39. 

 of tlie blastopo3:'e become especially active, dividing and crowding 



in arouad tlie large internal cell, wlLidi novr divides into nearly 



equal cells, figure 39. 



Most of tiie outer cells now take on a decidedly differ- 

 ent appearance from tiie internal cells. 



There is a rather densely staining layer of protoplasm 

 against the outer -wall of each cell, but the remainder of the cell 

 is occupied by vacuolated protoplasm. These vacuoles become 

 larger -with age. 



Beside the vaucolated cells, there is a group of cells, 

 figures 38 and 39 eg, and figure 40, ap, which are largely sur- 

 face colls and later give rise to the cerebral ganglia and the 

 apical plate. The apical plate, figure 40, Ifcp, is already dis- 

 t ingui shab 1 e . 



"Whether there is a general migration of surface cells 

 into the interior is doubtful, but it is certain that their num- 

 ber 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, figure 31, apical plate ac- 

 quires a tuft of apical cilia, ac, and the oiiter, vacuolated 

 cells, which will hereafter be referred to as test cells, arrange 



