical 

 cilia are bunched 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 blastopore are still 

 actively undergoing division and are quite small, Fig. 41. 



The cells of the apical plate, ap, and those from which the cerebral 



